seasonal interactions in the yellow warbler setophaga petechia): winter habitat...
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Seasonal interactions in the Yellow Warbler
(Setophaga petechia): winter habitat use,
migration and demography
by
Anna Evelyn Gray Drake
M.Sc. (Animal Science), University of British Columbia, 2007 B.Sc. (Ecology and Evolution), Simon Fraser University, 2003
Thesis Submitted In Partial Fulfillment of the
Requirements for the Degree of
Doctor of Philosophy
in the
Department of Biological Sciences
Faculty of Science
Anna Evelyn Gray Drake 2013
SIMON FRASER UNIVERSITY
Spring 2013
All rights reserved. However, in accordance with the Copyright Act of Canada, this work may
be reproduced, without authorization, under the conditions for "Fair Dealing". Therefore, limited reproduction of this work for the purposes of
private study, research, criticism, review and news reporting is likely to be in accordance with the law, particularly if cited appropriately.
ii
Approval
Name: Anna Evelyn Gray Drake
Degree: Doctor of Philosophy (Biology)
Title of Thesis: Seasonal interactions in the Yellow Warbler (Setophaga petechia): winter habitat use, migration and demography
Examining Committee: Chair: Arne Mooers
Professor
David J. Green
Senior Supervisor Associate Professor
David B. Lank Supervisor Adjunct Professor
Ronald C. Ydenberg
Supervisor Professor
Wendy J. Palen
Internal Examiner Assistant Professor Department of Biological Sciences
Peter P. Marra
External Examiner Adjunct Professor Department of Biology Queen’s University
Date Defended: April 16, 2013
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Partial Copyright Licence
Ethics Statement
The author, whose name appears on the title page of this work, has obtained, for the research described in this work, either:
a. human research ethics approval from the Simon Fraser University Office of Research Ethics,
or
b. advance approval of the animal care protocol from the University Animal Care Committee of Simon Fraser University;
or has conducted the research
c. as a co-investigator, collaborator or research assistant in a research project approved in advance,
or
d. as a member of a course approved in advance for minimal risk human research, by the Office of Research Ethics.
A copy of the approval letter has been filed at the Theses Office of the University Library at the time of submission of this thesis or project.
The original application for approval and letter of approval are filed with the relevant offices. Inquiries may be directed to those authorities.
Simon Fraser University Library Burnaby, British Columbia, Canada
update Spring 2010
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Abstract
Numerous bird species in the western hemisphere – Nearctic-Neotropical migrants –
breed and rear young in North America but spend two-thirds of the year south of the
Tropic of Cancer. The annual cycle of these species can be divided into a distinct
breeding, migratory and non-breeding period. Events in one period may alter
demographic rates in a subsequent period; however, the spatial and temporal separation
of these events and the difficulty of tracking individuals over the annual cycle make
detecting seasonal interactions/carry-over effects difficult. In this thesis I confirm that
13C and 15N isotopic signatures in Yellow Warbler (Setophaga petechia) tissues vary
with winter habitat use and can therefore be used as a means of inferring the wintering
conditions experienced by individuals captured on the breeding grounds. I show that
isotopic signatures suggestive of drier and possibly more southerly winter habitat use
are associated with delayed clutch initiation and lower productivity in young female
Yellow Warblers breeding in Revelstoke, British Columbia. In contrast, wintering isotopic
signatures do not influence the breeding phenology of other age/sex-classes in
Revelstoke or any age/sex-class in Inuvik, Northwest Territories, 2000 km further north.
Differences between Revelstoke and Inuvik in how productivity declines seasonally
suggest that one pathway through which carry-over effects can act is reduced or absent
in the north. I also examine the role of climate variables operating at discrete periods of
the annual cycle on the demography of Yellow Warblers in Revelstoke. I demonstrate
that wind conditions during spring migration are the best predictor of adult survival, male
arrival date and female clutch initiation date. These timing effects, in turn, impact
reproductive success. This finding indicates that the spring migration is a key period for
western Yellow Warblers, having a larger impact than either winter habitat use or
wintering conditions and influencing both demographic rates.
Keywords: migration; seasonal interactions; carry-over effects; breeding phenology; population demography; Setophaga petechia
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Dedication
To Bus for walking with me when I was small
and sharing with me your love of nature, and
to my Mom and Dad for always sharing with
us your engagement and interest in the world
and in life
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Acknowledgements
Thank you to my advisory committee, Dr. Ron Ydenberg and Dr. David Lank, for your
help throughout the process: Ron for playing devil’s advocate and therefore making the
arguments and the resulting science much stronger and Dov for thoughtful feedback and
difficult questions which helped me better frame my thoughts and arguments. Thank you
to my supervisor Dr. David Green, for all of your help and for always being invested and
supportive. Thank you for asking me after my first field season when I was going to take
a break rather than when I was going to start analyzing the data. Connie Smith, Monica
Court and Marlene Nguyen: without your help I couldn’t have completed this degree.
Thank you to my examining committee, Dr. Peter Marra and Dr. Wendy Palen and to my
defense chair, Dr. Arne Mooers for investing your time and energy in reviewing and
critiquing my work, and in the thesis exam itself.
I owe many thanks to my field assistants. Stephanie Topp for your hard work over two
seasons in Inuvik and for your strong ethical standards regarding both the research sites
and the birds. The quality and quantity of the data from Inuvik owes a lot to your
excellent field and observational skills. Matthew Pennell for your dedication and
enthusiasm in 2010: thank you for forging ahead through long hours and millions of
mosquito and blackfly bites. Thank you to Simon Valdez for your indispensible help in
2011 (89 birds in 41 days!). Thank you also for your company and support: you made
the degree easier and everything else a lot more fun.
The Nihtat Gwich’in Renewable Resources Council, the Inuvik Hunters and Trappers
Committee and the Inuvik Town Council allowed me to collect data in the Inuvik area for
which I am grateful. The staff and visiting researchers at the Aurora Research Institute
were a great help in 2009 and 2010 and their company made my time in Inuvik
enjoyable. Thank you especially: Don Ross, Annika Trimble, Lisa Greenland, Debbie
Gully, William Hurst, and Pippa Seccombe-Hett. Environment Canada provided in-kind
support that made the Inuvik work feasible. A special thank-you goes to Joel Ingram and
Blake Bartzen at Environment Canada who managed the administrative details for this,
provided local bird knowledge, and offered assistance. Thank you to Dr. Guillermo
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Fernández at the Universidad Nacional Autónoma de México (UNAM), who enabled me
to conduct work in Mexico and received, held, and exported samples for the project. I am
greatly appreciative of your help and of the time and effort you put into this. The staff and
researchers at the UNAM field stations made my stay in two beautiful reserves
productive and enjoyable. Specific thanks goes to Dr. Jorge Vega and Rosamond
Coates for sharing their knowledge of wintering migrant habitat use within the Chamela-
Cuixmala and Los Tuxtlas regions. Thank you also to Rosamond and to the local
farmers and ranchers at Punta Pérula and Emiliano Zapata, JA and La Barra and
Sontecomapan, VZ for generously providing access to your properties for surveys and
banding.
I am also grateful to the undergraduate work-study students and assistants who helped
in Inuvik and did data processing over the years: Kate Snow, Daniel and Andrew Fehr,
Gabrielle Pang, Lindsay Davidson, Amirhossein Manavi, Pipreesh Gaind and especially
Kristina Hunter, who did the work of two assistants in 2011.
To my parents, thank you for your constant encouragement, your love, and the many,
many ways you have offered me support over my graduate career. To the rest of my
family, particularly Emily and Jol, and to each of my friends: thank you for your
encouragement and support. My fellow grad students (past and present) offered
feedback, assistance, coffee breaks, and entertainment and made my time at the CWE
memorable. Special thanks to Danica Hogan, Michelle Martin and Heidi Currier: when
the options were to laugh or cry, you made laughing much easier.
My graduate work would not have been possible without scholarship and research
support from the Natural Sciences and Engineering Research Council of Canada
(NSERC), the Northern Studies Training Program (NSTP), the Centre for Wildlife
Ecology, and from Simon Fraser University.
Finally, thank you to the birds: for putting up with the stress of capture, handling, feather
pulling, harassment at the nest, and the oddity of plastic rings on your legs. Three
seasons of working with Yellow Warblers have made me conclude that they are a little
like tiny, feathered bulldogs: occasionally bad tempered and always determined and
tenacious. My respect goes to the pairs that attempted to lure us away from the nest with
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injury displays, the females who did not flush when approached, and particularly
‘BwDgP’ who valiantly stayed at the nest edge to battle me, only 7000× her size, over
nest measures by grabbing and attempting to toss the ruler.
Aldo Leopold said that despite intense study we can never know all of the salient facts
that relate to the hundreds of little dramas that occur in the woods and fields. I think that
this is true, but that there is something of spiritual value in the attempt. I was witness to
hundreds of dramas in my time in Inuvik: nestlings and adults lost to predators, late
clutches abandoned, cuckoldry, mates that disappeared for unknown reasons, a female
who brooded an empty nest, and, more happily, a breeding pair who - after having
previously lost two clutches and built nests in three different locations - hatched a single
egg at the tail end of the season: an egg which both I and my field assistants had been
sure was infertile. Anyone casually walking in the immediate vicinity of these events
would not know about them; in fact, without some effort of observation, this person might
not even know there were little yellow birds going about their lives a few feet away. My
work allowed me to make this effort, to be an observer within this parallel world, and to
attempt to see things from a new, non-human perspective. I hope many people get this
kind of opportunity, because it is a door out of a limited, human-centered worldview into
something much more diverse and fascinating.
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Table of Contents
Approval .......................................................................................................................... ii Partial Copyright Licence................................................................................................ iii Abstract .......................................................................................................................... iv Dedication ....................................................................................................................... v Acknowledgements ........................................................................................................ vi Table of Contents ........................................................................................................... ix List of Tables .................................................................................................................. xi List of Figures................................................................................................................ xiii Introductory Image ........................................................................................................xvi
Chapter 1. General Introduction .............................................................................. 1 The role of the non-breeding period in population dynamics............................................ 2 Measuring the influence of the non-breeding period ........................................................ 3 Objectives ....................................................................................................................... 4 References ...................................................................................................................... 6 Figures .......................................................................................................................... 11
Chapter 2. Habitat use differs by sex in wintering Yellow Warblers ................... 13 Abstract ......................................................................................................................... 13 Introduction ................................................................................................................... 14 Methods ........................................................................................................................ 16
Study species ....................................................................................................... 16 Study areas .......................................................................................................... 17 Habitat searches, point counts and targeted mist-netting ..................................... 18
13C and 15N Isotope Signatures ......................................................................... 20 Statistical Analysis ................................................................................................ 20
Results .......................................................................................................................... 23 Habitat Use........................................................................................................... 23 Variation in morphology ........................................................................................ 23 Abundance and habitat use by sex, age and body size ........................................ 23 Within-habitat variation in 13C and 15N signatures .............................................. 25
Discussion ..................................................................................................................... 26 References .................................................................................................................... 31 Tables and Figures........................................................................................................ 37
Chapter 3. Carry-over effects of winter habitat vary with age and sex in Yellow Warblers (Setophaga petechia) ............................................... 40
Abstract ......................................................................................................................... 40 Introduction ................................................................................................................... 41 Methods ........................................................................................................................ 44
Study Species and location .................................................................................. 44 Monitoring and breeding ....................................................................................... 45 Sexing and aging .................................................................................................. 45 Feather samples and stable isotope analysis ....................................................... 46 Winter habitat use and 13C and 15N ................................................................... 46
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Data Analysis ....................................................................................................... 48 Results .......................................................................................................................... 49
Age differences in phenology and breeding performance ..................................... 49 Winter habitat use and 13C and 15N ................................................................... 49 Carry-over effects ................................................................................................. 50
Discussion ..................................................................................................................... 51 References .................................................................................................................... 55 Tables and Figures........................................................................................................ 62
Chapter 4. Winter habitat use does not influence spring arrival dates or the reproductive success of Yellow Warblers breeding in the arctic ..................................................................................................... 67
Abstract ......................................................................................................................... 67 Introduction ................................................................................................................... 68 Methods ........................................................................................................................ 70
Study Species ...................................................................................................... 70 Study site and population monitoring .................................................................... 72 Sexing and aging .................................................................................................. 73 Tissue samples and stable isotope analysis ......................................................... 73 Carry-over effects ................................................................................................. 74
Results .......................................................................................................................... 75 Population traits .................................................................................................... 75 Carry-over effects ................................................................................................. 76 Comparison with mid-latitude sites ....................................................................... 77
Discussion ..................................................................................................................... 77 References .................................................................................................................... 81 Tables and Figures........................................................................................................ 87
Chapter 5. Wind speed during migration influences the survival, timing of breeding, and productivity of a Neotropical migrant..................... 92
Introduction ................................................................................................................... 92 Results and Discussion ................................................................................................. 95 Materials and Methods .................................................................................................. 96 References .................................................................................................................. 100 Tables and Figures...................................................................................................... 104
Chapter 6. Significance and Future Directions .................................................. 114 General Conclusions ................................................................................................... 114 Future directions ......................................................................................................... 116
Non-breeding effects – are there longitudinal patterns? ...................................... 116 References .................................................................................................................. 119 Tables and Figures...................................................................................................... 123
Appendices ............................................................................................................. 129 Appendix A. Derivation of wind data presented in Figure 6.1 .................................. 130 References: ................................................................................................................. 130
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List of Tables
Table 2.1 Morphometric traits (± SD) of Yellow Warblers wintering on the Pacific and Gulf coast of Mexico. Sample sizes are reported in brackets. The length of the 6th primary feather was measured in 2011 only and therefore the sample sizes for wing shape (P6/P9) are smaller than other metrics. ............................................................................................... 37
Table 3.1 Summary of stable isotope signatures (n, mean ±SD and range) from winter-grown feathers collected from Yellow Warblers breeding in Revelstoke, British Columbia. Yearling individuals are in their first breeding season; older birds are at least 2 years old. .................................. 62
Table 3.2 Results of general linear model analyses for each stage of the proposed pathway from winter habitat use to fledgling productivity. Relationships for yearling and older Yellow Warbler are reported separately for (A) males and (B) females. Standardized beta coefficients for significant continuous variables are reported in Figure 3.2. .............................................................................................................. 63
Table 4.1 Multiple regression analyses testing the influence of winter habitat
(inferred from 13C and 15N) on Inuvik Yellow Warbler breeding metrics. For predicted relationships see Figure 4.1. Age and Year plus second-order interaction terms were included in initial models and dropped sequentially if they did not contribute to fit. Below each model
we report the contribution of 13C and 15N to model fit (italics) if these variables were to be independently added back into the final model........................................................................................................... 88
Table 4.2 Location effects on the relationship between first-clutch initiation date and productivity in Yellow Warblers from the perspective of (A) breeding males and (B) breeding females. For both sexes the best fitting model predicts a negative relationship at our mid-latitude (Revelstoke) site and no relationship at our high-latitude (Inuvik) site. See also Figure 4.2. ..................................................................................... 89
Table 5.1 Models describing apparent annual survival of Yellow Warblers breeding in Revelstoke, British Columbia (n=279 individuals, 437 encounters). Models are ranked using QAICc. Model number refers to regionally-specific climate variables described in the text. Age was included in all models as a covariate (see methods). The number of parameters in the model (K), Akaike’s information criterion (QAICc), QAICc difference from the top model (ΔQAICc), and Akaike weight (ωi) are reported. ....................................................................................... 104
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Table 5.2. Ranked summary of AICc support for the candidate climate models and the null model (YEAR + AGE) describing male Yellow Warbler arrival date (n=183) in Revelstoke, British Columbia. Age (SY=1 yr; ASY>2 yrs) was included in all models as a covariate (see methods). Model adjusted r2, the number of parameters in the model (K), Akaike’s information criterion adjusted for small sample size (AICc), AICc difference from the top model (ΔAICc), and Akaike weight (ωi) are reported. .............................................................................................. 106
Table 5.3 Ranked summary of AICc support for the candidate climate models and the null model (YEAR + AGE) describing female Yellow Warbler clutch initiation date (n=155) in Revelstoke, British Columbia. Age (SY=1 yr; ASY>2 yrs) was included in all models as a covariate (see methods). Model adjusted r2, the number of parameters in the model (K), Akaike’s information criterion adjusted for small sample size (AICc), AICc difference from the top model (ΔAICc), and Akaike weight (ωi) are reported. ............................................................................ 107
Table 5.4. Correlation (r) matrix of explanatory climate variables and time (year) (n=7). Significant relationships (Spearman’s ρ) are starred (*=0.01 ≤ P ≤ 0.05). Variables where the predicted relationship with survival and breeding phenology would suggest a possible confound between models are marked with (‡) ....................................................................... 108
Table 6.1. Overview of cross-seasonal studies involving Neotropical migrant passerines. Study number indicates study location in Figure 6.2. .............. 124
Table 7.1. Region limits and night wind vectors (March-May averages, 2003-2012) presented in Figure 6.1(b) ................................................................ 130
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List of Figures
Figure 1.1 Data collection sites and their associated chapters within this thesis. Base map courtesy of Wikimedia Commons. ............................................... 11
Figure 1.2 Data collection sites shown within the Yellow Warbler breeding and wintering range (shaded regions). Dashed lines indicate the geographical limit of the eastern genetic line during the breeding (top) and wintering (bottom) period (Boulet et al. 2006). The probability of eastern haplotypes being present to the west of these lines is less than 0.1 (Boulet et al. 2006). ........................................................................ 12
Figure 2.1 Patterns of habitat segregation in Yellow Warblers on the (a) Pacific coast (“Chamela”) and (b) Gulf coast (“Los Tuxtlas”) of Mexico. Left columns represent point count results with individuals identified as older (ASY) males (solid dark grey) or ‘other’ (solid light grey). Right columns represent banding data where the ‘other’ category is separated into older females (solid white), yearling (SY) females (stippled), and yearling males (grey, diagonal lines). Sample sizes are indicated above each column. In Chamela, riparian habitat contained more males than agriculture and both of these habitats contained more males than scrub. In Los Tuxtlas, coastal and pasture habitats did not significantly differ in the proportion of males present. ....................... 38
Figure 2.2 Variation in Yellow Warbler blood-15N signatures within habitats in relation to body size. Habitat-types were classified as (a) agricultural habitat, Pacific coast, (b) riparian habitat, Pacific coast, (c) scrub/seasonal saline marsh, Pacific coast and (d) “Los Tuxtlas”, Gulf of Mexico. This last category merges data from birds using coastal and pasture habitats on the Gulf of Mexico as habitats were not isotopically distinct. Unfilled circles indicate females, filled circles indicate males. ............................................................................................. 39
Figure 3.1 Biplot of the mean (±SD) 15N (‰) and 13C (‰) values of red blood cells from Yellow Warblers over-wintering in different habitat-types on the southern Gulf coast (Veracruz) and the Pacific slope (Jalisco) of Mexico. Bracketed letters indicate signature overlap: birds from habitats with shared letters were not significantly different from each
other in terms of tissue 15N (A-C) and/or 13C (D-F). .................................. 64
Figure 3.2 Model pathways for Yellow Warbler males and females by age class. Black solid lines indicate significant relationships between variables (thickest line P<0.0001, mid-thickness P<0.001, thin line P<0.05). Dashed lines indicate no statistically-detectable relationship between variables. Standardized beta coefficients for significant continuous variables are shown above each arrow. Whole-model r2 values are reported in parentheses above the dependent variables for significant relationships only. ........................................................................................ 65
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Figure 3.3 Relationship between winter isotope signatures andclutch initiation date in yearling female Yellow Warblers. Initiation dates are presented
as residuals from a model controlling for year and 15N (a) or for year
and 13C (b). ............................................................................................... 66
Figure 4.1 Suggested pathways through which winter-habitat-driven carry-over effects could act. For females in our study population, clutch initiation date was used as a proxy for arrival date (see text) and therefore condition- and timing-pathways to clutch initiation are not separate.
Depleted 13C and enriched 15N tissue signatures are expected to be associated with high quality winter habitat use. ............................................ 90
Figure 4.2 The relationship between first-clutch initiation date and annual productivity (total number of young fledged) among Yellow Warbler females within our high-latitude breeding population (Inuvik, top) and our mid-latitude breeding population (Revelstoke, bottom). See also Table 4.2. .................................................................................................... 91
Figure 5.1 The location of our study (A), the Yellow Warbler wintering range (blue), and the area used to calculate wind speed values during migration (yellow). Image courtesy of NASA. ............................................. 109
Figure 5.2 Relationship between westerly wind speed during migration and annual survival of Yellow Warblers. Points are apparent survival (± SE) of young (1 yr, open points) and older (≥2 yrs, filled points) birds (7 years, n=279 individuals, 437 encounters). Solid lines and shading represent predicted survival (ϕ) ± 95% CI from the top model assuming an average southerly wind vector. ............................................. 110
Figure 5.3 Predicted relationship between southerly wind speed during migration and annual survival of Yellow Warblers in Revelstoke, British Columbia (see also Fig. 1). Lines and shading represent predicted survival (ϕ) ± 95% CI for young (1 yr, dashed) and older (≥2 yrs, solid) birds from the top model assuming an average westerly wind vector (7 years, n=279 individuals, 437 encounters). ............................................ 111
Figure 5.4 Male arrival date (circles) and female clutch initiation date (diamonds) for Yellow Warblers as a function of westerly wind speed during migration. Points represent mean dates ± SE for young (1yr, open points) and older (≥2yrs, filled points) birds (6 years, n=338). .................... 112
Figure 5.5 Within-individual changes in arrival date (males; n=49) and clutch initiation date (females; n=21) associated with inter-annual variation in average westerly wind vectors. Box-plots show the median change, 10th and 90th percentile, and outliers. Positive values represent delayed breeding phenology in stronger wind years; negative values represent advanced breeding phenology in stronger wind years. Data was restricted to individuals that were ≥ 2 years of age in both years. Males and females exhibited a median shift of +2 and +4 days respectively................................................................................................ 113
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Figure 6.1 (A) The suggested migratory routes used by Neotropical migrants between temperate and tropical regions (Lincoln 1998) and (B) nighttime wind vectors encountered by migrants during the March-May period (10-year average (2003-2012)). Average vectors were calculated within regions bounded by a longitudinal and latitudinal grid (see Appendix A). Boxed regions indicate the approximate area for which vectors were calculated, but are unadjusted for projection. Arrow length corresponds to wind strength with the exception of (*) where length was halved relative to other vectors in the interest of space. ........................................................................................................ 127
Figure 6.2 The geographical distribution and main findings of cross-seasonal studies involving Neotropical migrant passerines. Evidence gathered from wintering isotopic signatures are summarized by age- and sex-class in (A), evidence from wintering climatological data is summarized in (B). Study details are presented in Table 6.1. .................... 128
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Introductory Image
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Chapter 1. General Introduction
Each year North America experiences a dramatic increase in bird abundance, driven by
the seasonal movement of Nearctic-Neotropical migrants from wintering habitat in
Mexico, Central, and South America to breeding locations in the United States and
Canada. This movement involves 252 landbird species and amounts to 3.5 billion
individuals redistributing themselves across an 18.5 million km2 region (Rich et al. 2004).
Within weeks, the number of birds in Canada and the United States quadruples (Rich et
al. 2004; e.g. Holmes 2007). In the subsequent months, these individuals transform the
abundant food of the northern summer into young and their numbers briefly jump to
about 5 billion (Michel et al. 2011). This tide of birdlife then recedes as migrants move
southward to the wintering grounds where they will spend the remaining two-thirds of the
year.
A feature of seasonal movement is that single populations are dependent on
conditions within several regions. For both migrant and resident species, demographic
rates in one period may be influenced by events in an earlier period (“seasonal
interactions”) (Webster and Marra 2005). For migrants, however, this earlier period is
geographically separate. Similarly, factors that limit resident numbers occur within their
breeding range; migrant numbers may be limited by conditions encountered at any
geographic location used during the annual cycle (Sutherland 1996; Newton 2004, 2006)
The role of non-breeding period in regulating and limiting migrant numbers is of
particular interest to ecologists and managers. Migrants tend to be well-studied on the
breeding grounds but they receive little attention during the wintering period and are
difficult to track during the migratory period (Faaborg et al. 2010). This creates a gap in
knowledge that makes migratory birds more vulnerable than resident species: we don’t
often know where or when populations are most limited, or what actions will better
facilitate the recovery of those species in decline (Kelly and Finch 1998).
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The role of the non-breeding period in population dynamics
How might the non-breeding period influence survival and productivity? Migrants
can die at any point in the annual cycle; however, some periods may have a stronger
influence on demography than others. While mortality during migration is high (Sillett and
Holmes 2002), the overwintering period may play a larger role in population limitation.
Winter habitat availability and density dependence may determine population carrying
capacities (“winter-limited” populations; Newton 2004). Sexual habitat segregation and
competition on the wintering grounds may result in more young and female birds dying
than older males (Sherry and Holmes 1995, 1996; Marra and Holmes 2001). These
processes can limit population growth by lowering over-winter survival and juvenile
recruitment rates, and by producing male-biased sex ratios on the breeding grounds that
reduce population productivity (Marra and Holmes 2001; Steifetten and Dale 2006). The
non-breeding period may also alter productivity via a more complex route. The quality of
habitat occupied by individuals on the wintering grounds has been linked to the
maintenance of body weight in the period prior to spring migration (Marra and Holmes
2001; Studds and Marra 2005). Birds in high quality habitat are able to gain mass and
depart the wintering grounds earlier in spring (Marra et al. 1998; Studds and Marra
2005). These birds then arrive earlier on the breeding grounds and initiate reproduction
before their conspecifics (Norris et al. 2004). Because reproducing earlier in the season
tends to result in higher productivity, the repercussions of winter habitat use “carries-
over” to impact individual breeding success (Norris et al. 2004; Reudink et al. 2009;
Tonra et al. 2011). In this thesis I refer to this type of carry-over effect as “time-mediated”
because it is produced by wintering events that act entirely through the timing of
individual arrival on the breeding grounds. Winter habitat use can also influence the
condition of individuals when they arrive on the breeding grounds (González-Prieto
2012). Good wintering habitat may confer physical advantages such as greater fat
reserves, improved muscle condition or higher quality winter-moulted plumage
(González-Prieto 2012; Lindsay 2008; Jones, Drake, and Green, unpublished data). In
this thesis I refer to carry-over effects that do not act via arrival date as “condition-
mediated”. Birds arriving on the breeding grounds in better condition than conspecifics
may pair earlier, pair with a higher quality mate, initiate clutches earlier, or be better
providers. Productivity advantages for these birds follow from the advancement of
3
breeding phenology after arrival and/or from direct benefits. Individual carry-over effects
can scale-up to influence population productivity when a large proportion of the
population experiences poor non-breeding conditions.
Broad-scale climatic variation plays a role in the environment encountered by
migratory birds during their annual cycle (e.g. Saino et al. 2004; Mazerolle et al. 2011).
The effects of climate can be additive and/or interact with local-scale habitat quality to
influence the conditions faced by migrant populations (Brown and Sherry 2006; Studds
and Marra 2007). Climate conditions during the non-breeding period can affect migrant
survival and productivity. For example, links between climate cycles such as the El Niño
Southern Oscillation (ENSO) and the abundance, apparent annual survival, and
breeding phenology of migrant birds have been shown in multiple studies (Sillett et al.
2000; Mazerolle et al. 2005; Macmynowski et al. 2007). This association is often
attributed to variation in winter habitat quality produced by variation in rainfall. Similar
patterns where direct measures of wintering-ground rainfall or primary productivity are
used offer support for this interpretation (e.g. Nott et al. 2002; Saino et al. 2004;
Rockwell et al. 2012).
Measuring the influence of the non-breeding period
Migratory birds must be followed in space as well as over time in order to model
population dynamics accurately. For small-bodied species, spatial tracking is extremely
difficult: mark-resight/recapture is ineffective over large geographical scales (Wassenaar
and Hobson 2001) and remote monitoring devices are currently too heavy and generate
too much drag for species under 40g (Bowlin et al. 2010; Fraser et al. 2012). Ideally
technological advancements in the near future will allow us to follow individuals through
the whole annual cycle (e.g. Wikelski et al. 2007); in the meantime, alternative
approaches must be used. One approach is to infer habitat use or geographic origin
from intrinsic (naturally occurring, tissue-based) markers such as stable isotopes
(Hobson et al. 2010). Stable isotope ratios (particularly those of carbon (13C), nitrogen
(15N), and hydrogen (D)) provide a means of collecting cross-seasonal data because
they vary spatially in relation to physical, climatological and biological processes (Kelly
2000; Hobson et al. 2010). These ratios are incorporated into tissues grown while an
4
animal resides and feeds in a given area; individuals retain these signatures until this
tissue is replaced (Hobson 1999). For example, birds that undergo pre-alternate moult
on the wintering grounds incorporate wintering signatures into plumage that they will
retain for a full a year (Hobson 1999). Winter feathers collected on the breeding grounds
therefore offer a means of inferring certain aspects of non-breeding habitat use. A
second option for collecting cross-seasonal data is to use our current (albeit inexact)
knowledge of migrant population movements and distributions (e.g. Boulet et al. 2006;
Norris et al. 2006). By matching this data with climate data we can assess the role of
conditions at different points of the annual cycle in altering migrant phenology and in
limiting their numbers. In this approach, it is necessary to test regionally-specific models
as climate cycles influence large areas and can impact breeding, wintering and migration
conditions simultaneously (Nott et al. 2002; Forchhammer et al. 2002).
The influence of the non-breeding period on populations can be measured in
terms of between-individual differences (after controlling for annual effects), or it can be
measured in terms of within-individual or within-population differences between years. A
between-individual approach requires there to be variation in the types of non-breeding
habitat occupied by individuals within a single population (e.g. Marra et al. 1998). This
variation is detected using isotope signatures in tissue samples and the strength of the
habitat effect is then measured by contrasting the breeding performance of individuals
within the population. A between-year approach requires there to be year-to-year
variation in the climatic conditions faced by populations in non-breeding regions (e.g.
Saino et al. 2004; Rockwell et al. 2012). This variation is detected using regional climate
data and the strength of the effect is then measured by contrasting the performance of
the same individual or population between years. Differences in survival probabilities
with changing non-breeding conditions may also be detected using the second
approach, and be indicative of the role of these periods in population limitation (Baillie
and Peach 1992; Wilson et al. 2011).
Objectives
In this thesis, I seek to expand our current knowledge of how migratory bird
populations are influenced by events occurring outside of the breeding season. To do
5
this, I make use of both stable isotope and regional climate data to quantify the
conditions experienced by my study populations in the non-breeding period. I measure
the impact these conditions might have on individuals or breeding populations as a
whole by using between-individual (Chapter 3 and 4) and between-year approaches
(Chapter 5). My study species is the northern Yellow Warbler (Setophaga petechia).
Yellow Warblers are a good model species with which to investigate seasonal
interactions: they breed throughout North America and their migratory patterns and
wintering distributions have been delineated to a greater extent than those of other small
migrants (Boulet et al. 2006).
In Chapter 2, I collect data on Yellow Warblers wintering in on the Pacific and
Atlantic coasts of the isthmus region of Mexico (Figure 1.1). I document habitat use, and
age- and sex-distributions within habitat types. I then analyze blood collected from
wintering birds for its carbon (13C) and nitrogen (15N) values in order to determine the
extent to which habitat use can be inferred from tissue in this species.
In my third chapter I measure the strength of time- and condition-mediated carry-
over effects among Yellow Warblers breeding in Revelstoke, British Columbia (50°58’N,
118°12’W; Figure 1.1). To do this, I relate between-individual differences in breeding
phenology and performance to variation in winter habitat use inferred from 13C and 15N
signatures obtained from winter-grown plumage. In Chapter 4, I move 2000 km further
north - to the limit of the Yellow Warbler summer range - and collect breeding data from
birds within the Mackenzie watershed near Inuvik, Northwest Territories (68º 21’N, 133º
45’W; Figure 1.1). The different costs and benefits incurred by breeding at different
latitudes would be expected to influence the strength of carry-over effects. I therefore
compare my breeding populations in terms of phenology, productivity and the
relationship between these parameters and inferred winter habitat use.
Finally, in Chapter 5, I use a long-term dataset from the Revelstoke population to
assess the role of climate, acting at different points in time and in different regions, on
breeding phenology and apparent annual survival. To reduce the confounding effects
produced by broad climate cycles that impact multiple regions at the same time, I
compete regionally-specific hypotheses.
6
The data in this thesis focuses on the annual cycle of migrants in western North
America. Both breeding sites in this study are used by Yellow Warblers of the western
genetic lineage and sampling sites in Mexico were selected due to the prevalence of
western migrants in these areas (Boulet et al. 2006; Figure 1.2). Chapter 3 and 4 are the
first studies to measure carry-over effects in a western landbird, while Chapter 5
assesses the role conditions in western Mexico play in breeding success and in survival.
Such information is needed, as most of the current North American literature is based on
research conducted with eastern birds. There is increasing evidence that eastern
migratory systems differ from western ones in terms of how strongly they are influenced
by wintering conditions (e.g. Wilson et al. 2011; LaManna et al. 2012). This suggests
that the migrant story is complex, and I offer my interpretation of the existing patterns
and the results of my research in the General Conclusion.
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10
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11
Figures
Figure 1.1 Data collection sites and their associated chapters within this thesis. Base map courtesy of Wikimedia Commons.
12
Figure 1.2 Data collection sites shown within the Yellow Warbler breeding and wintering range (shaded regions). Dashed lines indicate the geographical limit of the eastern genetic line during the breeding (top) and wintering (bottom) period (Boulet et al. 2006). The probability of eastern haplotypes being present to the west of these lines is less than 0.1 (Boulet et al. 2006).
13
Chapter 2. Habitat use differs by sex in wintering Yellow Warblers
Anna Drake, Simon O. Valdez Juarez, David J. Green1
Abstract
Sex-biases in habitat use are common among wood warblers (Parulidae) that are
territorial over winter. We show that in tropical deciduous forest on the Pacific coast of
Mexico, male Yellow Warblers predominate in riparian habitat and to a lesser extent in
agricultural field margins while females predominate in scrub/seasonal saline marsh
habitat near the coast. In contrast, we find no evidence of sexual habitat segregation
between the pasture and coastal habitats used by Yellow Warblers on the Gulf of
Mexico. Body size does not play a significant role in predicting habitat occupancy on the
Pacific coast, suggesting that segregation is produced either by differences in
preference or by competitive asymmetry produced by sex-specific traits other than
physical size. Additional, more subtle, differences in habitat use are suggested by blood-
isotope samples. While variation in individual 13C and 15N signatures are most strongly
related to occupancy of a particular habitat-type and region, body size (but not age or
sex) explains additional variation in 15N signatures within scrub and riparian habitats on
the Pacific coast. This pattern may be the product different foraging behaviours or
differences in microhabitat use within broader habitat-types. We discuss sexual habitat
segregation in relation to regional climate and suggest that the seasonally dry climate of
1 A version of this chapter has been submitted to The Condor
14
the northern and western portion of the Yellow Warbler wintering range might create
steeper habitat quality gradients that result in segregation.
Introduction
Wood warblers (Parulidae) exhibit a diversity of over-wintering strategies (Stutchbury et
al. 2005; Faaborg et al. 2010). Species that exploit predictable, defendable food sources
such as the American Redstart (Setophaga ruticilla) and Black-throated Blue Warbler
(Setophaga caerulescens) maintain permanent territories throughout the wintering
period and show site fidelity between years (Brown and Long 2007; Holmes, et al. 1989;
Wunderle and Latta 2000). Territoriality in other species is more flexible. Northern
Waterthrush (Parkesia noveboracensis) will aggressively defend winter territories in high
quality areas but allow extensive home-range overlap to occur in poor habitat (Smith et
al. 2011) and Tennessee Warblers (Oreothlypis peregrina) will form ephemeral territories
around productive nectar sources (Greenberg 1986). Winter flock formation is observed
in many species. Coniferous forest breeders such as Bay-breasted (Setophaga
castanea), Yellow-rumped (Setophaga coronata) and Blackpoll (Setophaga striata)
Warblers that feed on insects, fruits and nectar in the tropics will form intraspecific flocks
(Greenberg 1979) while species feeding primarily on insects, such as Golden-cheeked
(Setophaga chrysoparia), Black-throated Grey (Setophaga nigrescens), and Black-and-
white (Mniotilta varia) Warblers, will join mixed-species flocks (Rappole et al. 1999; Hutto
1994).
Winter habitat use varies with sex and age in many territorial warbler species.
Use patterns tend to be consistent, with males associated with forest, females found in
more open and early successional habitat (Ornat and Greenberg 1990; Parrish and
Sherry 1994; Stutchbury et al. 2005) and older individuals tending to occupy high quality
habitats (Latta and Faaborg 2002; Marra and Holmes 2001). These sex- and age-biases
among habitats can be driven by differing preference or can be the product of the
competitive exclusion of certain classes (females and young birds in all existing
examples) from high quality or less variable habitat. For example, studies suggest that
differing habitat preference produces sexual habitat segregation in Hooded Warblers
(Setophaga citrina). Male Hooded Warblers appear to exhibit a preference for vertical
15
landscape elements (Morton 1990) and natural disturbances that produce changes in
structure within forested territories can result in sites being abandoned by males and
occupied by females (Morton et al. 1993). Removal experiments within male-dominated
habitats do not result in females occupying vacated territories (Morton et al. 1987). In
contrast, studies involving American Redstarts suggest that sexual habitat segregation in
this species is a product of competitive exclusion (Marra and Holmes 2001). Male-
biased, mangrove habitat has greater and less variable insect biomass over the
wintering period (Studds and Marra 2005, 2007). Males removed from territories in
mangrove are readily replaced by females (Marra et al. 1993; Studds and Marra 2005),
who subsequently maintain better condition over the wintering period than females in
second-growth scrub habitat (Studds and Marra 2005).
Habitat segregation by sex or age can have a diversity of consequences for
populations. Where preference produces segregation, differences in the availability of
preferred habitat may mean that winter carrying capacities differ between age- and sex-
classes. Where competition for high quality habitat produces segregation, outcomes are
more complex. In competitive exclusion scenarios, classes without access to good
quality habitat may occupy marginal habitat and consequently be in poorer condition and
experience reduced over-winter survival (Sherry and Holmes 1995; Marra 2000). If
juveniles are forced to winter in more marginal habitat, juvenile survival and recruitment
into breeding populations may be reduced. Similarly, higher female mortality may skew
operational sex ratios on the breeding grounds (Sherry and Holmes 1995; Marra and
Holmes 2001; Townsend et al. 2009) and reduce population productivity (Steifetten and
Dale 2006). Meanwhile, population responses to landscape changes will be complex:
exclusion rates (and therefore the condition and survival) of subordinate age- and sex-
classes will vary with both the availability of high quality habitat and with overall
population size (Sherry and Holmes 1995; Marra and Holmes 2001). Despite the
potential to influence population demography, we know very little about the habitat use
patterns exhibited by most warbler species (Morton and Stutchbury 2005; Faaborg et al.
2010).
Yellow Warblers are one of the most abundant North American warblers, with an
estimated population size of 30 million (Rich et al. 2004). They have been the subject of
many breeding-ground studies (from Bigglestone 1913 to Quinlan and Green 2012) but
16
there is still relatively little is known about their wintering ecology. In this study, we
compare the morphology of Yellow Warbler wintering within the Pacific and Gulf coast
regions of the isthmus of Mexico, assess habitat use within these regions, and examine
the extent to which habitat use varies with sex, age and body size. Body size is not the
only factor that could determine the outcome of intraspecific competitions (Holmgren and
Lundberg 1993; Arizaga and Bairlein 2011), however, body size differences among
habitats could indicate competitive exclusion. Previous work has shown that the type of
winter habitat occupied by Yellow Warblers can be discerned from 13C and 15N
signatures in tissue samples (Drake et al. 2013; Chapter 3). We use 13C and 15N to
further evaluate whether there is evidence of different within-habitat, microhabitat use
associated with sex, age, or body size.
Methods
Study species
Yellow Warblers are small, insectivorous birds that breed in riparian margins and
wet deciduous forest throughout North America. They winter primarily in lowland regions
from the southern Baja peninsula (25°N) to northern Brazil (1°N) where they occupy both
natural and human-modified habitats (Hutto 1980, 1989; Binford 1989; Lynch 1989;
Estrada et al. 1997). Molecular and stable isotope data suggests that eastern and
western populations of Yellow Warblers differ in their overwintering distributions, with
eastern populations occupying South and Central America and western populations
wintering in Mexico and Central America (Boulet et al. 2006). In the regions where we
conducted our study, wintering populations are composed largely of individuals who
breed in northwestern North America and the probability of eastern haplotypes being
present is <0.1 (Boulet et al. 2006).
Several studies indicate that Yellow Warblers hold territories over winter.
Territorial behaviour coupled with a high degree of interspecies aggression has been
documented among individuals using riparian trees bordering pasture in Chiapas,
Mexico (Greenberg et al. 1994; Greenberg and Ortiz 1994), sun-coffee plantations in
Tucurú, Guatemala (Greenberg et al. 1996), and second-growth scrub forest in Panama
17
(Neudorf and Tarof 1998). However, Morton (1976) reports that young Yellow Warblers
in Panama will join wandering mixed-species flocks. There is also evidence from band
returns (n=7; Lowther et al. 1999) and marked populations (Valdez Juarez, unpublished
data) that Yellow Warblers show winter site fidelity.
Study areas
We examined the habitat use of Yellow Warblers within the Pacific and the Gulf
coast regions of the isthmus of Mexico. On the Pacific coast, we worked within and near
the Chamela-Cuixmala Biosphere Reserve, in the state of Jalisco (19° 28' 44" N 105° 2'
38" W; Figure 1.1; hereafter “Chamela”). Chamela is predominantly lowland tropical
deciduous forest (Leguminosae, Euphorbiaceaea, and Sapindacea (Hutto 1994)) with
narrow bands of semi-deciduous tropical forest in riparian areas along the Cuixmala
River and Chamela Arroyo (including Salix gooddingii, Acacia hindsii, Bursera arborea,
and Astianthus viminalis). Coastal regions are predominantly dry scrub (e.g. Prosopis
juliflora) and seasonal saline marshland (Conocarpus erecta and Avicennia germinans)
(Lott 1993). Local agricultural areas (in the communities of Punta Pérula and Emiliano
Zapata) consist mostly of papaya, chili pepper, corn, and tomatoes bordered by shrubby
fence margins or tree rows planted as wind breaks. During the Yellow Warbler wintering
period (September-May) the region has mean temperatures of 24.4 ± 2.4°C and a total
of 380.4 ± 216.7 mm of rainfall. Rainfall is highly seasonal and only 3% of precipitation in
the wintering period falls between February and May (3.3 mm month-1) (UNAM 2007).
On the Gulf of Mexico, we worked within the Los Tuxtlas Biosphere Reserve, on
the Catemaco coast, Veracruz (18° 33' 41"N, 95° 1' 56”W; Figure 1.1; hereafter “Los
Tuxtlas”). Coastal regions in Los Tuxtlas are dominated by short (<4m) vegetation
including Randia laetevirens and Hibiscus tiliaceus. Shoreline vegetation within the
Laguna de Sontecomapan is predominantly red mangrove (Rhizophora mangle). Inland
areas would naturally be lowland tropical moist- and wet-forest (Gutiérrez-García and
Ricker 2011), however, the majority of the region is deforested (80-90% of the landscape
(Estrada et al. 1997)) and the landscape is primarily cattle pasture bounded by live-
fences (lines of Bursera simaruba and Gliricidia sepium trees strung with barbed wire).
Primary forest remains within the boundaries of the protected zone of the reserve and
some patches of second-growth exist near the coastline. Agricultural crops account for
18
approximately 3% of the landscape in Los Tuxtlas (Estrada et al. 1997). Within our study
area, plantings were citrus, corn, beans and chili. During the Yellow Warbler wintering
period, Los Tuxtlas has mean temperatures of 24.3 ± 2.3°C and a total of 2371 ± 137.8
mm of rainfall. As in Chamela, conditions become drier just prior to spring migration,
however, rainfall does not decrease as dramatically. The period between February and
May accounts for 16% of rainfall within the Yellow Warbler wintering period (93.8mm
month-1) (Gutiérrez-García and Ricker 2011).
Habitat searches, point counts and targeted mist-netting
We surveyed for Yellow Warblers in Chamela within a 220-km2 region between
February 2 and February 14, 2011. In Los Tuxtlas, we sought birds within a 120-km2
region between February 25 and March 12, 2011. We conducted surveys within lowland
deciduous forest, riparian semi-deciduous forest, agricultural land, and scrub/seasonal
saline marsh in Chamela, and, in coastal vegetation, red mangrove, primary lowland
tropical forest, second-growth, and cattle pasture in Los Tuxtlas. Agricultural plantings
within our study area in Los Tuxtlas were small (<4 ha) and situated within pasture
habitat. We therefore did not distinguish these areas from pasture during surveys. For
each habitat-type, we sought Yellow Warblers at two to four locations separated by at
least 4 km. Surveys were conducted over one to two hours and consisted of walking a
minimum of 0.5 km through the given habitat, seeking birds by sight. Visual surveys
were interspersed with playback of male song and chip calls (recorded during the
breeding season) followed by silent, stationary observation. Playback attracted wintering
Yellow Warblers in both regions; male and females responded aggressively to the audio
in areas where they were seen foraging.
Where Yellow Warblers were found within a given habitat-type, we subsequently
conducted point counts at two replicate sites in order to determine the density of
individuals and the proportion of adult males present (identifiable on the basis of their
bright plumage). Distances between replicate sites averaged 13.1 km (range: 4.6-28.5
km). Ten counts were done per replicate between 7:00 and 10:00 CST. Points were
placed 200m apart along the length of two parallel, 1km-long transects in broadly
distributed habitat or along one, 2km-long transect in areas where available habitat was
narrowly distributed (riparian sites in Chamela, and one coastal site in Los Tuxtlas).
19
Limited access to private property meant that agricultural point counts in Chamela were
randomly placed (≥ 200m apart) at the edges of fields. Counts consisted of 10 minutes
of silent observation followed by five minutes of male song and chip playback (Sliwa and
Sherry 1992). For each location we noted the distance of observed Yellow Warblers
from the center of the point during both silent and playback periods. Playback increased
our detection of individuals and drew birds closer to the observer. Yellow Warblers
cannot be aged except in the hand, and it is not possible to distinguish between females
and young males by sight. We therefore noted if birds were bright (older males), dull
(young males or females of both age-classes), or “unknown” (when birds were heard but
not seen).
Targeted mist-netting was also conducted within each habitat-replicate in order to
provide an alternative dataset with which to examine sex, age and body size differences
between habitats. Yellow Warblers in Chamela were captured between February 3 and
February 20, 2011 and between January 8 and April 7, 2012. Yellow Warblers in Los
Tuxtlas were captured between February 26 and March 14, 2011. In 2011, 7-10 birds
were captured per habitat-replicate: 52 birds in Chamela and 32 birds in Los Tuxtlas. In
2012, 8-13 additional individuals were captured per habitat-replicate in Chamela (n=52
birds).
Individuals were drawn to the net using playback and decoy. Once extracted,
they were fitted with a Canadian Wildlife Service-issued aluminum band and a unique
combination of three color bands (AC Hughes, UK). We aged and sexed individuals
based on physical and plumage characteristics. Birds hatched in the previous summer
(“second year” or SY) were identified by their pale lower mandibles, tapered primary
coverts and retrices and the limited yellow coloration on the inner webs of their outer tail
relative to older (“after-second year” or ASY) birds (Pyle 1997). Once aged, birds could
be sexed using plumage coloration (brightness and degree of rufous streaking) with
males being the brighter bird within their age-class. Prior to release, we measured the
individual’s tarsus and head-bill length (nearest 0.1mm) and tail and wing chord (9 th
primary) (nearest 1mm). We subsequently defined individual “size” as the first principal
component score (PC1) derived from these metrics (n=133 birds). This score was
normally distributed and described the independent metrics fairly well, capturing 36, 46,
57, and 65% of their variation respectively.
20
13C and 15N Isotope Signatures
Small birds have a whole blood half-life of approximately 4-6 days for 13C
(Hobson and Bairlein 2003; Pearson et al. 2003) and approximately 11 days for 15N
(Hobson and Bairlein 2003; Evans Ogden et al. 2004). Isotopic signatures in blood
should therefore reflect the diet/foraging area of an individual within 12 days prior to
sampling for 13C and within 22 days prior to sampling for 15N (or two half-lives; Hobson
and Clark 1993). To infer habitat use, we collected an average of 30 µl of blood from the
brachial vein of each bird banded in 2011. Blood was cooled in the field then separated
into its components and frozen within 5.5 hours. In the lab, RBCs were dried at 60°C for
3 days. 1 mg (±0.2) of material was then analyzed at the University of California Davis
Stable Isotope Facility in California, USA to determine its 13C and 15N isotope ratios.
Delta values are expressed relative to international standards: V- PDB (Vienna PeeDee
Belemnite) for carbon and Air for nitrogen; the estimated measurement precision was
0.2‰ for 13C and 0.3‰ for 15N. Within-sample replicates indicated that repeatability
was high for both isotopes (13C: r=0.96, F19,1=75.9, P<0.001; 15N: r=1.0, F19,1=129.6,
P<0.001) (Lessells and Boag 1987).
Statistical Analysis
We first used ANOVA to assess whether Yellow Warbler morphology differed
between our study regions. Work by Boulet et al. (2006) indicates that wintering Yellow
Warbler populations on the Pacific and Gulf coast of Mexico differ in their average
breeding latitude. Morphological differences between Chamela and Los Tuxtlas might
therefore be expected as a product of subspecies differences and/or differences in
subspecies mixing. Sex was included in all analyses, as female Yellow Warblers tend to
be smaller than males. Age was included in wing and tail measures as the primaries and
tail feathers of young (SY) birds tend to be shorter than those found in adults. In 2011
we measured 6th primary length in addition to wing chord (9th primary length). The 6th to
9th primary ratio is reported to vary with breeding latitude in Yellow Warblers
(Wiedenfield 1991) and we used this ratio as an additional estimate of regional
differences.
21
We subsequently examined how warbler densities varied with habitat type in
Chamela and Los Tuxtlas. Yellow Warblers were recorded at less than half of point
counts during silent observation (39/100). Playback enabled us to detect individuals at
significantly more points (63/100; one-sided binomial test P<0.001). We therefore
calculated habitat densities using the maximum number of individuals seen or heard
within a 50m radius during both silent and playback periods. We used a generalized
linear mixed model (GLMM; Poisson distribution) to examine habitat-within-region
differences in individuals counted per point, with replicate site as a random factor and
habitat as a fixed effect.
Within each region, we used point count data to calculate the ratio of older males
(bright individuals) to young males and females (dull individuals) within habitat types. We
supplemented these results with capture data where we could assign individuals to a
definitive sex- and age-class (Figure 2.1). Sex- and age-ratio differences between
habitats were assessed using GLMMs: for point count data, the sample replicate was the
point while for capture data the sample replicate was the individual. All models were fit
using a binomial distribution (logit-link) with sampling site as a random effect and habitat
type as a fixed effect. Within each sex we assessed age distributions across habitats.
Finally, a between-region comparison of habitat sex ratios was done using GLMM and
capture data.
We examined how body size (PC1) influenced habitat occupancy within regions
using nominal logistic regression. Our initial analysis for Chamela included age- and sex-
class as covariates plus ‘body size×class’ interaction terms. No significant interactions
were detected (both P>0.70) and the model was rerun using only main effects. For Los
Tuxtlas we did not test interactions as our sample size was small. In our second
analysis, we examined if body size alone influenced habitat occupancy. We predicted
that, if competitive exclusion based on body-size was driving sex distributions, physically
larger birds would be more likely to hold territories in male-dominated habitats. If
physical size alone determines occupancy, body size in our second model would
account for most of the variance explained by sex and age in our first model. In contrast,
if sex- and age-specific traits other than body size (such as aggression, experience, or
innate preference) play a role in habitat use, our second model would explain
substantially less variance than our first.
22
Finally, we examined whether 13C and 15N signatures in Yellow Warbler RBC
samples varied with an individual’s sex, age, or body size, after controlling for habitat
type. Previous work shows that Yellow Warbler 13C and 15N signatures at our study
sites are related to broad-scale habitat use (Drake et al. 2013; Chapter 3). Briefly, 13C is
most enriched among individuals captured in Los Tuxtlas and in agricultural areas in
Chamela. Birds using scrub habitat in Chamela are more 13C depleted, and individuals
in riparian habitat in Chamela have the most 13C-depleted signatures in our dataset.
15N signatures are more enriched in Chamela. Significant differences in 15N exist
between Los Tuxtlas birds, birds using scrub habitat in Chamela, and birds using
agricultural or riparian areas in Chamela. Together 13C and 15N signatures enable
differentiation between habitats used in Chamela and between Chamela and Los
Tuxtlas, but do not allow for the separation of the coastal and pasture habitats used by
birds in Los Tuxtlas (Drake et al. 2013; Chapter 3). We hypothesized that, as habitat-
types are relatively heterogeneous, individuals might show non-random differences in
small scale, microhabitat use within-habitat. Such differences might be reflected in
blood-isotope signatures. To test this, we divided habitats into four, isotopically-distinct
categories: scrub/seasonal saline marsh, agriculture, and riparian in Chamela plus a
merged pasture and coastal habitat category for the Gulf of Mexico (“Los Tuxtlas”). We
then examined small-scale variance in blood-13C and -15N signatures using linear
mixed models (LMM (REML)) with sampling site included as a random effect and habitat
category, sex, age, and body size as fixed effects. We anticipated that heterogeneity
might differ between habitat-types and therefore included habitat interaction terms with
our predictive variables. Non-significant interaction terms (P>0.10) were dropped from
our final model.
All statistical analyses were run using JMP 9.0.2 (JMP 2010). Values reported in
the Results are means ± SD.
23
Results
Habitat Use
In Chamela, Yellow Warblers were detected in riparian semi-deciduous forest,
agricultural areas, and scrub/seasonal saline marsh. They were not detected within the
lowland deciduous forest. In Los Tuxtlas, Yellow Warblers were found in coastal shrub
habitat, cattle pasture, and agricultural plantings. They were not detected within forested
areas (including forested riparian or lake margins) or within mangrove that did not border
(within 50m) on cattle pasture. Playback in forested regions yielded responses from
Wilson’s Warblers (Cardellina pusilla) and American Redstarts (Setophaga ruticilla)
indicating that the dense growth in these areas did not obstruct sound transmission.
Variation in morphology
Yellow Warbler morphology varied with sex and age. Males were physically
larger than females (head-bill: P=0.01, tarsus: P=0.01, wing: P<0.001 tail: P<0.001), and
young birds had truncated plumage (wing: P<0.001, tail: P<0.001). After controlling for
sex and age, population differences existed between our Pacific and Gulf coast
populations (Table 2.1). Birds in Chamela had shorter wing chords (P=0.03) and
possibly shorter tarsi than birds in Los Tuxtlas (P=0.07). Head-bill and tail length did not
differ between regions (P=0.44 and P=0.56 respectively). Wing morphology did not differ
between sex or age classes but did differ between regions. On average, Los Tuxtlas
birds had lower 6th to 9th primary ratios (P6/P9) (F1,81=23.9, P<0.001) and there was a
trend toward less variance in wing shape within this population relative to individuals in
Chamela (Brown-Forsythe test: F1,81=3.10, P=0.08; P6/P9 range, Chamela: 0.93-1.05,
n=51, Los Tuxtlas: 0.98-1.02, n=32).
Abundance and habitat use by sex, age and body size
The number of Yellow Warblers sighted at point counts differed significantly
between habitats in Chamela (χ22=6.8, P=0.03) and in Los Tuxtlas (χ2
1=7.9, P=0.005). In
Chamela, the number of individuals found in riparian and agricultural habitats were
approximately double that found in scrub (2.4 ± 1.0 and 2.3 ± 1.1 versus 1.2 ± 0.8
24
individuals per point; Z=2.8, P=0.005, Z=2.5, P=0.01 respectively). In Los Tuxtlas,
densities were higher in pasture than along the coast (1.6 ± 1.2 versus 0.4 ± 0.8
individuals per count; Z=3.6, P<0.001).
Bright individuals (i.e. older males) were not disproportionally drawn to playback
relative to dull individuals (paired-t99=0.6, P=0.54) and we therefore used total birds
sighted per point to calculate the ratio of bright to dull individuals by habitat. Habitat type
was a significant predictor of the proportion of ASY males present at different point
counts in Chamela (χ22=6.2, P=0.04). Counts conducted in riparian and agricultural
habitats did not differ significantly in their proportion of adult males (Z=0.8, P=0.43) but
scrub habitat had fewer adult males than riparian (Z=2.9, P=0.004) or agricultural habitat
(Z=2.3, P=0.02) (Figure 2.1). Habitat type did not predict the proportion of ASY males at
different point counts in Los Tuxtlas (χ21=0.5, P=0.48).
Capture data mirrored our point count data. Habitat in Chamela was a significant
predictor of the proportion of ASY males captured (χ2 2=6.5, P=0.04). Riparian and
agricultural habitats did not differ in their proportion of adult males (Z=0.25, P=0.80) but
both had more adult males than scrub habitat (agriculture: Z=2.8, P=0.005; riparian:
Z=2.6, P=0.009). As dull individuals could be reliably sexed, we could also compare
overall sex ratios between habitats. These differed significantly by habitat in Chamela
(χ22=10.6, P=0.005): more males were present in riparian habitat than in either
agricultural (Z=2.1, P=0.03) or scrub habitat (Z=4.2, P<0.001) and males were also more
common in agricultural sites than in scrub (Z=2.7, P=0.006). Young (SY) individuals in
Chamela accounted for 34% of birds banded (25% in 2011 and 42% in 2012); age ratios
within sex did not differ significantly between habitats (female age: χ22=2.1, P=0.34, male
age: χ22=4.2 P=0.12) (Figure 2.1).
In Los Tuxtlas capture data did not indicate differences in the proportion of ASY
individuals within coastal and pasture habitats (χ21=2.1, P=0.15). Sex and age-within-sex
ratios also did not differ between habitats (sex: χ21=1.0, P=0.31; male age: χ2
1=0.52,
P=0.47; female age: χ21=0.0, P=0.92). Young individuals in Los Tuxtlas accounted for
30% of the total number of individuals banded (Figure 2.1).
Comparisons of habitat sex ratios between regions using capture data indicated
that males were more common in pasture on the Gulf of Mexico than in scrub habitat on
25
the Pacific coast (Z=-2.1, P=0.03) but less common than in riparian habitat on the Pacific
(Z=1.9, P=0.05). Males were less abundant in coastal habitat on the Gulf of Mexico than
in riparian habitat on the Pacific (Z=2.9, Z=0.004) but did not differ significantly from
other Pacific habitat types.
Body size was not a determinant of habitat use in Chamela after controlling for
sex biases in habitat use (whole model: χ26=29.0, P<0.001, n=101; sex: χ2
2=24.2,
P<0.001, age: χ22=2.0, P=0.36, body size: χ2
2=1.2, P=0.55). Body size alone was also
not a significant predictor of habitat use (χ22=2.4, P=0.31, n=101). Together these
models indicate that (1) sex distributions across habitats in Chamela were not explained
by size differences between males and females and (2) that physical size was poor
predictor of habitat use.
In Los Tuxtlas body size did not predict habitat use (whole model: χ23=2.2,
P=0.53, n=32; sex: χ21=0.0, P=0.85, age: χ2
1=0.6, P=0.44, body size: χ21=0.4, P=0.50;
reduced model (body size): χ21=1.6, P=0.21, n=32).
Within-habitat variation in 13C and 15N signatures
Much of the variance in 13C and 15N was explained by the habitat-type used by
an individual (13C whole model: r2=0.65, n=81; habitat: F=9.1, P=0.01; 15N whole
model: r2=0.84, n=81; habitat: F=101.2, P<0.001; see also Drake et al. 2013; Chapter 3).
After accounting for this variation, neither sex, age, nor body size explained additional
variance in blood-13C values (sex: F=2.1, P=0.15; age: F=0.6, P=0.44; body-size:
F=0.2, P=0.64). 15N signatures also did not vary with sex (F=0.0, P=0.92) or age
(F=1.5, P=0.23). However, body size was correlated with 15N signatures in two habitats
(body size: F=0.0, P=0.84; body size×habitat: F=4.6, P=0.005). Physically larger birds in
riparian habitat tended to have more depleted 15N signatures than smaller birds while,
in scrub habitat, the pattern was reversed: larger birds were 15N enriched (Figure
2.2b,c).
26
Discussion
Yellow Warblers are reported to use a diversity of natural and anthropogenically-
modified habitats over winter (Dunn and Garrett 1997). Previous work on the Pacific
slope of Mexico indicates that this species makes use of six of eight broad vegetation
zones found within the lowland areas of the region (Hutto 1980). They have not been
observed within tropical deciduous forest or as members of mixed-species insectivorous
flocks that use the forest canopy during the non-breeding season (Hutto 1980, 1989,
1994). On the Gulf of Mexico, diversity surveys conducted across habitats in Los Tuxtlas
captured the majority of Yellow Warblers (n=102) within citrus plantations and live-
fences (30% and 35% of captures) and the minority within mixed crops and forested
areas (4% and 6% of captures) (Estrada et al. 1997). Our habitat surveys in Chamela
and Los Tuxtlas were not exhaustive and multi-year data is needed to assess changes
in occupancy patterns associated with variation in climate and bird abundance between
years. Despite this fact, our results are consistent with the above studies. We found
Yellow Warblers in natural and man-made landscapes but they were largely absent from
the dominant (or historically dominant) forest cover in both regions. Birds in our study
foraged in defined areas and were attracted by and aggressive toward call playback,
suggesting territoriality. Contrary to more southerly observations by Morton (1976), this
behaviour extended to the 45 young birds we captured.
Sex-biases in habitat use are a common phenomenon among territorial
overwintering wood-warblers (Stutchbury et al. 2005). Two previous studies that
assessed sexual habitat segregation in Yellow Warblers, however, differed in their
findings. Greenberg et al. (1997) conducted surveys in the Ocosingo valley, Chiapas,
Mexico that indicated that Yellow Warblers in gallery forest along arroyos (equivalent to
what we define as riparian habitat) were predominately male (83%; n=12), while birds in
drier acacia plantations were predominantly female (76%; n=23). As Greenberg’s were
survey results, it is likely that the observed males in gallery forest were older individuals
and that birds identified as females in acacia plantations were actually a mixture of
females and young males that could only be distinguished in the hand. In contrast to the
sexual habitat segregation found in Chiapas, Lopez-Ornat and Greenberg (1990) found
an even distribution of sexes among Yellow Warblers captured in scrub and open
habitats in the Sian Ka’an Biosphere Reserve, Quinta Roo, Mexico (n=62). Yellow
27
Warblers in our study showed regional differences in how sexes were distributed in the
landscape. Birds in Chamela exhibited sexual habitat segregation. Riparian areas were
predominately occupied by males (92% of individuals), while almost two thirds of
occupants of coastal scrub/seasonal saline marsh areas were female (63%). Agricultural
areas were also male dominated (71%) but were not as severely skewed as riparian
habitat. Of the Yellow Warblers we banded in scrub, 79% would be visually identified as
dull-coloured, suggesting that the distributions we observed are very similar to those
found acacia plantations in Chiapas (Greenberg et al. 1997). In Los Tuxtlas, we found no
significant differences in sex distributions between pasture and coastal habitats. Males
made up 71% and 53% of the individuals captured in in these habitats respectively. Our
sample size in Los Tuxtlas was relatively small (n=32). This may have limited our ability
to detect sex-differences between habitat types on the Gulf coast. Between-region
comparisons indicate that a sex-skew in pasture habitat, if present, is more moderate
than observed in riparian habitat on the Pacific coast. This could support within-region
results (i.e. reduced habitat biases in Los Tuxtlas) but it may also reflect differences in
regional sex ratios and/or population density.
Climate differences between study locations could offer a possible explanation
for differences in the degree of sexual habitat segregation between regions. The Pacific
slope of Mexico, where Chamela is located, has a highly seasonal climate (García-Oliva
et al. 1991). Low rainfall would be expected to produce sharp quality differences
between habitat-types at the end of the wintering period (Studds and Marra 2007). As
male-dominated riparian corridors are the only areas in Chamela with continuous access
to fresh water and as these corridors make up a very small percentage of the landscape,
we might expect greater competition for access. In contrast, the more moderate climate
of the southern Gulf coast might be expected to reduce quality differences between
habitats in the region. Our inability to identify habitat use by 13C or 15N in Los Tuxtlas
might further suggest that these habitats are qualitatively similar (Drake et al. 2013;
Chapter 3). Differences in how female and male Yellow Warblers distribute themselves
within the Ocosingo valley (Greenberg et al. 1997) and Sian Ka’an Biosphere Reserve
(Lopez Ornat and Greenberg 1990) are consistent with a climate explanation. The
Ocosingo valley, like Chamela, receives very little rainfall in the spring (21mm month-1
(February-May)) while Sian Ka’an falls within the same climate zone as Los Tuxtlas
(Kottek et al. 2006) and receives more rain than either Ocosingo or Chamela (although
28
less than Los Tuxtlas) in the same period (45mm month-1 (February-May)). Increased
competition in dry wintering regions would also be consistent with the reported latitudinal
segregation of Yellow Warblers on the wintering grounds (Komar et al. 2005). The
northern regions of the Yellow Warbler winter range have seasonal climates with low
mean annual precipitation (Legates and Willmott 1990; Kottek et al. 2006) and males
may exclude females to a greater degree from good quality habitat in these areas.
Females may, as a result, experience lower mortality if they overwinter in the wetter
southern and Caribbean-coast regions.
We predicted that, if sexual habitat segregation within habitats in Chamela was a
product of competition, body size might be a determinant of habitat occupancy. One
source of competitive asymmetry between sexes is size, as males tend to be physically
larger than females. In American Redstarts, habitat occupancy was a significant
predictor of female size: individuals in high quality (mangrove) habitat tended to be
larger than those in lower quality (scrub) habitat (Marra 2000). However, we found no
evidence that body size predicted habitat occupancy or explained the differences in
habitat use between sexes in Chamela. Age effects were also absent in this model
suggesting that experience or prior occupancy was not a determining factor in habitat
use. Our data therefore does not allow us to rule out the possibility that Yellow Warbler
distributions in Chamela are the product of sex-specific preference. It is also possible
that sex biases within habitats are due to competitive asymmetries based on factors
other than size or experience (Arizaga and Bairlein 2011). For example, males may be
physiologically more capable than females of expressing aggressive behaviours during
territory formation. Male American Redstarts are more aggressive than females in
response to simulated territorial intrusions (Marra 2000). Similarly, wintering
observations indicate that male Cape May Warblers (Setophaga tigrina) are more
aggressive than females when defending resources (Latta et al. 2001). In (breeding)
male Song Sparrows (Melospiza melodia), the presence of challengers triggers
increased testosterone secretion, a process that Wingfield et al. (1987) suggests could
facilitate the expression of aggression over an extended timeframe. Females, with lower
testosterone production (Møller et al. 2005), may not be able to mount a similar
response and may therefore be more likely to lose contests through attrition.
29
Broad-scale habitat use in wintering Yellow Warblers can be partially
differentiated using 13C and 15N signatures in tissue samples (Drake et al. 2013;
Chapter 3). After accounting for this source of variance, we investigated whether fine-
scale differences in 13C or 15N signatures might be non-random in relation to sex, age
or body size. 13C signatures did not suggest additional differences in habitat use
between classes or between large and small individuals once broad-scale habitat use
was accounted for. There was, however, an apparent relationship between 15N and
body size in riparian and scrub habitats in Chamela. These trends opposed each other
and explained 31% of the variation in 15N within riparian corridors and 21% of the
variation in 15N within scrub habitat. Increasing 15N with increasing body size was
evident in both females and males in scrub habitat, indicating that the relationship was
not the product of more subtle sex-differences in habitat use (Figure 2.2c). In
investigating this pattern, we found that birds with similar 15N signatures in scrub (but
not riparian) habitat were clustered geographically, suggesting that scrub habitat was
patchy. Larger, 15N enriched birds tended to border on higher vegetation mixed with
trees while smaller, 15N depleted birds tended to be in low-lying vegetation. Differences
in 15N could indicate that the mean trophic level of prey items consumed by Yellow
Warblers differed between these vegetation-types or that differences in plant-level 15N
exist between patches (Post 2002). Plant-level 15N differences could occur if high-
vegetation patches contain more deeply rooted species than shrubby areas (Hyodo et al.
2010). Foraging behaviour might also produce a relationship between 15N and body
size if differences in foraging strategy meant individuals consumed prey from different
trophic levels. Foraging behaviours such as hover-picking are more common in smaller,
more agile species (Greenberg 1979; Forstmeier and Keßler 2001) and might also be
more common among smaller individuals within-species (Nyström 1991). Identical shifts
foraging strategies could produce opposing trends where prey communities differ. While
our results remain ambiguous without supporting behavioural and insect sampling data,
they do indicate that blood-15N data can be used as a tool for identifying potential
differences in microhabitat use in passerines, once broad habitat variables are taken into
account.
Point count data indicate that Yellow Warblers density varied between habitats
and between regions with the greatest number of individuals per count sighted in riparian
30
and agricultural habitat in Chamela. Scrub in Chamela, and pasture and coastal areas in
Los Tuxtlas all had low densities. Our point counts were conducted primarily to
determine age and sex distributions. As density estimates, these results are crude. The
use of playback means that the number of individuals per count could reflect real
differences in bird number, but also behavioural differences between habitats. For
example, riparian and agricultural birds in Chamela may be more responsive to
perceived territorial intrusions than birds in scrub or in Los Tuxtlas (Silwa and Sherry
1994). Estimates of habitat-specific territory size are needed to achieve better habitat-
density estimates for this species.
Genetic and isotopic work indicate that Yellow Warblers who breed in the
Northwest Territories and Yukon are more common on the Gulf coast of Mexico than the
Pacific coast during the winter (Boulet et al. 2006). In addition to northern breeders,
Pacific wintering populations contain individuals from southern Canada and from the
USA (Boulet et al. 2006). Where we found morphology differences between our study
regions, they were in agreement with these patterns. Birds in Los Tuxtlas tended to have
longer wing chords and more pointed wing morphology (Table 2.1), which is suggestive
of more northern subspecies (Wiedenfeld 1991). Similarly, higher average values (i.e.
more rounded wing morphology) and possibly greater variance in P6/P9 measures in
Chamela are consistent with the presence of southern breeders and a wider range of
breeding origins among Pacific birds.
Our data suggest that Yellow Warblers exhibit regional differences in sexual
habitat segregation on their wintering grounds in Mexico. Differences in habitat use by
sex were evident on the Pacific but not detected on the Gulf coast. Where sexual habitat
segregation did occur, males were most common in mesic, forested habitats while
females were most common in drier, open habitats. This type of distribution is consistent
with patterns reported in other wintering warbler species (Stutchbury et al. 2005). Further
work is needed to determine the mechanism by which sex-distributions in Yellow
Warblers are produced. If it is a response to regional climate conditions as we propose,
and if differences in habitat quality and competitive asymmetry are responsible for
female- and male-skewed habitats on the drier Pacific coast (this study) and in the state
of Chiapas (Greenberg et al. 1997), then we would predict from climate maps (Kottek et
al. 2006) that segregation occurs over approximately 40% of the Yellow Warbler
31
wintering range. However, dry habitats account for approximately 80% of the region
occupied by Yellow Warblers within Mexico. As western breeding populations
predominate in Mexico, sexual habitat segregation on the wintering grounds could be
much more common phenomenon in this lineage. This warrants further investigation, as
such differences could contribute to demographic and population-response differences
between eastern and western Yellow Warbler populations.
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Tables and Figures
Table 2.1 Morphometric traits (± SD) of Yellow Warblers wintering on the Pacific and Gulf coast of Mexico. Sample sizes are reported in brackets. The length of the 6th primary feather was measured in 2011 only and therefore the sample sizes for wing shape (P6/P9) are smaller than other metrics.
Morphometrics Pacific coast (Chamela-Cuixmala) Gulf coast (Los Tuxtlas)
Sex ASY SY ASY SY
Wing Chord (mm) Female 59.2 ± 2.2 (20) 57.6 ± 1.4 (13) 59.9 ± 1.6 (7) 58.8 ± 1.3 (5)
Male 62.0 ± 2.1 (49) 60.2 ± 1.7 (20) 62.5 ± 1.6 (15) 62.2 ± 1.1 (5)
Tail (mm) Female 42.4 ± 1.8 (20) 41.8 ± 1.0 (13) 42.7 ± 1.2 (7) 40.6 ± 2.3 (5)
Male 44.8 ± 2.0 (48) 42.8 ± 2.5 (21) 44.0 ± 1.6 (15) 44.2 ± 1.9 (5)
Wing shape (P6/P9) Female 1.02 ± 0.02 (12) 1.00 ± 0.03 (5) 1.00 ± 0.01 (7) 1.00 ± 0.01 (5)
Male 1.01 ± 0.02 (27) 1.02 ± 0.02 (7) 0.99 ± 0.01 (15) 0.99 ± 0.01 (5)
Head- bill (mm) Female 28.6 ± 0.6 (20) 28.4 ± 0.6 (13) 28.7 ± 0.4 (7) 28.6 ± 0.4 (5)
Male 28.8 ± 0.6 (49) 28.8 ± 0.6 (22) 28.9 ± 0.6 (15) 28.9 ± 0.5 (5)
Tarsus (mm) Female 18.8 ± 0.5 (20) 18.8 ± 0.5 (13) 18.7 ± 0.6 (7) 19.1 ± 0.5 (5)
Male 19.0 ± 0.6 (49) 19.0 ± 0.6 (22) 19.3 ± 0.6 (15) 19.2 ± 0.7 (5)
Mass (g) Female 8.2 ± 0.4 (17) 8.1 ± 0.4 (13) 8.7 ± 0.3 (7) 8.8 ± 0.4 (5)
Male 8.5 ± 0.5 (45) 8.3 ± 0.4 (21) 9.1 ± 0.5 (15) 9.6 ± 0.3 (5)
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Figure 2.1 Patterns of habitat segregation in Yellow Warblers on the (a) Pacific coast (“Chamela”) and (b) Gulf coast (“Los Tuxtlas”) of Mexico. Left columns represent point count results with individuals identified as older (ASY) males (solid dark grey) or ‘other’ (solid light grey). Right columns represent banding data where the ‘other’ category is separated into older females (solid white), yearling (SY) females (stippled), and yearling males (grey, diagonal lines). Sample sizes are indicated above each column. In Chamela, riparian habitat contained more males than agriculture and both of these habitats contained more males than scrub. In Los Tuxtlas, coastal and pasture habitats did not significantly differ in the proportion of males present.
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Figure 2.2 Variation in Yellow Warbler blood-15N signatures within habitats in relation to body size. Habitat-types were classified as (a) agricultural habitat, Pacific coast, (b) riparian habitat, Pacific coast, (c) scrub/seasonal saline marsh, Pacific coast and (d) “Los Tuxtlas”, Gulf of Mexico. This last category merges data from birds using coastal and pasture habitats on the Gulf of Mexico as habitats were not isotopically distinct. Unfilled circles indicate females, filled circles indicate males.
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Chapter 3. Carry-over effects of winter habitat vary with age and sex in Yellow Warblers (Setophaga petechia)
Anna Drake, Christine Rock, Sam P. Quinlan, David J. Green2
Abstract
We use stable isotope data to investigate the role of winter habitat use in altering
the breeding phenology of Yellow Warblers (Setophaga petechia). We first confirm that
13C and 15N isotopic signatures vary with winter habitat use in this species. We then
examine the relationship between winter habitat use, breeding phenology and
productivity within four age-sex-classes, since life history theory would predict that carry-
over effects should vary with age and gender. The 13C signatures of Yellow Warblers
using riparian habitats over winter were more depleted than the signatures of those
using agricultural or scrub habitat. Individuals on the Pacific coast of Mexico were also
more 15N enriched than those on the southern Gulf of Mexico. 13C and 15N signatures
were only correlated with earlier clutch initiation and subsequent higher productivity in
first-breeding-season females. We estimate that shifts in 13C equivalent to a shift from
scrub to riparian winter habitat would be associated with the production of 0.8 more
fledglings by yearling females. Pre-breeding events that influence the timing of breeding
could also influence the reproductive performance of older males and females, but we
found little evidence that winter habitat use influenced breeding season phenology in
these birds. 2 A version of this chapter is published in the Journal of Avian Biology doi: 10.1111/j.1600-
048X.2013.05828.x.
41
Introduction
Carry-over effects are defined as events and/or processes that occur in one
period of the annual cycle and affect an individual’s fitness in a subsequent period due to
the individual making the transition between periods in a different state (Harrison et al.
2011). Carry-over effects in migratory birds, particularly those driven by winter habitat
use and affecting subsequent breeding productivity, have received considerable
attention over the past decade. To date, such effects have been documented in four
migratory bird species and indicated in a fifth: Black-tailed Godwits (Limosa limosa
islandica; Gill et al. 2001; Gunnarsson et al. 2005, Gunnarsson et al. 2006) and Barn
Swallows (Hirundo rustica; Saino et al. 2004) from populations breeding in Europe, and
American Redstarts (Setophaga ruticilla; Marra et al. 1998, Norris et al. 2004, Reudink et
al. 2009, Tonra et al. 2011), Kirkland’s Warblers (Setophaga kirtlandii; Rockwell et al.
2012), and potentially Black-throated Blue Warblers (Setophaga caerulescens; Bearhop
et al. 2004) from populations breeding in eastern North America. These studies indicate
that loss, degradation, or climate-driven variation in the quality of winter habitat could
influence migratory populations beyond simply impacting over-winter survival. However,
few of these studies included age as a factor in their models: three limited their data to
older individuals (Norris et al. 2004; Reudink et al. 2009, Tonra et al. 2011) and five
combined age-classes in their analysis (Marra et al. 1998; Gill et al. 2001; Norris et al.
2004; Gunnarsson et al. 2005; Gunnarsson et al. 2006).
Life history theory would suggest that the strength of carry-over effects should
vary with both age- and sex-class. This is because the ultimate effect of state on an
individual’s seasonal fitness will differ depending on the costs and constraints faced by
that individual (as seen in staging Brent Geese (Branta bernicla bernicla); Ebbinge and
Spaans 1995). As different age- and sex-classes are likely to vary predictably in the
costs and constraints they face (Brown and Roth 2002), it follows that the presence and
magnitude of carry-over effects may also vary predictably between classes within a
population.
Habitat-driven carry-over effects act through non-fatal differences in the physical
condition of individuals. Such differences may have a direct impact on fitness or they
may result in variance in biologically relevant events such as onset of migration, the date
42
at which individuals arrive on the breeding grounds and establish a territory, the timing of
breeding, and/or the timing of moult. In the latter case, it is the variance in these events
that ultimately impact fitness and can be considered the mechanism through which
carry-over effects act. In this paper we use the term “time-mediated” carry-over effect, to
distinguish this mechanism from where physical condition directly impacts fitness.
Theoretically, time-mediated carry-over effects resulting from winter habitat use
should be more evident in males than in females. The timing of arrival on the breeding
grounds typically has a stronger impact on male breeding success: arrival-date-
dependent changes in within-pair and extra-pair paternity, polygyny, and greater odds of
failing to acquire a territory or mate (and any reproductive success) mean that males
may experience a greater range in productivity than females (Lozano et al. 1996; Marra
and Holmes 1997; Hasselquist 1998; Langefors et al. 1998; Reudink et al. 2009). In
contrast, females are likely to experience less dramatic fitness declines with later arrival
dates, driven by decreasing territory and mate quality, mate-sharing, and later clutch
initiation dates (Alatalo et al. 1986; Bensch and Hasselquist 1992; Rowe et al. 1994;
Brown and Roth 2002; Smith and Moore 2005; Huk and Winkel 2006). Additionally, if
female migration is influenced by other factors under stronger selection (such as
improving body condition at stop-over sites prior to arrival (e.g. Lavee et al. 1991; Yong
et al. 1998)), winter-habitat quality may show no relationship to arrival date. Empirical
data offers support for this concept: winter habitat quality affected male but not female
arrival dates in both American Redstarts (Marra et al. 1998; Norris et al. 2004), and
Black-tailed Godwits (Gunnarsson et al. 2006).
Time-mediated carry-over effects may also be more evident in older birds for
several reasons. First, familiarity with breeding sites may allow early-arriving,
experienced males to select high-quality territories before leaf emergence; lack of prior
knowledge may reduce the advantages of early arrival for yearling males (Lozano et al.
1996). Secondly, older individuals may reduce or forgo reproductive investment late in
the breeding season in favour of improving the odds of over-winter survival and future
reproduction (Brown and Roth 2002). This behaviour should produce a strong negative
relationship between clutch initiation date and fledge number. Yearling birds, with poorer
over-winter survival, may continue to invest in current reproduction and thus show a
weaker negative relationship between clutch initiation date and fledge number (Brown
43
and Roth 2002). Finally, younger, inexperienced birds generally have lower reproductive
success than older individuals (reviewed in Forslund and Pärt 1995; Fowler 1995) and
poor overall performance may simply overwhelm carry-over effects.
It is also possible that greater experience enables older birds to compensate for
some of the negative effects of late arrival. For example, late-arriving older females may
be able to initiate and complete nest-building more rapidly than late-arriving yearling
birds. Such compensation would result in weaker carry-over effects for older birds and
could be differentiated from other age differences by weaker effect-slopes for older birds
for events occurring after arrival.
Understanding whether the strength of carry-over effects vary among age- and
sex-classes is necessary for accurate population modeling. For example, breeding
populations of many migratory songbirds contain a large proportion of yearling
individuals (e.g. Setophaga caerulescens: 38% of males (Graves 1997); Seiurus
aurocapilla: 39% of males (Porneluzi and Faaborg, 1999); Hylocichla mustelina: 48% of
males and 54% of females (Brown and Roth 2002)). These individuals tend not to be
evenly distributed throughout the species range (Graves 1997; Rohwer 2004).
Differences in carry-over effect strength between age classes could therefore create
regional variation in how a species responds to shifts in winter habitat quality.
In this study we examine whether the quality of winter habitat used by Yellow
Warblers in Mexico/Central America, influences their breeding performance in western
Canada and whether the strength of time-mediated carry-over effects varies between
sex- and age-classes. Specifically we ask whether winter habitat type, measured
indirectly through 13C and 15N isotope ratios in tissue samples, influences when males
establish territories, when females initiate clutches, and whether the timing of these
events impact breeding performance. We expect the relationship between breeding
season phenology and performance to be negative: late territory establishment and
clutch initiation will result in fewer fledged young. We predict that the strength of time-
mediated carry-over will vary with age and sex; specifically, that (1) both winter habitat to
territory establishment/territory arrival date and habitat to total productivity effects will be
stronger in males, and that (2) carry-over effects will be weaker in yearling individuals.
44
Methods
Study Species and location
Yellow Warblers are small, insectivorous, Nearctic – Neotropical migrants that
commonly breed in wet deciduous and riparian habitat. Their breeding range covers the
majority of the North American continent, and extends as far north as the arctic tree-line
(Lowther et al. 1999). Recent genetic and isotopic work suggest that eastern and
western populations undergo parallel migration with western populations wintering in
Mexico and Central America and eastern populations wintering in Central and South
America (Boulet et al. 2006).
Yellow Warblers are found in a diversity of natural winter habitats ranging from
mangrove, swamp and riparian forests to dry coastal scrub and are also found in a
variety of human modified habitats such as pasture, sun/semi-shade coffee plantations,
and other cropland (Binford 1989; Greenberg et al. 1996; Garrett and Dunn 1997). The
species undergoes a pre-alternate moult prior to spring migration, replacing a variable
number of greater covert, body, and crown feathers (Pyle 1997; Quinlan and Green
2010). As Yellow Warblers are territorial and exhibit winter site fidelity (Greenberg and
Ortiz, 1994; Greenberg and Salewski, 2005; Morton, 1976) these feathers are expected
to incorporate local isotopic signatures that reflect habitat use (Hobson 1999).
We have studied a banded population of Yellow Warblers near Revelstoke,
British Columbia (50.97 °N, 118.20°W; Figure 1.1) since 2004. Birds are monitored at
three, 30-39 ha study plots in riparian habitat bordering the northern section of the Upper
Arrow Lakes Reservoir (elevation 435-441 m). These sites consist of seasonally flooded
grassland, grading into isolated willow thickets (Salix sp.) and ultimately mature black
cottonwood (Populus trichocarpa) forest at elevations above the high-water level
(>438m). The average breeding density of Yellow Warblers within the study sites is 0.7
pairs/ha. In the years encompassed by this study, yearling (first-breeding season or
“SY”) females accounted for 43.8% of the breeding females at our study sites (annual
range 25-57%, n=5 years) and yearling (SY) males accounted for 30.0% of the breeding
males (annual range 13-48%, n=5 years).
45
Monitoring and breeding
Study sites were monitored from early-May until late-July. Territory establishment
dates were determined during surveys of the three study plots conducted every 1-2
days. Males were easily detected and captured, as they would begin to sing once on
territory and were highly aggressive toward song-playback. Males of both age-classes
were typically caught within three days of their appearance using targeted mist-netting.
Females were more difficult to detect and capture as they were less conspicuous and
less responsive to playback. Females were either caught with their mates or later in the
season by placing mist-nets across flight paths used during nest-building, incubation, or
while provisioning young. Upon capture, all previously unmarked individuals were fitted
with a Canadian Wildlife Service-issued aluminum band and a unique combination of
three colour bands (AC Hughes, UK).
Pairs were monitored from early May to late July in 2005-2006, and 2008-2010 in
order to determine the onset of breeding and the fate of all nesting attempts. Only pairs
that initiated clutches were included in the data that were analysed. Nests were located
through close observation of females during nest building and checked every 3 days to
determine clutch initiation dates, clutch size, brood size and fledging success. Clutch
initiation date was defined as the laying date of the first egg in a female’s first nesting
attempt of the season. Nestlings were banded seven days after hatching. We assumed
that all nestlings present on day 7 successfully fledged unless signs of predation were
found in or around the nest after fledging. Fledging was confirmed, in most cases, by
observing parents feeding or defending fledglings. Annual productivity for an individual
bird was defined as the number of fledglings produced over all nesting attempts.
Although rare, some males (3.9%, 7/178 males over 6 years) obtained two social mates,
and it was possible for their seasonal productivity to be higher than that of females.
Sexing and aging
Birds were sexed using plumage (brightness/degree of rufous streaking),
behaviour and breeding characteristics (brood patch/cloacal protuberance). Birds were
classified as yearling or older birds based on plumage characteristics. Yearling (SY)
birds have narrow, tapered primary coverts, tapered retrices with less yellow colouration
on the inner webs of the outer tail, and worn prebasic plumage; older (ASY) birds have
46
broad, truncate primary coverts with narrow yellow-olive edging, and truncate retrices
with more extensive yellow colouration on the inner webs (Pyle 1997).
Feather samples and stable isotope analysis
We collected three new inner greater covert feathers from all individuals captured
on the breeding grounds in 2008-2010. These feathers are easily identifiable because of
their brighter pigmentation and lack of wear and carry isotopic signatures from habitat
used during pre-alternate moult prior to spring migration (Pyle 1997; Quinlan and Green
2010).
Feathers were washed in 2:1 chloroform:methanol solution for 24hrs, drained,
and then air-dried in a fume hood for an additional 24h to remove excess solvent. One
mg (±0.2) of tissue was placed in 9×5mm smooth-walled tin capsules (Elemental
Microanalysis, UK). Tissue samples were analyzed using a PDZ Europa ANCA-GSL
elemental analyzer interfaced to a PDZ Europa 20-20 isotope ratio mass spectrometer
(Sercon Ltd., Cheshire, UK) at the University of California Davis Stable Isotope Facility in
California, USA. Delta values are expressed relative to international standards: V-PDB
(Vienna PeeDee Belemnite) for carbon and air for nitrogen; estimated measurement
precision was 0.2‰ for 13C and 0.3‰ for 15N.
Winter habitat use and 13C and 15N
In February and March 2011, we took tissue samples (n=89) from territorial
Yellow Warblers wintering on the Pacific slope (Chamela, Jalisco; 19.52 °N, 105.08°W),
and the southern Gulf coast (Los Tuxtlas, Veracruz; 18.57°N, 95.05°W) of Mexico. As
birds had not undergone pre-alternate moult we collected red blood cell (RBC) samples
to obtain local habitat signatures. Samples were collected from replicate habitat-types
across the range of habitat used to determine associated 13C and 15N signatures.
Within each habitat-replicate we collected 30 µl of blood from the brachial vein of 7-10
birds captured using targeted mist-netting. Blood was held in a heparinized
microcapillary tube and cooled on ice for a maximum of 5 1/2 hours (average of 3 hours)
before being spun down for 5 minutes at 11,000 rpm in a ZIPocrit centrifuge (LW
Scientific Inc., USA). Red blood cells were separated and frozen. Once in the lab
47
(Burnaby, Canada) samples were thawed, transferred to eppendorf tubes and placed in
a drying oven at 60°C for 3 days. Dried RBCs then weighed and analyzed in the same
fashion as feather tissue described in the preceding section.
13C patterns are largely driven by plant photosynthetic pathways that differ with
water availability (Hobson 2005). Use of cooler, mesic winter habitats is expected to
result in depleted 13C signatures. 15N in terrestrial animals can show complex variation
related the trophic level of dominant prey items and to local environmental factors (e.g.
marine inputs, waste, cultivation, and artificial fertilizer) (Kelly 2000; Amundson et al.
2003). However, 15N also varies more predictably with factors such as climate,
temperature, and water and/or nutritional stress (van der Merwe et al. 1990; Hobson et
al. 1993; Cormie and Schwarcz 1996; Kelly 2000; Amundson et al. 2003) and enriched
15N may suggest xeric and/or poor habitats.
Mesic (13C depleted) habitats have been shown to be better over-wintering sites
for American Redstarts (Studds and Marra 2005), Cape May Warblers (Setophaga
tigrina; Latta and Faaborg 2002), and Northern Waterthrushes (Parkesia
noveboracensis; Smith et al. 2010; Smith et al. 2011). Such habitat has greater insect
biomass than drier habitat (Studds and Marra 2005; Smith et al. 2010, Smith et al.
2011), resulting in less mass-loss and higher muscle mass for warblers during the dry
season (Marra and Holberton 1998; Latta and Faaborg, 2002; Smith et al. 2010), earlier
fat deposition prior to spring migration (Smith et al. 2010) and earlier spring migration
dates (Marra et al. 1998). Mesic riparian habitats in Chiapas, and Jalisco, Mexico
contain a greater proportion of older male Yellow Warblers than drier habitat-types,
suggesting that riparian habitat is of higher quality (Greenberg et al. 1997, Drake, in
preparation). We therefore expected that depleted 13C would be associated with earlier
territory establishment and clutch initiation in our breeding population (Marra et al. 1998;
Norris et al. 2004). We predicted that Yellow Warblers from poor wintering habitat might
have enriched 15N signatures if the use of such habitat results in nutritional deficiencies
by the end of the wintering period (Hobson et al. 1993). If poor habitats are also dry
habitats, then feather samples should be further 15N enriched and we would predict
high 15N values to be associated with high 13C values and later territory establishment
and clutch initiation dates.
48
Data Analysis
We first assessed whether 15N and 13C signatures in blood samples collected
from birds in Mexico differed across winter habitat-types using Analysis of Variance and
Tukey-Kramer HSD post-hoc tests for each element (Figure 3.1). We then evaluated
whether 13C or 15N of winter-grown feathers collected from birds in Revelstoke varied
by sex or age-within-sex-classes (Table 3.1). Revelstoke signatures were non-normally
distributed and we therefore used Wilcoxon tests for comparisons between classes. We
subsequently used general linear models to evaluate whether winter habitat use
influences the territory establishment dates, onset of breeding, and productivity of male
and female Yellow Warblers within different age classes.
The proposed pathways for carry-over effects in male and female warblers are
illustrated in Figure 3.2. For males we hypothesized that winter habitat could influence
territory establishment dates, because occupation of high quality habitat would allow
individuals to initiate spring migration earlier and/or in better condition, allowing them to
arrive on the breeding grounds earlier. We also tested whether winter habitat use could
directly affect clutch initiation, based on the hypothesis that males in high quality habitat
might have greater access to resources during winter moult and birds with superior
plumage might acquire a mate more rapidly (Studd and Robertson 1985a, 1985b). We
used a simpler pathway for females because female arrivals on territories were more
difficult to detect than males and later capture and banding made female identity prior to
clutch initiation less certain (Figure 3.2). Since the female model connects winter habitat
use directly to clutch initiation, it may incorporate winter-habitat effects on bird condition
in addition to those acting through arrival-on-territory date (see Discussion).
Data for yearling and older birds were analyzed separately because age
influences all aspects of breeding (see Results). Year was included as a potential
explanatory variable in all models. Sample sizes for the different steps in the proposed
pathways differ as tissue samples were not obtained from all individuals in all years.
Where winter habitat quality carried-over and impacted breeding productivity, we
used standardized partial regression coefficients (s) for each step in the pathway in
order to calculate the total effect of 13C and 15N on fledgling number. We then used the
shift in mean 13C and 15N values between wintering habitat-types with the greatest
49
isotopic difference to estimate the expected total shift in offspring number associated
with variation in winter habitat use (see Bart and Earnst 1999; Norris et al. 2004).
Results
Age differences in phenology and breeding performance
We collected data on male territory establishment dates, female nest initiation
dates and annual productivity for 132 males and 121 females over f ive years (2005-
2006; 2008-2010). Older males established territories an average of 8.1 days before
yearling males (partial F1,131=31.4, P<0.0001) and their social mates initiated nesting an
average 8.4 days before those of yearling males (partial F1,123=39.4, P<0.001). Older
males fledged twice as many young as yearling males (2.4 ± 1.9 vs. 1.1 ± 1.6 (partial
F1,129=11.7, P<0.001)). Older females initiated nests an average of 5.3 days before
yearling females (partial F1,115=18.0 P<0.001) and produced significantly more fledglings
(2.4 ± 0.22 vs. 1.4 ± 0.27 (partial F1,120=7.7, P=0.006)).
Winter habitat use and 13C and 15N
Yellow Warblers on the Pacific slope of Mexico wintered in riparian forest
corridors, agricultural field margins and coastal scrub habitat. Those on the southern
Gulf coast wintered within cattle pasture and along coastal habitat. Red blood cell 13C
and 15N signatures differed significantly among these habitats (13C: F4,78=22.16,
P<0.0001; 15N: F4,78=81.49, P<0.0001; Figure 3.1). Tukey HSD tests showed that 15N
signatures were significantly more depleted on the southern Gulf coast than on the
Pacific slope (P<0.0001). 13C signatures on the Pacific slope varied with habitat use
(P=0.05), being most depleted in riparian habitats. 13C signatures in pasture and
coastal habitat on the Gulf of Mexico did not differ (P=0.52).
Within our Revelstoke breeding population, 13C and 15N signatures were only
weakly correlated (r=0.13, P=0.09, n=171) indicating that these signatures were
reflecting different aspects of wintering habitat use. 13C and 15N values did not differ
significantly between sex- and age-within-sex-classes (Table 3.1; 13C: Gender, Z=0.09,
50
P=0.93; Age-class females, Z=-0.36, P=0.72; Age-class males, Z=-0.84, P=0.40; 15N:
Gender, Z=-1.51, P=0.13, Age-class females, Z=0.30, P=0.76, Age-class males, Z=-
0.05, P=0.96).
Carry-over effects
Winter habitat use only had a carry-over effect on the productivity of yearling
females (Table 3.2, Figure 3.2). More depleted 13C signatures and more enriched 15N
signatures were correlated with earlier clutch initiation dates in this group (Table 3.2,
Figure 3.3). Earlier clutch initiation dates were, in turn, associated with the production of
more fledglings by yearling females (Figure 3.2). Using the isotopic values obtained from
wintering birds of known habitat origin in Mexico, this result suggests that birds
originating in mesic habitat within a dry/seasonal climate region performed better than
those with 13C and 15N signatures consistent with wetter climate regions.
The total effect of 13C on the productivity of yearling females was -0.17. Thus, a
shift in one standard deviation of 13C would be expected to result in a shift of (-0.17)
standard deviations in fledglings. Using our wintering isotope data, our model would
predict that yearling females occupying habitat at the mesic end of the 13C gradient (i.e.
13C depleted Pacific riparian forest) would be able to produce 0.8 more fledglings the
following summer than counterparts in 13C enriched coastal or pasture habitat in the
southern Gulf of Mexico. Similarly, the total effect of 15N on the productivity of yearling
females was 0.15. Our model would thus predict that females with 15N signatures
equivalent to the average of our Pacific slope site would produce 0.6 more fledglings
than birds with signatures equivalent to the southern Gulf coast.
Although we found no evidence of winter habitat effects in our other age-sex
classes, early territory establishment was associated with early onset of reproduction
and greater annual productivity in older males (Table 3.2). Similarly, early onset of
reproduction in older females was associated with greater annual productivity (Table
3.2). Although territory establishment dates were related to the onset of reproduction in
yearling males, clutch initiation date had no impact on reproductive success (Table 3.2,
Figure 3.2).
51
Annual differences (“year”) had significant direct effects on older male territory
establishment dates and on yearling female clutch initiation dates (Figure 3.2).
Discussion
Carry-over effects have generated considerable attention because of their ability
to explain some of the observed variation in individual fitness within populations and in
population processes between years (Webster and Marra 2005). Our study offers
additional support for the concept that events occurring in one season can affect
important biological processes in a subsequent season. We expand upon previous work
(e.g. Marra et al. 1998; Gill et al. 2001; Norris et al. 2004; Gunnarsson et al. 2005;
Gunnarsson et al. 2006; Reudink et al. 2009; Tonra et al. 2011) by investigating age-
and sex-class effects. We predicted that males would experience stronger time-
mediated carry-over effects due to the importance of arrival date with regards to territory
acquisition and the greater range in productivity experienced by this sex on breeding
grounds. Similarly, we predicted that age-dependent differences in experience and
performance would make time-mediated carry-over effects weaker in yearling birds.
Contrary to our predictions, there was no evidence that winter habitat, inferred using
13C and 15N, influenced the territory establishment date of Yellow Warbler males or the
clutch initiation dates of their social mate. This differs from research on American
Redstarts (Marra et al. 1998, Norris et al. 2004) but is consistent with other Yellow
Warbler work in Ohio that found no relationship between male winter 13C signatures
and the clutch initiation dates of their social mates (Lindsay 2008).
Time-mediated carry-over effects did act on yearling female Yellow Warblers. For
this age-sex class, depleted 13C and enriched 15N wintering signatures were
associated with earlier clutch initiation dates and higher subsequent productivity. Our
13C results indicate the yearling females originating from mesic winter habitats have
higher fitness than those with signatures suggesting drier habitat use. This finding is
consistent with previous migration phenology and carry-over work (Marra et al. 1998,
Norris et al. 2004, Smith et al. 2010 (departure delay inferred from body condition)).
52
The relationship between breeding season phenology and wintering 15N
signatures has not been previously investigated in landbirds. Our blood 15N signatures
of Yellow Warblers sampled on the wintering grounds align with predicted soil 15N
enrichment patterns produced by regional temperature and precipitation gradients
(Amundson et al. 2003; Handley et al. 1999). Coastal Jalisco falls within the As/Aw
climate zones (Köppen-Geiger climate designation; Kottek et al. 2006) where high
temperatures and low rainfall would produce 15N enriched soil. Southern Veracruz (Am
climate zone) receives 3 times more precipitation and its soil 15N should be
correspondingly depleted. As/Aw climate zones cover the northern portion of the Yellow
Warbler wintering range and follow the Pacific slope of Central America as far south as
Panama. Wetter climate regions first arise at the southern-most portion of the Gulf of
Mexico and then continue southward on the Caribbean coast of Central America. If 15N
signatures in our Yellow Warblers reflect regional soil 15N signatures, we would expect
birds using the northern portion of the wintering range to be uniformly 15N enriched,
while birds wintering further south would have mixed signatures. Delayed breeding
associated with depleted 15N may therefore be related to wintering latitude, with
yearling females wintering further south arriving later than counterparts from the northern
portion of the wintering range.
Tissue 13C values in our study population showed a similar range to those
reported for American Redstarts (Marra et al. 1998; Norris et al. 2005; Reudink et al.
2009) (Table 3.1). This, when coupled with our wintering data, would suggest that there
is sufficient among-individual variation in winter habitat use to produce detectable carry-
over effects in other age-sex classes within our population. It is possible that similar
habitat variation does not influence spring migration phenology in western songbirds as
strongly as it does in eastern populations of American Redstarts. Such differences could
be driven by regional differences in habitat tolerance or by flyway geography.
Western migrants make greater use of seasonal tropical environments that are
mostly avoided by their eastern counterparts (Terborgh 1989). It has been suggested
that this pattern is driven by eastern populations becoming specialized with regards to
the mesic forests that dominate eastern North America (Terborgh 1989; Rotenberry et
al. 1995). Yellow Warblers occupy a much broader breeding range than American
53
Redstarts, which might suggest that they are less specialized in the tropics as well. An
additional feature of western systems is the absence of large geographical barriers.
Birds wintering on Caribbean islands, or those crossing the Gulf of Mexico must obtain a
threshold mass before initiating migration across open water (Schaub et al. 2008). This
could produce a close relationship between the rate of mass gain on wintering territories
and departure dates in eastern populations, a pattern which may not be present in the
west.
Lack of support for winter habitat-driven carry-over effects in males was not due
to the absence of timing effects on the breeding grounds. Territory establishment dates
had a large effect on the date by which male Yellow Warblers had attracted a mate and
that mate had initiated reproduction. For older males, the timing of clutch initiation
ultimately influenced productivity; for yearling males, it did not, principally because of the
low overall reproductive success in this age class. Because breeding season phenology
did influence the reproductive success of older males, it is possible that seasonal
interactions mediated through migration phenology could be operating in this older age-
sex class. Factors such as inter-annual variation in climate (Nott et al. 2002; Vähätalo et
al. 2004) deserve exploration, as “year” had a significant effect on the territory
establishment dates of older males in this study.
We predicted that time-mediated carry-over effects would be weaker in female
Yellow Warblers. In American Redstarts 13C had no effect on female arrival dates but
instead had a direct effect on the fledging date of those females that successfully reared
a clutch (Norris et al. 2004). Fledge date subsequently influenced the productivity of
those females. Although female condition would be the most likely mechanism through
which winter habitat could influence fledge date, no such correlation was found (Norris et
al. 2004) and this relationship may instead be linked to intrinsic quality. As in the merged
female age classes assessed in Norris et al. and Lindsay (2008), we found no evidence
of a winter habitat effect on the clutch initiation dates of older Yellow Warbler females.
Contrary to our expectations, we did find evidence of carry-over effects operating via
clutch initiation date in yearling females. Genders did not differ significantly in terms of
their 13C and 15N values and yearling females did not differ from older females, making
it unlikely that the presence of carry-over effects in this class is due to these individuals
occupying a wider range of habitats or relatively worse habitat than other classes (Table
54
3.1). It possible that carry-over effects are not seen in older, experienced, females
because they are better able to compensate for late arrival on territories by initiating
clutches more rapidly. However, our female arrival data does not support this, with lag
times to clutch initiation not differing significantly by age. Older females may also differ
from yearling females in their migration behaviour (with differing departure schedules or
stop-over timing) in a way that reduces their spread in arrival dates and subsequent
clutch initiation dates. Again, such a hypothesis was not supported by the available data:
older females showed a wider absolute range of dates as a product of earlier clutch
initiations than yearling females.
That time-mediated carry-over effects vary by age and by sex in Yellow Warblers
has population-level implications. In the 5 years encompassed by this study, 43.8% of
breeding females were yearling birds. As a result, changes in mean winter habitat quality
(Norris and Taylor 2006) could be expected to have productivity repercussions for
roughly half of the pairs at our study site. The total effect score of 13C and 15N on the
productivity of yearling females was -0.17 and 0.15 respectively, independently
corresponding to an estimated shift of 0.8 and 0.6 fledged young across the range of
mean isotope values presented in Figure 3.1. This response is less than that reported for
female American Redstarts, where a comparable shift in 13C resulted in an estimated
shift of 2 fledged young. However, like American Redstarts, Yellow Warblers generally
rear a single brood per season and have an average first clutch size of four offspring.
Thus, even small shifts in fledgling number represent a large shift in total productivity for
an individual. An additional implication of age-specific carry-over effects is geographic. If
a greater proportion of yearling birds are found at the periphery of a species breeding
range (Graves 1997; Rohwer 2004) and carry-over effects are present in yearling but not
older individuals, winter habitat degradation may lead to reduced productivity at the
edges of the breeding range but have little effect on productivity within higher-quality
breeding areas.
As with all among-individual assessments of carry-over effects, our results have
a caveat. In this type of correlational study it is not possible to isolate winter habitat
effects from the potential effect of the intrinsic quality of individuals (Harrison et al.
2011). It is possible that intrinsic quality may be driving both winter habitat occupancy
and reproductive success, leaving these factors confounded. Within-individual
55
differences in breeding season phenology and winter signatures across years, or studies
that incorporate winter habitat alteration (such as food supplementation) and record
migration date would be needed to confirm a causal link.
Our data are consistent with other work that has shown sex differences in carry-
over effect presence or strength (Marra et al. 1998, Norris et al. 2004, Gunnarsson et al.
2005, Gunnarsson et al. 2006). However, these previous studies found arrival-mediated
carry-over in males and not in females. Our study also indicates that carry-over effects
can vary between age classes. Saino et al. (2004) found differences in between-year
carry-over effects by age; to our knowledge this is the first paper to show within-year
differences.
The wintering ecology of many Neotropical warblers is poorly understood. The
majority of our current knowledge is based on research conducted on eastern
populations wintering in the Caribbean. Carry-over effects and age-sex class variation in
carry-over effects may be more evident (and more important from a conservation
standpoint (Norris et al. 2004)) among migratory bird species that prefer mesic winter
habitat. Such habitats buffer the impact of the late-winter dry season experienced in
many regions of the Neotropics (Rotenberry et al. 1995) and marked fitness differences
may exist between individuals with access to it and those occupying drier habitat.
Additional work is needed within the wintering range of western populations and with
western warbler species. These populations face different winter climate regimes, and
possibly possess a greater tolerance for drier winter habitat types than their eastern
counterparts. We advocate incorporating biologically relevant differences such as age-
class and sex into carry-over analyses and population models that include carry-over
effects in order to better understand how these effects act on individuals within a
population and to better predict population effects associated with habitat loss.
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Tables and Figures
Table 3.1 Summary of stable isotope signatures (n, mean ±SD and range) from winter-grown feathers collected from Yellow Warblers breeding in Revelstoke, British Columbia. Yearling individuals are in their first breeding season; older birds are at least 2 years old.
Age-Sex Class N 13C (‰) 15N (‰)
min Q1 med Q3 max min Q1 med Q3 max
Yearling Females 42 -24.6 -22.3 -21.5 -20.9 -18.1 5.1 7.6 8.2 9.6 13.3
Older Females 41 -23.6 -22.3 -21.7 -21.1 -18.8 5.3 7.6 8.4 9.6 11.4
Yearling Males 21 -23.7 -22.5 -21.8 -21.1 -17.4 6.4 7.2 8.5 10.9 12.5
Older Males 64 -24.8 -22.3 -21.5 -21.0 -16.8 5.2 7.6 8.9 10.2 15.5
Pooled 168 -24.8 -22.3 -21.6 -21.0 -16.9 5.1 7.6 8.5 9.9 15.5
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Table 3.2 Results of general linear model analyses for each stage of the proposed pathway from winter habitat use to fledgling productivity. Relationships for yearling and older Yellow Warbler are reported separately for (A) males and (B) females. Standardized beta coefficients for significant continuous variables are reported in Figure 3.2.
A) Males Yearling (SY) Older (ASY)
r2 n p F ratio Prob>F r2 n p F ratio Prob>F
Arrival 0.18 19 0.55
0.15 63 0.05
Year 0.12 0.89 4.70 0.01
13C 0.00 0.97 1.82 0.18
15N 2.28 0.15 0.42 0.52
Clutch Initiation 0.61 15 0.09
0.42 58 <0.001
Year 0.18 0.83 1.86 0.17
Arrival 13.23 0.005 29.21 <0.001
13C 0.24 0.64 2.48 0.12
15N 1.02 0.34 0.07 0.79
Fledge number 0.14 39 0.55
0.13 114 0.02
Year 0.84 0.53 1.76 0.13
Clutch initiation 1.09 0.3 8.89 0.004
B) Female Yearling (SY) Older (ASY)
r2 n p F ratio Prob>F r2 n p F ratio Prob>F
Clutch Initiation 0.33 38 0.009 0.10 39 0.47
Year 3.37 0.05 1.54 0.23
13C 5.45 0.03 0.52 0.48
15N 4.35 0.04 0.37 0.55
Fledge number 0.27 61 0.007
0.13 84 0.08
Year 1.48 0.21 1.71 0.14
Clutch initiation 14.93 <0.001 5.65 0.02
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Figure 3.1 Biplot of the mean (±SD) 15N (‰) and 13C (‰) values of red blood cells from Yellow Warblers over-wintering in different habitat-types on the southern Gulf coast (Veracruz) and the Pacific slope (Jalisco) of Mexico. Bracketed letters indicate signature overlap: birds from habitats with shared letters were not significantly different from
each other in terms of tissue 15N (A-C) and/or 13C (D-F).
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Figure 3.2 Model pathways for Yellow Warbler males and females by age class. Black solid lines indicate significant relationships between variables (thickest line P<0.0001, mid-thickness P<0.001, thin line P<0.05). Dashed lines indicate no statistically-detectable relationship between variables. Standardized beta coefficients for significant continuous variables are shown above each arrow. Whole-model r2 values are reported in parentheses above the dependent variables for significant relationships only.
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Figure 3.3 Relationship between winter isotope signatures andclutch initiation date in yearling female Yellow Warblers. Initiation dates are
presented as residuals from a model controlling for year and 15N
(a) or for year and 13C (b).
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Chapter 4. Winter habitat use does not influence spring arrival dates or the reproductive success of Yellow Warblers breeding in the arctic
Anna Drake, Michaela Martin, David J. Green1
Abstract
Winter-habitat use can influence the breeding success of migratory songbirds in
temperate regions due to its impact on bird condition and breeding phenology. How such
carry-over effects vary with latitude and therefore how they are incorporated into
migration trade-offs is unknown. To address this question, we examined how winter
habitat use, inferred from 13C and 15N signatures in winter-grown feathers, influenced
the breeding phenology and productivity of Yellow Warblers (Setophaga petechia) at the
extreme north of their range in the Canadian arctic (68ºN) and compared this population
to mid-latitude Yellow Warbler (50ºN) and American Redstart (Setophaga ruticilla; 44ºN)
populations reported in previous studies. In the arctic, we examined male arrival dates,
female clutch initiation dates and the relationship between these timing variables and the
number and quality of offspring produced within the season. In contrast to warblers
breeding at mid-latitudes, we find no support for an impact of winter habitat use on
timing or productivity measures. Male arrival dates and female clutch initiation dates in
both young and older individuals were not correlated with isotopic signatures acquired
on the wintering grounds. Males with enriched 15N signatures paired more rapidly after
arrival, indicating a possible relationship between winter habitat use and condition. This
1 A version of this chapter has been submitted to the Journal of Polar Biology
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relationship did not translate into productivity advantages as the negative relationship
between breeding phenology and reproductive success was significantly weaker in our
northern population when compared to Yellow Warblers breeding further south. This
reduction or absence of timing effects on productivity effectively removes one pathway
through which carry-over effects can act.
Introduction
Among migratory birds, individuals breeding at high latitudes are expected to
have higher annual productivity, but lower inter-annual survival odds than their more
southern counterparts (Alerstam et al. 2003). Increased productivity is expected because
of reduced nest predation at higher latitudes (McKinnon et al. 2010; Salgado-Ortiz et al.
2008) and greater resource availability (Cox 1985; Alerstam et al. 2003). Conversely,
energetic cost and presumably mortality risk will increase with traveling distance (Bell
1996; Wikelski et al. 2003). Breeding latitude may therefore be seen as a trade-off
between movement costs and breeding benefits. In the absence of range shifts,
productivity and mortality should balance each other and populations at different
latitudes should perform equally (Alerstam and Enckell 1979; Sutherland 1996).
Studies have shown that the quality of winter habitat occupied by migrant birds
can influence bird condition (Sherry and Holmes 1996; Studds and Marra 2005; Smith,
et al. 2011). Poor winter conditions and delayed pre-migratory fattening can delay spring
migration and therefore the timing of events on the breeding grounds (e.g. Marra et al.
1998; Saino et al. 2004). Birds arriving late may produce fewer offspring and this chain
of events therefore represents a cross-seasonal or “carry-over” effect (Norris et al.
2004). The influence of wintering-ground events on individual breeding performance
might be expected to vary with latitude for several reasons. First, the role of breeding
phenology in individual productivity should vary with latitude. Suitable conditions for
rearing young exist for a progressively smaller window of time as latitude increases
(Newton 2008). A shortened breeding season should increase the reproductive costs
associated with delayed arrival at northern breeding sites. Secondly, conditions at the
start of the breeding period are harsher at more northern latitudes and body condition
and energy reserves may become more important with increasing latitude (Sandberg
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1996). Thirdly, for birds wintering in the same region, the energetic costs of migration will
be greater with increasing breeding latitude (Wikelski et al. 2003). We might therefore
predict that the importance of bird condition at the onset of migration would increase with
breeding latitude (Marra 2012) and that time- and condition-mediated carry-over effects
will be more pronounced in northern populations. However, it is also possible that, with
increasing migration distance, events occurring during migration increase in their relative
contribution to individual arrival date and condition. This would be expected to dampen
the impact of winter events on breeding processes and make winter to breeding-ground
carry-over effects weaker in northern populations.
While carry-over effects have been investigated at mid-latitudes using songbird
populations (e.g. Saino et al. 2004; Norris et al. 2004; Rockwell et al. 2012; Drake et al.
2013; Chapter 3). All high-latitude studies have been conducted with geese and waders
(e.g. Ebbinge and Spaans 1995; Gunnarsson et al. 2005; Legagneux et al. 2011).
Differences in breeding biology complicate comparisons between these groups and
single-species work is needed. Yellow Warblers have one of the largest breeding ranges
of all wood-warblers (Parulidae), making them an ideal species to examine latitudinal
variation in the strength of carry-over effects. We have previously demonstrated that
stable 13C and 15N isotope signatures in winter-grown tissue is related to habitat use
among Yellow Warblers wintering in Mexico and that winter habitat quality influences the
breeding performance of young females within a mid-latitude population (50°N,
Revelstoke, BC; Drake et al. 2013; Chapter 3). Here, young females with isotopic
signatures consistent with the use of dry winter habitat (enriched 13C) experienced
breeding delays and lower productivity than those with signatures suggestive of more
mesic winter habitat use (Drake et al. 2013; Chapter 3). Birds breeding in northwestern
regions of North America (such as Revelstoke) share their wintering range with birds
which breed at higher latitudes (Boulet et al. 2006). We therefore asked whether carry-
over effects in Yellow Warblers breeding at the extreme north of the species range in
Inuvik, Northwest Territories, Canada (68°N) were stronger or weaker than those
indicated in these conspecifics further south.
We hypothesized that winter habitat quality could carry-over to impact breeding
success in our northern breeding birds through two, non-exclusive, mechanisms:
individual timing (time-mediated) and individual condition (condition-mediated) (Figure
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4.1). In the first scenario, individuals occupying high-quality winter habitat are able to
fatten more rapidly and depart the wintering grounds earlier in the spring (Marra et al.
1998; Smith et al. 2010 (departure inferred)). This allows them to arrive at the breeding
grounds earlier (Marra et al. 1998; Norris et al. 2004) and obtain the productivity
advantages that are associated with advanced breeding phenology (Lozano et al. 1996;
Smith and Moore 2005; Reudink et al. 2009). In the second scenario, individuals
occupying high-quality winter habitat are in better physical condition than their
counterparts when they reach the breeding grounds. For males, larger energy reserves,
more muscle mass, or higher quality plumage could offer an advantage in territory
acquisition/defense and influence female choice, both of which should result in more
rapid pairing (Studd and Robertson 1985; Gottlander 1987; Yezerinac and Weatherhead
1997). For females, larger energy reserves might result in more rapid egg-laying
(Sandberg and Moore 1996 but see Smith and Moore 2003). Both of these factors could
advance breeding phenology in both sexes from the point of clutch initiation rather than
from arrival. In addition, habitat-based variation in parental condition could have a direct
influence on fledging state and number because females in good condition may produce
larger, higher quality clutches (Smith and Moore 2003) and parents in good condition
may provision at a higher rate. We evaluated evidence for time- and condition-mediated
carry-over effects in Yellow Warblers breeding in Inuvik using information on winter
habitat use inferred from 13C and 15N signatures in winter-grown feathers, the dates of
male arrival on territories, the timing of breeding, and number and condition of offspring
produced by breeding pairs. We subsequently evaluated whether our results differed
from those reported in the carry-over effect literature for Yellow Warblers and American
Redstarts breeding in British Columbia and Ontario (Drake et al. 2013; Norris et al.
2004).
Methods
Study Species
Yellow Warblers are insectivorous Nearctic-Neotropical migrants in the family
Parulidae. During the summer they are found in riparian and wet deciduous habitat
throughout North America as far north as the Arctic tree-line (Lowther et al. 1999).
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Genetic and isotopic work suggest that eastern and western populations undergo
parallel migration with western populations wintering in Mexico and Central America and
eastern populations wintering in Central and South America (Boulet et al. 2006).
Substantial wintering overlap is seen in mid- and high-latitude populations in the west
(Boulet et al. 2006) indicating that northern populations migrate further to reach their
breeding grounds than mid-latitude populations.
Overwinter, Yellow Warblers occupy native habitat ranging from mangrove,
swamp and riparian forests to dry coastal scrub as well as a diversity of human-modified
habitat such as pasture and cropland (Dunn et al. 1997; Greenberg et al. 1996; Binford
1989). The species undergoes pre-alternate moult prior to spring migration, replacing a
variable number of greater covert, body, and crown feathers (Pyle 1997; Quinlan and
Green 2010). Yellow Warblers are territorial on the wintering grounds (Greenberg and
Ortiz 1994, Valdez Juarez, unpublished data) and the habitat used by an individual
influences the 13C and 15N signatures in their blood (Drake et al. 2013; Chapter 3) and
ultimately the feathers they grow during this period (Hobson 1999).
Depleted 13C is indicative of more mesic habitat use (Hobson et al. 2010).
Previous work suggests this type of habitat is of higher quality for many wintering
warblers (Studds and Marra 2005; Latta and Faaborg 2002; Smith et al. 2011) including
Yellow Warblers (Drake et al. 2013; Chapter 3). 15N ratios are influenced by regional
temperature and precipitation (Amundson et al. 2003), and, at the local-level, by factors
such as soil cultivation, and marine and artificial fertilizer inputs (Hobson 1999;
Amundson et al. 2003; Croll et al. 2005). 15N in animal tissue may become further
enriched with water and/or nutritional stress (Hobson et al. 1993; Kelly 2000) and with
trophic level (Post 2002). Among Yellow Warblers overwintering in Mexico, the greatest
variation in tissue 15N was regional and we suggest that depleted 15N is indicative of
wintering origins in higher rainfall regions such as eastern Mexico and southern Central
America, while enriched 15N suggests wintering origins in drier northern and Pacific
coastal regions of the Yellow Warbler wintering range (Drake et al. 2013; Chapter 3).
We predicted that the relationship between 13C, 15N and breeding phenology
and performance in Inuvik would be the same as in our southern study (Drake et al.
2013; Chapter 3): winter habitats conferring more depleted 13C and more enriched 15N
72
signatures would be associated with earlier arrival and clutch initiation dates and better
breeding performance by individual birds.
Study site and population monitoring
Between 2009 and 2011, we monitored a banded population of Yellow Warblers
in the taiga plains ecozone near Inuvik, Northwest Territories (68º 21’ N, 133º 45’ W;
Figure 1.1; hereafter Inuvik). Our study site covered 20.0 ha of riparian habitat bordering
the Mackenzie River (3-6 m elevation). Low-lying portions of this habitat are dominated
by several species of willow shrubs (Salix spp.) and by Green Alder (Alnus crispa) all of
which experience seasonal flooding during spring ice-break. These species are
intermixed with White Spruce (Picea glauca) at higher elevations (Gill 1973). Yellow
Warblers experience an average of 23 hrs of daylight per day during the breeding period
and, in the years of our study, mean daily temperatures of 11.5 ± 0.4°C, range: -2.2-
25.5°C (Environment Canada 2012).
Dates of male arrival on breeding territories were determined during surveys
conducted every 1-2 days beginning in mid-May. Male Yellow Warblers sing when
establishing territorial boundaries making them easy to detect. Males are also highly
aggressive toward song-playback and were typically caught within 1-2 days of arrival
using targeted mist-netting. Females were more difficult to detect, but were either caught
with their mates or later in the season by placing mist-nets across flight paths used
during nest-building, incubation, or while provisioning young. Upon capture, all
previously unmarked individuals were fitted with a Canadian Wildlife Service-issued
aluminum band and a unique combination of three colour bands (AC Hughes, UK).
Breeding pairs were monitored through the entire season (mid-May to late-July).
Nests were located by following females during the building period and then checked
every 3 days to determine clutch initiation dates, clutch size, brood size and fledgling
number. Clutch initiation date was defined as the laying date of the first egg in a female’s
first nesting attempt of the season. Nestlings were banded, weighed and had tarsus
measures taken seven days after hatching. We calculated the average condition of
nestlings within a clutch by calculating the mean tarsus length (mm) and mass (g) for all
individuals in the nest. We then regressed mean mass against mean tarsus for the
73
population and used the residuals (i.e. body-size controlled mass) as a unit-less metric
of average clutch ‘condition’. We assumed that all nestlings present on day 7
successfully fledged unless signs of predation were found in or around the nest after
fledging. Successful fledging was usually confirmed by observing parents feeding or
defending young out of the nest. Annual productivity for an individual bird was defined as
the number fledglings produced over all nesting attempts. Although rare, some males
(4%, n=5/128 males over 3 years) obtained two social mates, and it was possible for
their seasonal productivity to be higher than that of females.
Sexing and aging
Birds were sexed using breeding characteristics (brood patch/cloacal
protuberance), plumage (brightness/degree of rufous streaking) and behaviour. They
were then classified as yearling or older birds based on plumage. Yearlings (“second
year” or SY birds) have narrow, tapered primary coverts, tapered retrices with less
yellow colouration on the inner webs of the outer tail, and worn prebasic plumage; older
(“after-second year” or ASY) birds have broad, truncate primary covets with narrow
yellow-olive edging, and truncate retrices with more extensive yellow colouration on the
inner webs (Pyle 1997).
Tissue samples and stable isotope analysis
To obtain winter habitat 13C and 15N signatures we collected three newly
moulted greater covert feathers (identifiable by their brighter pigment and lack of wear)
from all individuals captured in 2009-2011. Feathers were washed in 2:1
chloroform:methanol solution for 24hrs, drained, and then air-dried in a fume hood for an
additional 24h to remove excess solvent. 1mg (±0.2) of tissue was placed in 9x5mm
smooth-walled tin capsules (Elemental Microanalysis, UK) and samples were then
analyzed using a PDZ Europa ANCA-GSL elemental analyzer interfaced to a PDZ
Europa 20-20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, UK) at the
University of California Davis Stable Isotope Facility in California, USA. Delta values are
expressed relative to international standards: V-PDB (Vienna PeeDee Belemnite) for
carbon and air for nitrogen; estimated measurement precision was 0.2‰ for 13C and
0.3‰ for 15N.
74
Carry-over effects
We used general linear models (JMP 9.0.2, SAS Institute Inc.) to evaluate the
effect of winter habitat use on: male arrival date, the onset of breeding in males and
females, and productivity and offspring condition for both genders. Predicted habitat
effects follow the pathway illustrated in Figure 4.1. We did not examine the relationship
between winter habitat and female arrival as lower detection probabilities and later
banding dates made female arrival dates less certain and female identity unreliable until
nest construction had begun. Clutch initiation is therefore used as proxy for arrival date
in females. Connecting winter habitat use directly to clutch initiation merges condition-
vs. timing-mediated effects in the female pathway.
All models included ‘Age’ and ‘Year’ effects as covariates as we predicted that
timing effects could vary between age-classes and among years. We also predicted that
the relationships between timing variables could vary with age and year and included
‘Arrival×Age’ and ‘Arrival×Year’ in models predicting male clutch initiation dates, and
‘Clutch Initiation×Age’ and ‘Clutch Initiation×Year’ in models predicting fledgling number
and condition. Carry-over effect strength has been shown to vary by age classes (Drake
et al. 2013; Chapter 3) and by year (Norris et al. 2004). We therefore included ‘13C /15N
×Age’ and ‘13C/ 15N ×Year’ interaction terms in all models.
Contributing variables for each step in the path analysis were selected using a
stepwise procedure where the probability to enter and to leave the model was ≤ 0.05
and ≥ 0.1 respectively. Interaction terms were restricted such that they could only enter
the model with their precedent terms. We determined the contribution of 13C and 15N to
model fit by independently reintroducing them to the final model if they were dropped
during the stepwise procedure.
Power to detect carry-over effects observed at mid-latitude sites
We used the linear multiple regression test in G*Power 3.1 (Faul et al. 2007) to
calculate the power of our Inuvik data to detect isotope effect sizes equivalent to those
found in warbler populations breeding at mid-latitude sites. First, we calculated our
power to detect a 13C and 15N effect equivalent to that found in young female Yellow
Warblers breeding in Revelstoke, British Columbia (50ºN) (Drake et al. 2013; Chapter 3).
75
In Revelstoke, variation in 13C and 15N values independently explained 11.1% and
8.9% of the variation in young female clutch initiation dates. Jointly these isotopes
explained 17.1% of the variation in this data (n=38; for detailed study description see
Drake et al. 2013; Chapter 3). We used these values and the residual variance in
reduced Inuvik models that included only young females (‘Year + 13C or 15N’ and
‘Year’) to calculate the predicted effect size (f2) of 13C and 15N in Inuvik if these
isotopes explained an equivalent amount of variance as documented in Revelstoke. We
then used f2 and the sample size of young females in Inuvik to calculate Power (1- β).
We additionally calculated our power to detect a 13C effect equivalent to that shown in
older American Redstart males at Chaffey’s Lock, Ontario (44ºN; Norris et al. 2004).
Here variation in 13C explained 32% of the variation in arrival dates (n=57). We used
this value and the residual variance in arrival date within a reduced Inuvik model (‘Year’)
for older males to calculate f2. We then used the sample size of older males in Inuvik to
calculate Power (1- β).
We also compared the strength of the relationship between clutch initiation date
and productivity for males and females in Inuvik to that shown within our mid-latitude site
by combining Inuvik breeding data with breeding data collected in Revelstoke (for
detailed study description see Drake et al. 2013) in the same manner and over the same
time period (2009-2011; n=69 females and 76 males). We used a multiple linear
regression analysis to test for a ‘Location×Clutch Initiation’ interaction in models
predicting fledgling number after accounting for ‘Year’ and ‘Year×Location’ effects.
Results
Population traits
Breeding density in Inuvik was 3.8 pairs/ha. This was higher than in our
comparison population in Revelstoke, B.C. (0.7 pairs/ha; Drake et al. 2013; Chapter 3)
but lower than that reported for some other mid-latitude populations (e.g. 10 pairs/ha;
Mazerolle et al. 2005). First breeding season (SY) individuals accounted for 41% of the
female breeding population (range 38-47%, n=3 years) and 10% of the male breeding
76
population (range 9-12%, n=3 years). Breeding pairs fledged an average of 3.3±2.1
offspring per year (n=130 pairs over 3 years).
Male and female Yellow Warblers in Inuvik did not differ significantly in their
winter 13C signatures (13C (±SD): -23.3 ± 2.0 vs. -23.1 ± 2.3, t(229)=-0.81, P=0.42.
Males, however, had more enriched winter 15N signatures than females (15N (±SD):
8.9 ± 1.5 vs. 8.4 ± 1.4, t(230)=2.64, P(two-tailed)=0.009). Wintering signatures of young
and older birds did not differ (13C: ‘Age’, P=0.52, ‘Age×Sex’, P=0.60; 15N: ‘Age’,
P=0.40, ‘Age×Sex’, P=0.92). Winter 13C and 15N signatures were not correlated
(Spearman’s ρ=0.10, P=0.12, n=231).
Carry-over effects
13C and 15N signatures in winter-grown tissue showed no relationship to either
male arrival or female clutch initiation date. Although local weather conditions in May
were not notably different among years, male arrival dates were later in 2009 than in
2010 or 2011 (‘Year’, Table 4.1). Female clutch initiation dates were delayed in 2011 in
response to a period of freezing temperatures in early June (‘Year’, Table 4.1). After
controlling for arrival date, males with enriched 15N signatures obtained mates who
initiated clutches earlier than the mates obtained by males with depleted 15N signatures
(P=0.01; Table 4.1). Male 13C signatures showed no direct relationship with clutch
initiation date.
13C and 15N signatures were not directly related to the number of fledglings
produced by either sex. Productivity of males and females varied among years and was
significantly lower in 2011 when sub-zero temperatures in June killed eggs and led to the
abandonment of clutches initiated before the freeze (Table 4.1). Unexpectedly, we found
little evidence that the productivity of male or female warblers declined significantly with
later clutch initiation dates (Table 4.1). There was some suggestion that, in males,
seasonal effects on productivity varied among years (‘Year×Clutch Initiation’, P=0.06;
Table 4.1). However, this interaction was not caused by seasonal declines in productivity
in any of the three years, but rather a positive relationship between clutch initiation date
and male productivity in 2011 when males whose social mates initiated their first clutch
after the freeze experienced greater success.
77
The 13C signature of either social mate was not directly related to the average
condition of their nestlings at day 7 post-hatch. For males, none of the variables we
considered explained any of the variation in nestling condition (Table 4.1). Among
females, there was an indication that 15N influenced nestling condition in some years
but not others (‘Year×15N’, P=0.05; Table 4.1). This interaction was the product of a
negative relationship between 15N and nestling condition in 2010. Overall, the model did
not explain a significant amount of the variation in the dataset.
Comparison with mid-latitude sites
Our power to detect winter habitat effects on the breeding phenology of young
female Yellow Warblers in Inuvik of the same magnitude as that observed in young
females in Revelstoke, British Columbia was high (13C alone: 1-β=0.97; 15N alone: 1-
β=0.92; joint effect: 1-β=1.0; n=50). Similarly a winter habitat effect on arrival date
among older males in Inuvik of comparable strength to that observed in older male
American Redstarts in Chaffy’s Lock, Ontario would have been detectable given our
sample sizes (1-β=1.0; n=106).
The relationship between clutch initiation date and productivity differed
significantly between Inuvik and Revelstoke, confirming that the timing of breeding in
Inuvik had little impact on productivity. The number of offspring fledged decreased with
clutch initiation date in Revelstoke but did not vary with clutch initiation date in Inuvik
(‘Location×Clutch Initiation’: males, P=0.02; females, P=0.03; Table 4.2, Figure 4.2).
Discussion
We predicted that carry-over effects of winter habitat use would be more
pronounced among migrants breeding the arctic. We anticipated this because northern
breeders travel further during migration and encounter harsher environmental conditions
upon arrival. Both factors should result in higher energetic costs. Migrants in the north
additionally experience a shorter breeding season, which should strongly penalize
delayed reproduction. Instead we found little support for an effect of winter-habitat use
on the breeding phenology or productivity of Yellow Warblers in the far north. Our study
78
instead offers convincing support for the concept that increasing migration distance
dampens carry-over effects.
We found no evidence that 13C and 15N signatures in winter grown tissue
influenced male arrival, female clutch initiation dates, or pair productivity in Inuvik. Power
analysis indicated that 13C effects in older males equivalent to those found in American
Redstarts breeding at mid-latitudes in eastern Canada (Norris et al. 2004) would have
been detectable if present. The absence of a winter habitat effect on older males in our
northern population was not unexpected as we also found no support for such an effect
among older male Yellow Warblers breeding further south in Revelstoke, British
Columbia (Drake et al. 2013; Chapter 3). For young females, our ability to detect a
relationship between 13C, 15N and clutch initiation date equivalent to that found in
Revelstoke (Drake et al. 2013; Chapter 3) was also high. Differences in the relationship
between 13C and arrival date in adult male warblers between populations in eastern and
western North America may relate to differences in migration routes rather than
migration distance (see Chapter 5). However, differences in the relationship between
13C and 15N and young female clutch initiation dates between our arctic site and
Revelstoke would suggest that migration distance reduces the importance of wintering
habitat use.
Evidence of condition-mediated carry-over effects in Inuvik was limited. 13C had
no effect on the speed with which males obtained a mate and initiated reproduction. 13C
and 15N also had no impact on male or female productivity or nestling condition after
controlling for breeding phenology. Enriched 15N signatures were associated with
earlier clutch initiation in males once arrival date was accounted for. Enriched feather
15N is associated with higher feather chroma in Inuvik (Jones, Drake and Green,
unpublished data). This plumage trait may have enabled 15N-enriched males to pair
faster, but it remains unclear why 15N and chroma itself are related. Despite advances
in breeding phenology through clutch initiation date for 15N enriched males, this shift did
not represent a condition mediated carry-over effect as males did not experience higher
productivity with earlier clutch initiation. Similarly, clutch initiation date in Inuvik was not
directly associated with nestling condition. Such an effect might have produced a carry-
over effects if nestlings that leave the nest in better condition are more likely to survive
79
and recruit into the breeding population (Magrath 1991). We could not test two additional
avenues through which winter habitat use might impact fitness. It is possible that 15N-
enriched males experience higher rates of extra-pair paternity if advanced pairing is the
product of increased attractiveness. It is also possible that rapid pairing is the product of
rapid territory acquisition. This may mean that 15N-enriched males experience less
male-male conflict, which could in turn influence survival.
It was expected that individuals who arrived on the breeding grounds and
initiated clutches early would have higher productivity than individuals arriving later in the
breeding season (e.g. Smith and Moore 2005; Rockwell et al. 2012). We did not observe
this pattern in Inuvik Yellow Warblers. Clutch initiation date in Inuvik was not significantly
correlated with productivity even after excluding data from 2011, where freezing
temperatures penalized individuals who began nesting earlier in the season (males:
Spearman’s ρ=-0.16, P=0.15, n=83; females: Spearman’s ρ=-0.14, P=0.25, n=75). In
contrast, declines in seasonal productivity are observed among Yellow Warblers in
Revelstoke (Drake et al. 2013; Chapter 3). That seasonal productivity declines in Inuvik
were indeed weaker than those in more southerly populations was confirmed by
significant slope differences in the relationship between clutch initiation and fledgling
number between Revelstoke and Inuvik (Table 4.2). This result is similar to that reported
for ptarmigan (Lagopus spp.) where later first clutch dates had less of an influence on
clutch size for birds breeding in the arctic than for those in the alpine at lower latitudes
(Martin and Wiebe 2004) but counter to consistent seasonal declines in first clutch size
reported for American Pipits (Anthus rubescens) across latitudes (Hendricks 1997).
There are several reasons why productivity declines might not be observed in our Inuvik
population. First, inhospitable conditions in the early spring may mean that northern
birds arrive and initiate reproduction more synchronously than more southerly
populations. In the three years of our study, Inuvik birds initiated nests over a 22-day
period while Revelstoke birds began to breed a week earlier in the season and initiated
nests over a 27-day period (Figure 4.2). This reduced spread in clutch initiation dates in
Inuvik would reduce productivity differences among individuals if timing effects were
present. Secondly, Inuvik Yellow Warblers, like other birds breeding in the north,
experience lower predation rates than more southern populations (Martin 2013;
McKinnon et al. 2010). This means that fewer first-nests fail (39% (Inuvik, n=128 nests
over 3 years) vs. 52% (Revelstoke, n=152 nests over 6 years)) and, consequently,
80
additional time for re-nesting is less of an advantage for the majority of individuals.
Among females in Inuvik who failed in their first nesting attempt, individuals who initiated
their first clutch later in the season were less likely to successfully fledge a subsequent
clutch (χ21,45=12.9, P<0.001), however, these individuals represented too small a
proportion of the breeding population to influence overall trends. Yellow Warblers in
Inuvik are almost always single-brooded; in three years only one female in our study
reared a second clutch after successfully fledging a first. Fledging first nests early
therefore does not result in increased productivity through second broods. Finally, clutch
size in Inuvik is a quadratic function of clutch initiation date, with early clutches
containing fewer eggs than mid-season clutches (Martin, 2013). This pattern suggests
that early clutch initiation dates are costly, possibly due to the shorter window of suitable
climatic conditions and food availability in the north. Small, early clutch-sizes followed by
a mid-season peak and decline may flatten any negative linear relationship between
clutch initiation date and fledge number.
Yellow warblers breeding in Inuvik (particularly males) had more depleted 13C
signatures (indicative of more mesic habitat use (Hobson 2007)) and higher productivity
than reported for Yellow Warblers breeding in Revelstoke (Martin 2013). Mesic habitat is
considered to be higher quality habitat for wintering warblers relative to drier habitats
(e.g. Bearhop et al. 2004; Studds and Marra 2005; Smith et al. 2010) and signature
differences could indicate that Yellow Warblers in Inuvik are more socially dominant on
the wintering grounds than Revelstoke birds. This is similar to seasonal matching
documented among Black-tailed Godwits (Limosa limosa islandica) in the U.K., where
individuals from high quality winter habitat also occupy high quality breeding locations.
However, seasonal matching has not been documented in songbirds that breed across a
large latitudinal gradient and wintering isotope signatures and productivity data from
multiple breeding populations across the Yellow Warbler range would be needed to test
this concept.
In conclusion, we found no evidence of an effect of wintering habitat use on
individual productivity among Yellow Warblers at northerly extreme of their range. This
was due to (1) any potential effect of winter habitat use on breeding phenology being
weaker than that observed among warblers at mid-latitudes and (2) productivity patterns
in the north that undermine the time-mediated pathway through which carry-over effects
81
have previously been shown to act in passerines (Norris et al. 2004, Saino et al. 2004,
Drake et al. 2013; Chapter 3). We found some support for a non-breeding season effect
on male condition, which advanced male breeding phenology from the point of clutch
initiation. This effect did not translate into increased productivity or improved nesting
condition. In total, the absence of a detectable winter-habitat effect on fledgling number
in Inuvik indicates that timing effects are weaker in this population than in populations
where declines in fledgling number have been shown. Increased migratory distance
therefore appears to dampen the impact of winter habitat use on breeding phenology
while productivity advantages associated with breeding in the far north appear to reduce
the role of phenology in breeding success.
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Tables and Figures
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Table 4.1 Multiple regression analyses testing the influence of winter habitat
(inferred from 13C and 15N) on Inuvik Yellow Warbler breeding metrics. For predicted relationships see Figure 4.1. Age and Year plus second-order interaction terms were included in initial models and dropped sequentially if they did not contribute to fit. Below
each model we report the contribution of 13C and 15N to model fit (italics) if these variables were to be independently added back into the final model.
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Table 4.2 Location effects on the relationship between first-clutch initiation date and productivity in Yellow Warblers from the perspective of (A) breeding males and (B) breeding females. For both sexes the best fitting model predicts a negative relationship at our mid-latitude (Revelstoke) site and no relationship at our high-latitude (Inuvik) site. See also Figure 4.2.
A) Males r2 df, n p Std B F ratio Prob>F
Fledge number: 0.15 7, 199 <0.001
Location (Revelstoke-Inuvik) 0.36 20.50 <0.001
Clutch Initiation -0.16 4.17 0.05
Location×Clutch Initiation 0.18 5.41 0.02
Year - 1.80 0.17
Year×Clutch Initiation - 0.77 0.46
B) Females r2 df, n p Std B F ratio Prob>F
Fledge number: 0.13 7, 190 <0.001
Location (Revelstoke-Inuvik) 0.32 13.76 <0.001
Clutch Initiation -0.16 3.78 0.05
Location×Clutch Initiation 0.19 4.99 0.03
Year - 2.99 0.05
Year×Clutch Initiation - 0.53 0.59
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Figure 4.1 Suggested pathways through which winter-habitat-driven carry-over effects could act. For females in our study population, clutch initiation date was used as a proxy for arrival date (see text) and therefore condition- and timing-pathways to clutch initiation are not
separate. Depleted 13C and enriched 15N tissue signatures are expected to be associated with high quality winter habitat use.
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Figure 4.2 The relationship between first-clutch initiation date and annual productivity (total number of young fledged) among Yellow Warbler females within our high-latitude breeding population (Inuvik, top) and our mid-latitude breeding population (Revelstoke, bottom). See also Table 4.2.
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Chapter 5. Wind speed during migration influences the survival, timing of breeding, and productivity of a Neotropical migrant
Anna Drake, Christine Rock, Sam P. Quinlan, Michaela Martin, David J. Green1
Abstract
Migratory bird populations can be limited by climatic conditions on breeding
grounds, wintering grounds or during migration. Identifying the timing and location of
events that alter demographic rates would improve the accuracy of population models
and aid conservation efforts. We examine how climate variables operating at discrete
periods of the annual cycle influence the demography of Yellow Warblers (Setophaga
petechia) that breed in western Canada and overwinter in Mexico. We demonstrate that
wind conditions during spring migration are the best predictor of adult survival, male
arrival date, female clutch initiation date and, via these timing effects, reproductive
success. Our study emphasizes that events that occur during the migratory period are
important for the population dynamics of Nearctic-Neotropical migrants.
Introduction
Climate events have been related to variation in the phenology, survival, and
reproductive success of migratory birds (Sillett et al. 2000; Nott et al. 2002; Dugger et al.
1 This chapter is modified from a manuscript formatted for PNAS.
93
2004; Studds and Marra 2011) and have consequently been incorporated into
conceptual models examining the population dynamics of Nearctic-Neotropical migrants
(Sherry and Holmes 1995). However, our ability to predict the relative importance of
climate events operating at different stages of the annual cycle has been limited by
several factors: 1) climate variables are often correlated between periods and across
regions (Nott et al. 2002; Forchhammer et al. 2002); 2) events in one period may impact
processes occurring in subsequent ones (Norris et al. 2004; Saino et al. 2004); and 3)
most studies have focused on species that winter in the Caribbean and breed in eastern
North America. In this study, we evaluate the relative importance of conditions during
the winter, migratory, and breeding season on the demographic rates of Yellow
Warblers breeding in western Canada.
Yellow Warblers have a broad breeding distribution in North America. Genetic-
isotopic work suggests eastern and western lineages have parallel migration systems
and differing wintering distributions (Boulet et al. 2006). Northwestern populations, such
as those at our study site near Revelstoke, British Columbia (50.97°N, -118.20°W;
Figure 1.1), winter primarily in lowland areas in the occidental and isthmus region of
Mexico. In spring, they migrate along the western and central flyway (>97°W) (Boulet et
al. 2006) arriving at northern breeding grounds by mid-May. Males establish territories in
riparian and wet deciduous habitat and the majority of pairs rear a single clutch of four
nestlings before migrating back to Mexico in late August.
Climatic conditions operating during the wintering period, spring migration and
breeding period could all explain annual variation in the survival, breeding phenology
and subsequent reproductive success of Yellow Warblers in Revelstoke. We therefore
developed seven regionally-specific climate models that could potentially describe this
variation. We assessed the relative support for these models using an Information
Theoretic approach (Burnham and Anderson 2004). Wintering period models. Primary
productivity and insect abundance in overwintering areas, which is linked to rainfall
(Brown and Sherry 2006; Studds and Marra 2007), has been shown to increase survival
and induce earlier spring migration in some species (Dugger et al. 2004; Studds and
Marra 2007). The El Niño Southern Oscillation (ENSO) impacts rainfall patterns in
Mexico and we used the Southern Oscillation Index (SOI) to infer wintering conditions
for our population. The majority of rainfall (>60%) in the occidental and isthmus regions
94
of Mexico falls during the summer monsoon (May-August) (Caso et al. 2007) and,
consequently, this period impacts the conditions encountered by birds on the wintering
grounds. Weak monsoon years are associated with El Niño (negative SOI) phases of
ENSO (Caso et al. 2007) and likely result in reduced food availability for wintering birds.
We therefore predicted that negative mean SOI values in the May-August period would
be associated with low survivorship and delayed phenology in our population the
following spring (Model 1). It is possible that late winter rainfall is disproportionately
important to Neotropical migrants. Such rainfall contributes significantly less moisture to
over-winter habitat than the monsoon, but occurs at a critical time, when regions are
experiencing drought conditions. In winter El Niño conditions (negative SOI) promote
greater precipitation (Caso et al. 2007). We therefore predicted that negative SOI values
in the December-February period would be associated with favourable conditions that
would improve survivorship and advance breeding phenology (Model 2). Spring
migration models. Strong winds and poor weather can impact energetic costs and speed
of movement during migration (Liechti 2006). Birds facing hostile conditions should have
lower survival or later arrival dates due to slowed movement and increases in stop-over
number or duration (Liechti 2006). We used mean nighttime wind vectors (westerly (U-)
and southerly (V-) components) and precipitation over the western flyway region (Figure
5.1) between March and May as measures of migration costs. High wind speeds
(westerly (Model 3), southerly (Model 4), and combined (U+V; Model 5) and greater
precipitation (Model 6) were expected to be associated with reduced survivorship and
delayed breeding phenology. Breeding period models. Local temperatures at the
beginning of the breeding season impact the timing of insect development and therefore
food availability (Visser et al. 2006). Migratory birds have been shown to adjust arrival
and/or clutch initiation partly in response to local/regional conditions (Mazerolle et al.
2011). Increased food availability and earlier breeding phenology may also enhance bird
condition at the end of the breeding season, reducing overwinter mortality. Higher mean
May temperatures at our study site were therefore expected to be associated with
earlier breeding phenology in the same year, and improved adult survivorship in the
following year (Model 7).
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Results and Discussion
Variation in survival in Revelstoke was best described by the three migration
wind speed models (3, 4, 5), which received 78% of the total model support (Table 5.1).
Model 5 best described survival estimates, which declined in association with increasing
westerly and southerly wind speeds. Adult apparent survival was predicted to decline
from 75 to 37% across the range of westerly (Figure 5.2) and 68 to 46% across the
range of southerly (Figure 5.3) wind speeds in our dataset.
Wind speed also best described male arrival and female clutch initiation dates.
Wind speed models describing male arrival date received 68% of total model support
(Table 5.2). The westerly wind vector during spring migration was the best predictor of
male arrival (Figure 5.4, Table 5.2). Wind speed models describing female clutch
initiation date received 80% of total model support with the westerly wind vector also
being the best predictor (Figure 5.3, Table 5.3). Individuals that returned in more than
one year showed similar delays in years where westerly winds were stronger (male
arrival: P=0.001, t test, n=49; female clutch initiation: P=0.002, t-test, n=21; Figure 5.5).
Wind speed effects on breeding phenology and annual productivity varied with
age. Older males anticipated young male arrival by 12 days under the most favourable
wind conditions and arrived at the same time as young males in the year with the
strongest wind (Figure 5.4). Westerly wind vectors on the flyway are weaker in May,
suggesting that delaying migration could reduce costs for individuals. It is possible that
older male Yellow Warblers trade-off migration costs against the productivity
advantages associated with early arrival (Reudink et al. 2009) while young males, who
arrive late regardless of wind conditions, may simply minimize migration costs. Over the
range of westerly wind speeds, clutch initiation varied by 8 days in young females and 3
days in older females (Figure 5.4). This difference may arise because older females are
better able to compensate for delayed arrival by initiating reproduction more rapidly. We
calculate that, across the range of observed wind speeds, delays in clutch initiation
would result in declines in productivity of 0.5 fledglings per year for young females and
0.2 fledglings per year for older females.
Mean SOI values between March and May are correlated with westerly wind
vectors in the same time period (r2=0.36, P<0.0001, n=50). Our data therefore
96
corroborates and offers an additional explanation for delayed movement of migrants
through California in La Niña years (Macmynowski et al. 2007) as well as increased
migrant productivity in the Pacific Northwest during El Niño years (Nott et al. 2002).
Research conducted on Neotropical migrants that over-winter in the Caribbean
and breed in the eastern US and Canada indicate that populations are limited by events
during the wintering and/or breeding period (e.g. Wilson et al. 2011; Sillett et. al. 2000).
In contrast, we find only weak evidence that climate factors acting during the wintering
and breeding periods influence Yellow Warblers using western migratory routes and far
more compelling support for migration effects (see also LaManna et al. 2012). We argue
that this difference is due to 1) wind patterns that oppose the north/north-westerly
course of western migrants while facilitating spring movement in the east (Boulet et al.
2006; Gauthreaux 1980) and 2) a weaker dependency on wintering habitat quality for
birds using over-land (western) versus over-water (eastern) migration routes. Birds
moving through western North America can compensate for poor departure conditions
through shorter flights and increased stopovers: a behavioural option not available to
eastern birds who must cross large ecological barriers (Alerstam 1979; Schaub et al.
2008). We suggest that whereas pre-migration fattening in wintering habitat may be
essential for the movement and survival of eastern populations, en route conditions and
habitats are more important for western birds.
Materials and Methods
Population Monitoring
Survival
Breeding birds were captured and fitted with a Canadian Wildlife Service-issued
aluminum band and a unique combination of three color-bands (AC Hughes, UK). Birds
were aged as either SY (first breeding season) or ASY (at least 2 years old) individuals
based on plumage characteristics (Pyle 1997). Apparent annual adult survival for the
period 2004-2011 was calculated as the probability of an adult returning to the study site
(φ) after adjusting for the re-sighting probability of banded individuals (p) using methods
described by Lebreton et al. (1992) implemented using program MARK version 5.1
97
(White and Burnham 1999). The dataset contained 279 individuals (141 females, 138
males) and 437 encounter histories. The global model that allowed adult survival to vary
as a function of gender, age (SY vs. ASY), and year and re-sighting probability to vary
as a function of gender and between years with extensive monitoring (2005-2006 and
2008-2011) and one year with less extensive monitoring (2007) fit the data well (median
procedure, ĉ=1.11). We first determined the best model structure for the resighting rate,
then modeled survival rates with candidate models containing gender, age, year, and all
possible interactions, and finally investigated the influence of climate variables. We used
Akaike’s Information Criterion corrected for small sample sizes and over-dispersion
(QAICc), to rank competing models (Table 5.1). Recapture rates were best modeled
with only a gender term; males were estimated to have higher resighting probabilities
than females (male=0.89 ± 0.04, female=0.63 ± 0.09). Apparent annual survival rates
were best described by a model that included year and age (AIC weight=0.53) and a
model that included year, age and gender (AIC weight=0.41). The final candidate model
set therefore included both these models, models that included age, gender and climate
variables as main effects, only age and climate variables as main effects, and models
that included main effects, age×climate interactions and/or gender×climate interactions
(Table 5.1 lists the 47 models in the final candidate set).
Breeding Phenology
Breeding data was collected between 2005-2006, and 2008-2011. Male arrival dates
(n=183) were determined during surveys conducted every 1-2 days from early-May to
late-June. Pairs were monitored every 3 days from early-May to late-July in order to
determine the clutch initiation date (n=155), clutch size, fate and fledging success of all
nesting attempts. Clutch initiation date was defined as the laying date of the first egg in
a female’s first nesting attempt of the season. Nestlings were banded seven days post-
hatch and the number of nestlings present at this date was assumed (in the absence of
evidence of predation) to be the number of young fledged from successful nests.
Successful fledging was usually confirmed by observing parents feeding or defending
young out of the nest.
Age has a strong influence on individual behaviour, timing and breeding success
(e.g. 28, 29) (Fowler 1995; Stewart et al. 2002). Models examining climate effects on
98
male arrival and female clutch initiation dates included age as a main effect and as an
interaction term. The candidate model set therefore included 15 models (Table 5.2 and
5.3).
Carry-over Effect on Female Productivity
Productivity was defined as the total number of young fledged across all nesting
attempts made by a given individual. Later initiation dates in our population resulted in
lower female productivity (SY females: r2=0.18, n=61, P<0.001; ASY female: r2=0.08,
n=92, P=0.004). We multiplied the standardized partial regression coefficients (βs) for
the two steps in the pathway between wind speed and fledgling number to calculate the
total effect (TE) of wind speed on productivity for each age class. The TE score for ASY
females was less than for SY birds (-0.03 vs. -0.11). TE and the standard deviation of
both wind speed and age-specific fledge numbers in our data set were then used to
calculate predicted change in fledge number across the range of wind speeds observed
(Norris et al. 2004; Bart and Earnst 1999).
Climate variable sources and derivation.
All climate variables were derived from publically available datasets. Wintering
period. SOI values were averaged standardized monthly SOI data for May to August
and December to February (NOAA SOI 2012). Spring migration. Wind and rainfall on
migration was derived from modeled climate data extracted from the National Center of
Environmental Prediction (NCEP) Reanalysis 1 data archives at the NOAA-CIRES
Climate Diagnostics Center at Boulder, Colorado, USA (NOAA NCEP 2012) using the
RNCEP program (Kemp et al. 2012). This data has a spatial resolution of 2.5° latitude
and longitude and temporal resolution of six hours. We defined the western flyway for
our population as the overland region west of the easternmost portion of the continental
divide (107°W), beginning at the northern extent of the Yellow Warbler wintering range
(25°N) and ending at the latitude of our study site (50°N). We used surface precipitation
data (Kg/m2/s) and wind vector data (U- and V-wind speed components (m/s)) averaged
from the 850mb (1500m) and 925mb (700m) level (which equates with the mean altitude
of passerine migration (1094±319 m (Alerstam et al. 2011) for the March-May period,
dropping noon values so that the series represented conditions encountered during
nighttime migration (between 18:00h and 6:00h). Points that corresponded to locations
99
over the Pacific were dropped while those over the Baja peninsula and Gulf of California
were retained. Breeding period. Averaged daily mean temperatures (°C) on the
breeding grounds during the month of May (Mazerolle et al. 2011) were drawn from
“Revelstoke A” weather station (ID: 1176749; 50°57’40N, -118°11’00W, elevation
444.7m (Environment Canada 2012).
There was some collinearity between the five climate variables (Table 5.4), but in
all but one case the predicted effects were in opposite directions allowing our
predictions to be evaluated. SOIDEC-FEB and U-wind were positively correlated (r(6)=0.74,
P=0.05) and both were predicted to be negatively correlated with breeding phenology
and survival. However, model support for SOIDEC-FEB was consistently low (Table 5.1-5.3)
and we did not consider it to be competitive explanation for survival or breeding
phenology. SOIMAY-AUG and U-wind were also positively correlated with sampling year in
our data set (Table 5.4).
We evaluated whether temporal trends in breeding phenology and survival could
have influenced model selection by creating an alternative candidate model set that
replaced climate variables with the residuals of a climate metric-year regression and a
continuous time variable.
Conclusions regarding climate effects on breeding phenology did not change
when de-trended climate metrics were used. De-trended wind models received more
support (48%) than the next best models (SOIMAY-AUG; 30%) in candidate sets examining
variation in male arrival. De-trended wind models also received more support (58%)
than the next best model (the null model, “continuous time + YEAR (nominal) + AGE”;
19%) in candidate sets examining variation in female clutch initiation date. Annual
variation in apparent survival could be explained by linear declines in survival over the 7
years of the study since the null model (the continuous time variable alone) received
more support than any model that also included de-trended climate metrics. However,
the magnitude of the annual variation in apparent survival observed is unlikely to reflect
population trends in survival and we interpret support for the null model as reflecting the
close correlation between westerly wind vectors and year over our sampling period.
We assessed the relationship between mean SOI values between March and
May and westerly wind speed in the same time period (r2=0.36, n=50, P<0.0001) by
100
deriving these values, as above, for the years 1962-2011. Westerly wind speed for the
850mb (1500m) and 925mb (700m) levels alone did not differ from each other in their
relationship with SOI (850mb: r2=0.37, n=50, P<0.0001; 925mb: r2=0.33, n=50,
P<0.0001).
Wind speed declined from March to May (U-wind: F2,147=3.13, P=0.05; V-wind:
F2,147=13.22, P<0.0001). Post-hoc comparisons showed both vectors were significantly
weaker in May relative to earlier in the migratory period. U-wind speed dropped only in
May (students-t: March vs. April, P=0.97; April vs. May, P=0.03; March vs. May, P=0.03)
while V-wind showed a steady decline over the three-month period (students-t: March
vs. April, P=0.001; April vs. May, P=0.09; March vs. May, P<0.0001).
Statistical programs
Survival QAICc were run in MARK version 5.1 (White and Burnham 1999).
NCEP data extraction and breeding phenology cAIC analyses were run in R version
2.14.1 (2011, The R Foundation for Statistical Computing). All other statistical analyses
were run in JMP (8.0, SAS Institute Inc.).
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Tables and Figures
Table 5.1 Models describing apparent annual survival of Yellow Warblers breeding in Revelstoke, British Columbia (n=279 individuals, 437 encounters). Models are ranked using QAICc. Model number refers to regionally-specific climate variables described in the text. Age was included in all models as a covariate (see methods). The number of parameters in the model (K), Akaike’s information criterion (QAICc), QAICc difference from the top model (ΔQAICc), and Akaike weight (ωi) are reported.
Period Model Variables K QAICc ∆QAICc ωi
Migration 5 U-WIND + V-WIND + AGE 6 526.46 0.00 0.168
Migration 5 U-WIND + V-WIND + AGE + SEX 7 526.72 0.26 0.148
Migration 3 U-WIND + AGE 5 527.41 0.96 0.104
Migration 3 U-WIND + AGE + SEX 6 527.65 1.20 0.092
Migration 3 U-WIND + AGE + SEX + U-WIND*SEX 7 528.43 1.97 0.063
Migration 5 U-WIND + V-WIND + AGE + SEX + U-WIND*SEX + V-WIND*SEX
9 528.55 2.09 0.059
- - YEAR(nominal) + AGE 10 528.73 2.27 0.054
- - YEAR(nominal) + AGE + SEX 11 529.07 2.61 0.046
Migration 3 U-WIND + AGE + U-WIND*AGE 6 529.32 2.86 0.040
Migration 3 U-WIND + AGE + SEX + U-WIND*AGE 7 529.66 3.2 0.034
Migration 3 U-WIND + AGE + SEX + U-WIND*AGE + U-WIND*SEX 8 530.39 3.93 0.024
Migration 5 U-WIND + V-WIND + AGE + U-WIND*AGE + V-WIND*AGE 8 530.46 4.00 0.023
Winter 1 SOIMAY-AUG + AGE 5 530.46 4.01 0.023
Migration 5 U-WIND + V-WIND + AGE + SEX + U-WIND*AGE + V-WIND*AGE
9 530.83 4.37 0.019
Winter 1 SOIMAY-AUG + AGE + SEX 6 530.87 4.41 0.019
Winter 1 SOIMAY-AUG + AGE + SEX + SOIMAY-AUG *SEX 7 532.22 5.76 0.009
Winter 1 SOIMAY-AUG + AGE + SOIMAY-AUG *AGE 6 532.36 5.90 0.009
Migration 5 U-WIND + V-WIND + AGE + SEX + U*AGE + V*AGE + U*SEX + V*SEX
11 532.71 6.25 0.007
Winter 1 SOIMAY-AUG + AGE + SEX + SOIMAY-AUG *AGE 7 532.77 6.31 0.007
Breeding 7 MAY MEAN°C + AGE 5 532.81 6.35 0.007
Breeding 7 MAY MEAN°C + AGE + SEX 6 533.30 6.84 0.005
105
Table 5.1 continued
Period Model Variables K QAICc ∆QAICc ωi
Winter 2 SOIDEC-FEB + AGE 5 533.61 7.15 0.005
Winter 2 SOIDEC-FEB + AGE + SEX 6 534.06 7.60 0.004
Breeding 7 MAY MEAN°C + AGE + MAY MEAN°C *AGE 6 534.07 7.61 0.004
Winter 1 SOIMAY-AUG + AGE + SEX + SOIMAY-AUG *AGE + SOIMAY-AUG *SEX
8 534.11 7.65 0.004
Breeding 7 MAY MEAN°C + AGE + SEX + MAY MEAN°C*SEX 7 534.30 7.84 0.003
Breeding 7 MAY MEAN°C + AGE + SEX + MAY MEAN°C*AGE 7 534.46 8.00 0.003
Winter 2 SOIDEC-FEB + AGE + SEX + SOIDEC-FEB*SEX 7 535.08 8.62 0.002
Breeding 7 MAY MEAN°C + AGE + SEX + MEAN°C*AGE + MEAN°C*SEX
8 535.22 8.76 0.002
Winter 2 SOIDEC-FEB + AGE + SOIDEC-FEB*AGE 6 535.57 9.11 0.002
Winter 2 SOIDEC-FEB + AGE + SEX + SOIDEC-FEB*AGE 7 535.927 9.47 0.001
Migration 6 MIGRATION RAIN + AGE + MIG RAIN*AGE 6 535.99 9.53 0.001
Migration 6 MIGRATION RAIN + AGE + SEX + MIG RAIN*AGE 7 536.03 9.57 0.001
Migration 6 MIGRATION RAIN + AGE 5 536.38 9.92 0.001
Migration 4 V-WIND + AGE 5 536.93 10.47 0.001
Migration 6 MIG RAIN + AGE + SEX 6 537.03 10.57 0.001
Winter 2 SOIDEC-FEB + AGE + SEX + SOIDEC-FEB*AGE + SOIDEC-FEB*SEX 8 537.10 10.64 0.001
Migration 6 MIGRATION RAIN + AGE + SEX + MIG RAIN*AGE + MIG RAIN*SEX
8 537.50 11.04 0.001
Migration 4 V-WIND + AGE + SEX 6 537.59 11.13 0.001
Migration 4 V-WIND + AGE + SEX + V-WIND*SEX 7 537.87 11.41 0.001
Migration 6 MIGRATION RAIN + AGE + SEX + MIG RAIN*SEX 7 538.20 11.75 0
Migration 4 V-WIND + AGE + V-WIND*AGE 6 538.88 12.42 0
Migration 4 V-WIND + AGE + SEX + V-WIND*AGE 7 539.61 13.15 0
Migration 4 V-WIND + AGE + SEX + V-WIND*AGE + V-WIND*SEX 8 539.92 13.46 0
- - YEAR(nominal)*AGE*SEX 30 551.93 25.47 0
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Table 5.2. Ranked summary of AICc support for the candidate climate models and the null model (YEAR + AGE) describing male Yellow Warbler arrival date (n=183) in Revelstoke, British Columbia. Age (SY=1 yr; ASY>2 yrs) was included in all models as a covariate (see methods). Model adjusted r2, the number of parameters in the model (K), Akaike’s information criterion adjusted for small sample size (AICc), AICc difference from the top model (ΔAICc), and Akaike weight (ωi) are reported.
Period Model Variables r2 K AICc ∆AICc ωi
Migration 3 U-WIND + AGE + U*AGE 0.23 5 1273.80 0.00 0.546
Winter 1 SOIMAY-AUG + AGE + SOIMAY-AUG*AGE 0.22 5 1275.15 1.35 0.278
Migration 5 V-WIND + U-WIND + AGE + V-WIND*AGE + U-WIND*AGE
0.22 7 1277.35 3.55 0.092
Migration 3 U-WIND + AGE 0.20 4 1279.73 5.93 0.028
Winter 1 SOIMAY-AUG + AGE 0.20 4 1280.35 6.55 0.021
Migration 5 V-WIND + U-WIND + AGE 0.20 5 1280.81 7.01 0.016
Winter 2 SOIDEC-FEB + AGE + SOIDEC-FEB*AGE 0.20 5 1281.56 7.77 0.011
- - YEAR + AGE 0.20 8 1283.82 10.03 0.004
Winter 2 SOIDEC-FEB + AGE 0.18 4 1283.98 10.18 0.003
Breeding 7 MAY MEAN°C + AGE 0.12 4 1296.18 22.38 0
Breeding 7 MAY MEAN°C + AGE + MAY MEAN°C *AGE 0.12 5 1297.19 23.39 0
Migration 4 V-WIND + AGE 0.11 4 1299.00 25.19 0
Migration 4 V-WIND + AGE + V-WIND *AGE 0.11 5 1300.32 26.52 0
Migration 6 MIGRATION RAIN + AGE 0.09 4 1303.22 29.42 0
Migration 6 MIGRATION RAIN + AGE + MIGRATION RAIN*AGE
0.08 5 1305.33 31.53 0
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Table 5.3 Ranked summary of AICc support for the candidate climate models and the null model (YEAR + AGE) describing female Yellow Warbler clutch initiation date (n=155) in Revelstoke, British Columbia. Age (SY=1 yr; ASY>2 yrs) was included in all models as a covariate (see methods). Model adjusted r2, the number of parameters in the model (K), Akaike’s information criterion adjusted for small sample size (AICc), AICc difference from the top model (ΔAICc), and Akaike
weight (ωi) are reported.
Period Model Candidate Model r2 K AICc ∆AICc ωi
Migration 3 U-WIND + AGE + U*AGE 0.16 5 1045.30 0.00 0.317
Migration 5 V-WIND + U-WIND + AGE 0.15 5 1046.30 1.00 0.192
Migration 3 U-WIND + AGE 0.14 4 1046.34 1.04 0.188
- - YEAR + AGE 0.16 8 1047.01 1.70 0.135
Migration 5 V-WIND + U-WIND + AGE + V-WIND*AGE + U-WIND*AGE
0.15 7 1047.79 2.48 0.091
Winter 1 SOIMAY-AUG + AGE 0.12 4 1050.32 5.02 0.026
Winter 1 SOIMAY-AUG + AGE + SOIMAY-AUG*AGE 0.12 5 1051.81 6.51 0.012
Migration 6 MIGRATION RAIN + AGE 0.11 4 1052.82 7.52 0.007
Winter 2 SOIDEC-FEB + AGE + SOIDEC-FEB*AGE 0.11 5 1052.93 7.62 0.007
Winter 2 SOIDEC-FEB + AGE 0.11 4 1053.02 7.72 0.007
Breeding 7 MAY MEAN°C + AGE 0.10 4 1053.44 8.14 0.005
Migration 4 V-WIND + AGE 0.10 4 1053.81 8.51 0.005
Migration 6 MIGRATION RAIN + AGE + MIGRATION RAIN*AGE
0.10 5 1054.75 9.45 0.003
Migration 4 V-WIND + AGE + V-WIND *AGE 0.10 5 1055.14 9.84 0.002
Breeding 7 MAY MEAN°C + AGE + MAY MEAN°C *AGE 0.10 5 1055.29 9.99 0.002
108
Table 5.4. Correlation (r) matrix of explanatory climate variables and time (year) (n=7). Significant relationships (Spearman’s ρ) are starred (*=0.01 ≤ P ≤ 0.05). Variables where the predicted relationship with survival and breeding phenology would suggest a possible confound between models are marked with (‡)
SOIMAY-AUG SOIDEC-FEB Mig. Rain U-Wind V-Wind Year
Mean May °C -0.50 -0.05 -0.52 -0.63 0.34 -0.64
SOIMAY-AUG 0.75* 0.28 0.74* 0.11 0.73*
SOIDEC-FEB -0.38 0.74*‡ -0.16 0.52
Mig. Rain -0.07 0.50 0.14
U-Wind -0.48 0.77*
V-Wind -0.07
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Figure 5.1 The location of our study (A), the Yellow Warbler wintering range (blue), and the area used to calculate wind speed values during migration (yellow). Image courtesy of NASA.
110
Figure 5.2 Relationship between westerly wind speed during migration and annual survival of Yellow Warblers. Points are apparent survival (± SE) of young (1 yr, open points) and older (≥2 yrs, filled points) birds (7 years, n=279 individuals, 437 encounters). Solid lines and shading represent predicted survival (ϕ) ± 95% CI from the top model assuming an average southerly wind vector.
111
Figure 5.3 Predicted relationship between southerly wind speed during migration and annual survival of Yellow Warblers in Revelstoke, British Columbia (see also Fig. 1). Lines and shading represent predicted survival (ϕ) ± 95% CI for young (1 yr, dashed) and older (≥2 yrs, solid) birds from the top model assuming an average westerly wind vector (7 years, n=279 individuals, 437 encounters).
Fig. S1. Predicted relationship between southerly wind speed during migration and annual
survival of Yellow Warblers in Revelstoke, British Columbia (see also Fig. 1). Lines and
shading represent predicted survival (ϕ) ± 95% CI for young (1 yr, dashed) and older (≥2 yrs,
solid) birds from the top model assuming an average westerly wind vector (7 years, n=279
individuals, 437 encounters).
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Figure 5.4 Male arrival date (circles) and female clutch initiation date (diamonds) for Yellow Warblers as a function of westerly wind speed during migration. Points represent mean dates ± SE for young (1yr, open points) and older (≥2yrs, filled points) birds (6 years, n=338).
Fig. 3. Male arrival date (circles) and female clutch initiation date (diamonds) for Yellow
Warblers as a function of westerly wind speed during migration. Points represent mean dates ±
SE for young (1yr, open points) and older (≥2yrs, filled points) birds (6 years, n=338).
113
Figure 5.5 Within-individual changes in arrival date (males; n=49) and clutch initiation date (females; n=21) associated with inter-annual variation in average westerly wind vectors. Box-plots show the median change, 10th and 90th percentile, and outliers. Positive values represent delayed breeding phenology in stronger wind years; negative values represent advanced breeding phenology in stronger wind years. Data was restricted to individuals that were ≥ 2 years of age in both years. Males and females exhibited a median shift of +2 and +4 days respectively.
Fig. S2. Within-individual changes in arrival date (males; n=49) and clutch initiation date
(females; n=21) associated with inter-annual variation in average westerly wind vectors. Box-
plots show the median change, 10th
and 90th
percentile, and outliers. Positive values represent
delayed breeding phenology in stronger wind years, negative values represent advanced breeding
phenology in stronger wind years. Data was restricted to individuals that were ≥ 2 years of age
in both years. Males and females exhibited a median shift of +2 and +4 days respectively.
114
Chapter 6. Significance and Future Directions
General Conclusions
My research shows that Yellow Warblers wintering in the isthmus of Mexico
exhibit sex- but not age-segregation (Chapter 2). As in American Redstarts (Setophaga
ruticilla), male Yellow Warblers in the Pacific slope region predominate in limited, wetter
habitat (riparian corridors) while females represent a larger proportion of individuals
using drier, more abundant, shrub habitat (coastal scrub/seasonal saline marsh).
However, sex-segregation appeared to vary by region and was not evident in habitats
used by Yellow Warblers on the Gulf of Mexico. Regional differences, if real, may arise
because of regional rainfall patterns. Water limitation is a more common within the
tropical deciduous forests of the Pacific slope than in the wetter Gulf region. This may
produce steeper habitat gradients and possibly greater competition in western Mexico.
In both regions wintering Yellow Warblers incorporated local isotopic signatures into
their tissues. This was evidenced by significant differences in 13C and 15N between
blood samples collected within different habitat types and within different regions. As
isotope ratios will be incorporated into feather tissue during pre-alternate moult (which
occurs in Yellow Warblers on the wintering grounds (Pyle 1997, pers. observation),
winter-grown feathers collected on the breeding grounds can be used as a source of
cross-seasonal data.
It is important to note that the inference of winter habitat use from feather
isotopes involves several levels of uncertainty. First, in this thesis I assume that isotopic
differences in Yellow Warbler tissue reflect winter habitat quality differences. This
assumption is based on isotope-habitat relationships reported for other species of
wintering Parulidae, but has not been explicitly demonstrated in the Yellow Warbler.
115
Secondly, “ground-truthed” wintering isotope values in Chapter 3 were obtained from
blood samples as most birds captured in Mexico had not completed pre-alternate moult
at the time of capture. Blood tends to be 13C and 15N depleted relative to feathers due
to tissue-specific differences in isotope discrimination (Caut et al. 2009). For 15N, this
means that only relative values can be compared between tissue types. For 13C, blood
discrimination values also increase slightly as the diet becomes more 13C depleted
(Caut et al. 2009). Feathers do not show this shift in discrimination making comparisons
of relative 13C values between tissue types more difficult. Feather signatures should
more accurately reflect habitat-13C while 13C differences between habitats presented
in Figure 3.1 are likely to be slightly underestimated. Finally, carry-over models assume
a linear relationship between wintering isotope signatures and breeding variables. If
wintering habitat use is, in fact, a discrete variable (e.g. “high” versus “low quality”
habitat) the relationship between habitat, isotope values, and breeding variables may be
non-linear. Further work on the wintering grounds is needed to (a) quantify the
relationship between 13C, 15N and winter habitat quality for Yellow Warblers and (b)
determine species-specific discrimination values for feather and blood samples.
In Chapter 3 I showed evidence for a relationship between winter habitat use
(inferred using 13C and 15N in winter-grown feathers), breeding phenology, and
subsequent productivity among young (first-breeding season) female Yellow Warblers in
Revelstoke, British Columbia. Within this age-sex class, depleted 13C and enriched
15N was associated with earlier clutch initiation dates. This suggested that young
females wintering in wetter, and possibly more northern locations were able to breed
earlier the following spring. As the timing of clutch initiation was associated with higher
productivity, winter habitat use carried-over to impact young female fitness. No similar
effects were seen in other age-sex classes.
An equivalent analysis involving Yellow Warblers breeding in Inuvik, Northwest
Territories in Chapter 4 yielded less support for winter habitat carry-over effects. Here I
found no evidence that 13C or 15N was associated with timing effects in this
population. Support existed for a 15N effect on male condition: earlier clutch initiation
dates occurred among pairs where males had enriched 15N signatures. However, this
effect did not carry-over to influence productivity. Comparisons with the existing
116
American Redstart data (Norris et al. 2004) and with the Revelstoke data presented in
Chapter 3, indicated that Yellow Warblers breeding in Inuvik differ from southern
populations in both how they are influenced by winter habitat use and in how the timing
of breeding influences their productivity. In total, carry-over effects were weaker in this
extreme northern population.
Finally, in Chapter 5 I show strong evidence that wind conditions during the
spring migratory period play an important role in the timing of breeding events in
Revelstoke and in annual differences in apparent survival. This same analysis provided
only weak support for an influence of the wintering and breeding periods on this
population. This chapter, and Chapters 3-4 align with recent work (Wilson et al. 2011;
LaManna et al. 2012) that suggests a longitudinal difference exists in the contribution of
non-breeding conditions to events that occur on the breeding grounds. This is an
important avenue for future study. Below, I review the existing research, discuss
possible reasons why a longitudinal effect might be expected, and make predictions
about how the non-breeding period might impact migrants across North America.
Future directions
Non-breeding effects – are there longitudinal patterns?
The current paradigm for carry-over effects in Neotropical migrants is largely
built upon studies of eastern American Redstarts wintering in the Greater Antilles. Here
birds in higher quality wintering territories maintain their weight over the wintering period
and, in spring, depart earlier than their conspecifics (Marra et al. 1998; Marra and
Holmes 2001; Studds and Marra 2005). Similarly, increased food availability in high
rainfall years is associated with earlier departure schedules (Studds and Marra 2007;
Studds and Marra 2011). Early departing individuals subsequently arrive on the
breeding grounds earlier, where they obtain reproductive advantages relative to later-
arriving individuals (Norris et al. 2004; Reudink et al. 2009; Tonra et al. 2011; McKellar
2013). However, this pattern does not appear to be universal among passerines
breeding in North America (Figure 6.2, Table 6.2). A strong influence of winter
conditions on eastern breeding populations is supported by the majority of studies (8:1).
Results from the central region of the continent are more ambivalent (3:2) and effects
117
are more strongly supported for species that winter predominantly in the Caribbean
(González-Prieto 2012). West of the Rocky Mountains, the minority of studies found
support for wintering effects (1:3): two instead found strong evidence of a migratory
effect. As briefly discussed in Chapter 5, there are two reasons why this may be the
case: (1) eastern migrants likely face different movement costs than western migrants
and (2) migrants wintering on islands may face unique constraints relative to mainland
individuals.
It is likely that the majority of eastern birds cross ocean during migration, either
by island-hopping or by making the uninterrupted 18-40 hr. flight across the Gulf of
Mexico (Figure 1 A; Moore and Kerlinger 1987). Flights over water allow no opportunity
to rest or refuel: to make the crossing, individuals must be in good physical condition
and they must be able to obtain a threshold mass prior to departure (Alerstam 1979;
Schaub et al. 2008). Island birds cross open water immediately after departing their
winter territories. For these birds, on-territory fattening to a set threshold is likely
essential in order to initiate spring movement. Among eastern populations wintering in
Central and South America, the ocean barrier is encountered later in migration. For
these individuals, on-territory fattening may be less crucial for the initiation of migration,
but food availability at pre-barrier stopover sites (such as the Yucatan) may be
particularly important. Mainland individuals may also choose to follow an overland
(circum-Gulf) route to the breeding grounds (e.g. Stutchbury et al. 2009). This
behavioural strategy may reduce dependency on any one fuelling location by allowing
birds to make shorter flights. Evidence of lower fat loads carried by migrants at
southwestern (fall) stopover sites relative to stopover sites along the Gulf Coast of
Mexico would support this (Kelly and Hutto 2005).
Western birds are more likely to use exclusively overland routes (Figure 1A) and,
as for mainland eastern populations, on-territory fattening may not be as essential for
the initiation of spring movement. While geographical barriers exist in the west in the
form of the southwestern deserts, these barriers are not as extreme (birds may rest
even if they cannot refuel), they occur later in migration (northern Mexico), and are they
interspersed with ‘habitat islands’ such as riparian corridors that are heavily used
(Skagen et al. 2005a; Kelly and Hutto 2005). For a subset of western migrants,
however, our data indicates that there is a climatological barrier to spring movement.
118
For individuals moving west of the Sierra Nevada, wind patterns at the altitude of
migrant movement oppose north and westward movement (Figure 1B). This would
indicate that migration becomes more costly for western birds once they enter the
western United States. In contrast, conditions for birds in the central and eastern regions
of the US are less hostile with wind vectors facilitating a north-easterly movement
(Figure 1B; Appendix A; Gauthreaux 1980).
Taking these geographical factors into consideration, I would make several
predictions with respect to longitudinal patterns: (1) for eastern migrant populations
wintering in the Caribbean, winter habitat will play a strong role in the timing of
movement; (2) for eastern populations that winter in Central and South America,
stopover sites before water-crossings will be of greater importance than wintering
conditions for timing; (3) for migrants that breed to the west of the continental divide,
wind conditions in California will have a stronger influence than wintering conditions on
timing. Finally, (4) for both eastern and western populations wintering on the mainland,
survival may also be strongly influenced by conditions at stopover sites, which may
mask evidence of a wintering-ground effect. No such masking will occur in Caribbean
populations because wintering habitat and pre-barrier fuelling sites heavily overlap. By
the same token, individual-level carry-over effects in populations that winter on the
mainland may stem from access to foraging opportunities at stopover sites, rather than
on winter territories.
Further work is needed to test this interpretation. Our data suggests that
conditions on migration are correlated with the El Niño Southern Oscillation (ENSO) and
are more hostile during La Niña years. Two western studies where timing and
productivity shifts could be attributed to wintering conditions might also be interpreted
from a migration perspective, as both were correlated with ENSO in the direction we
would predict (Nott et al. 2002; Macmynowski et al. 2007). Evidence of a positive
relationship between previous-year spring temperatures and American Redstart
abundance in the US Northern Rockies Bird Conservation Region (BCR) (Wilson et al.
2011) may also be related to a migration effect. Higher temperatures in this region occur
during El Niño years. In this case, a connection with migration conditions is tentative
given the time lag in the effect and as similar patterns were not seen further north.
119
LaManna et al.’s (2012) work on Swainson’s Thrush (Catharus ustulatus)
breeding in the northwestern United States indicated that higher rainfall in the
southwestern portion of the spring flyway is linked to higher apparent annual survival.
This suggests that wind-speed may not be the only variable influencing populations
during migration. I found no support for a rainfall model with Yellow Warblers (Chapter
5). A post-hoc analysis using the regions used by LaManna et al. (2012) also was not
competitive with wind models (unpublished data). However, the impact of rain may be
species-specific or additive. Higher rainfall is associated with El Niño and therefore
reduced wind-speed years; this confound may mask rain effects in the Yellow Warbler
system.
An understanding of how non-breeding events alter breeding bird abundance,
survival and productivity is needed in order to make effective conservation decisions.
Without this knowledge the outcome of management practices on the breeding and
wintering grounds will be unpredictable and effort and money may be wasted. If
migration is the most costly period for western birds, stopover habitats in the
southwestern United States may play a significant role in maintaining western
populations. These habitats, limited and threatened by development (Skagen et al. 2005
a,b; van Riper et al. 2008), may need to be prioritized.
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Studds, Colin E, and Peter P Marra. 2007. Linking Fluctuations in Rainfall to Nonbreeding Season Performance in a Long-distance Migratory Bird, Setophaga ruticilla. Climate Research 35: 115–122. doi:10.3354/cr00718.
Studds, Colin E., and Peter P. Marra. 2005. Nonbreeding Habitat Occupancy and Population Processes: An Upgrade Experiment with a Migratory Bird. Ecology 86 (9): 2380–2385. doi:10.1890/04-1145.
———. 2011. Rainfall-induced Changes in Food Availability Modify the Spring Departure Programme of a Migratory Bird. Proceedings of the Royal Society B: Biological Sciences 278 (1723): 3437 –3443. doi:10.1098/rspb.2011.0332.
Stutchbury, Bridget J. M., Scott A. Tarof, Tyler Done, Elizabeth Gow, Patrick M. Kramer, John Tautin, James W. Fox, and Vsevolod Afanasyev. 2009. Tracking Long-Distance Songbird Migration by Using Geolocators. Science 323 (5916): 896–896. doi:10.1126/science.1166664.
Tonra, Christopher M., Peter P. Marra, and Rebecca L. Holberton. 2011. Migration Phenology and Winter Habitat Quality Are Related to Circulating Androgen in a Long-distance Migratory Bird. Journal of Avian Biology 42 (5): 397–404. doi:10.1111/j.1600-048X.2011.05333.x.
Wilson, Scott, Shannon L. Ladeau, Anders P. Tøttrup, and Peter P. Marra. 2011. Range-wide effects of breeding- and nonbreeding-season climate on the abundance of a Neotropical migrant songbird. Ecology 92 (9): 1789–1798.
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Tables and Figures
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Table 6.1. Overview of cross-seasonal studies involving Neotropical migrant passerines. Study number indicates study location in Figure 6.2.
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Table 6.1. Continued
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Table 6.1. Continued
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Figure 6.1 (A) The suggested migratory routes used by Neotropical migrants between temperate and tropical regions (Lincoln 1998) and (B) nighttime wind vectors encountered by migrants during the March-May period (10-year average (2003-2012)). Average vectors were calculated within regions bounded by a longitudinal and latitudinal grid (see Appendix A). Boxed regions indicate the approximate area for which vectors were calculated, but are unadjusted for projection. Arrow length corresponds to wind strength with the exception of (*) where length was halved relative to other vectors in the interest of space.
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Figure 6.2 The geographical distribution and main findings of cross-seasonal studies involving Neotropical migrant passerines. Evidence gathered from wintering isotopic signatures are summarized by age- and sex-class in (A), evidence from wintering climatological data is summarized in (B). Study details are presented in Table 6.1.
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Appendices
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Appendix A. Derivation of wind data presented in Figure 6.1
As also described in Chapter 5, wind on migration was derived from modeled climate data extracted from the National Center of Environmental Prediction (NCEP) Reanalysis 1 data archives at the NOAA-CIRES Climate Diagnostics Center at Boulder, Colorado, USA (NOAA NCEP 2012) using the RNCEP program (Kemp et al. 2012). This data has a spatial resolution of 2.5° latitude and longitude and temporal resolution of six hours. Vector data (U- and V-wind speed components (m/s)) were averaged from the 850mb (1500m) and 925mb (700m) level for the March-May period between 2003 and 2012. Noon values were dropped so that the series represented conditions encountered during nighttime migration (between 18:00h and 6:00h). Data was then averaged spatially for regions bounded by the longitudinal and latitudinal limits described below (Table 7.1).
Table 7.1. Region limits and night wind vectors (March-May averages, 2003-2012) presented in Figure 6.1(b)
Region Latitude °N Longitude °W U-wind (m/s) V-wind (m/s)
South America 5-15 72.5-57.5 -7.2 0.1
Southern Central America 5-15 92.5-75 -3.8 -1.1
Caribbean 17.5-27.5 92.5-72.5 -3.0 1.4
Mexico 17.5-27.5 115-95 0.4 0.6
Southeastern USA 30-40 90-67.5 4.2 1.0
South-central USA 30-40 112.5-93 2.0 2.4
Southwestern USA 30-40 125-115 2.6 -2.8
Northeastern USA 42.5-52.5 90-67.5 2.6 -0.4
North-central USA 42.5-52.5 112.5-93 1.5 0.2
Northwestern USA 42.5-52.5 125-115 2.0 1.1
Northwest Territories 55-65 135-112.5 0.6 1.1
Alaska/Yukon 55-65 160-137.5 -0.9 1.0
References:
Kemp, Michael U., E. Emiel van Loon, Judy Shamoun-Baranes, and Willem Bouten. 2012. RNCEP: Global Weather and Climate Data at Your Fingertips. Methods in Ecology and Evolution 3 (1): 65–70. doi:10.1111/j.2041-210X.2011.00138.x.
NOAA NCEP 2012. NCEP/NCAR Reanalysis 1. Earth System Research Laboratory, National Oceanic and Atmospheric Administration. www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.html.
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