EFFECTS OF LIVESTOCK GRAZING ON FORAGING ECOLOGY OF WESTERN

WOOD-PEWEES IN THE SOUTHERN SIERRA NEVADA MOUNTAINS

by

Alissa M. Fogg

A Thesis

Presented to

The Faculty of Humboldt State University

In Partial Fulfillment

Of the Requirements for the Degree

Master of Science

In Natural Resources: Wildlife

May 2009

EFFECTS OF LIVESTOCK GRAZING ON FORAGING ECOLOGY OF WESTERN

WOOD-PEWEES IN THE SOUTHERN SIERRA NEVADA MOUNTAINS

By

Alissa M. Fogg

Approved by the Master’s Thesis Committee:

T. Luke George, Major Professor Date

Matthew D. Johnson, Committee Member Date

Kathryn L. Purcell, Committee Member Date

Gary Hendrickson, Graduate Coordinator Date

Gary A. Hopper, Interim Dean Date

Research, Graduate Studies & International Programs

iii

ABSTRACT

Effects of Livestock Grazing on Foraging Ecology of Western Wood-Pewees in the

Southern Sierra Nevada Mountains

Alissa M. Fogg

Montane meadows represent one of the most critical habitats in the Sierra

Nevada Mountains for breeding birds. Livestock grazing in and around montane

meadows can produce deleterious ecological effects including changes in the structure

and complexity of the herbaceous and shrub layer, declining water tables and increased

tree densities. Western Wood-Pewees (Contopus sordidulus) breed in high densities

along edges of meadows and exhibit mixed, but mostly negative, responses to cattle

grazing in western riparian habitats. I observed the foraging ecology of pewees to

investigate the reasons why a species that forages and nests in the canopy may respond

negatively to cattle grazing. I estimated foraging attack rate, aerial arthropod abundance,

territory density and foraging habitat selection of breeding Western Wood-Pewees on the

edges of seven montane meadows during 2007 and 2008 and compared habitat variables

between grazed and ungrazed meadows to investigate structural differences. Foraging

rates (n = 144 observations) and aerial insect biomass (n = 27 insect traps) were similar

between meadow types but territory density was consistently higher in ungrazed

meadows. Western Wood-Pewees foraged from large, dead or dying trees and used areas

that were closer to the meadow edge and with lower canopy cover but higher variation in

iv

tree diameter at breast height than paired random sites. Grazed meadows had higher tree

densities and lower mean diameter at breast height than ungrazed meadows. Because

foraging rates and insect biomass did not differ between grazed and ungrazed meadows,

lower territory densities in grazed meadows likely indicate a lack of foraging habitat

rather than limitations related to prey abundance.

In songbird foraging studies, data on monochromatic males and females are

frequently pooled. While this may increase sample size, it can also obscure important

differences in foraging behavior and habitat use. To investigate intersexual variation in

foraging habitat use and behavior in relation to effects of livestock grazing, I compared

foraging rates, use of foraging substrates and behavior between male and female Western

Wood-Pewees. I located 42 nests during the study and compared nest locations with

foraging locations to examine whether females foraged closer to nests. I also examined

changes in foraging rates by males and females over the course of the breeding season.

Females foraged on average 2.83 ± 1.8 (n = 69) times per minute with elevated rates

during incubation while males foraged 1.09 ± 0.1 times per minute (n = 75) and rates did

not vary through the breeding season. Males perched higher in the canopy, flew longer

distances to capture prey and foraged from larger trees than females. Both sexes foraged

at similar distances from their nest while males tended to perch higher and foraged from

snags and sugar pines while females foraged lower in the understory from logs,

hardwoods and snags. Increased densities of small trees in forests surrounding grazed

meadows may make foraging in the understory less suitable for female pewees.

v

ACKNOWLEGEMENTS

I would like to sincerely thank my advisor, Luke George, for his advice, patience,

support and confidence to help make all the hard decisions. I would also like to thank my

committee members Matthew Johnson and Kathryn Purcell for their useful comments

and instruction. Kathryn provided generous support and loan of equipment and has been

my mentor, caring friend and an adept problem solver for many years now. I would like

to thank Steve Byrd at Southern California Edison for access to SCE land and for

providing me with maps and advice on where to find pewees. I would like to thank

everyone at Dinkey Creek Work Center, specifically all the members of the Fisher Crew,

for their friendship, advice, good humor and help with fieldwork. The PSW Fresno

office, specifically Doug Drynan, provided essential help with fieldwork, GIS and

administrative duties. Luke George and Rick Golightly gladly provided employment

during the school year. I am indebted to Timothy Meehan for use of the insect traps and

and his excellent guidance on applying TangleTrap. My labmate Greg Brown patiently

mentored me through my first year of graduate school and honorary labmates, Lisa

Eigner, Kyle Spragens and Mike Cunha, provided advice and support through regular

meetings. Lastly I would like to thank my parents, John and Nancy, who gave me

endless encouragement to continue school and my husband, Chad, for putting his career

on hold and supporting me as I pursued my dream to be a full-fledged wildlife biologist.

Funding for this project was provided by the Sierra Nevada Research Center, a Richard

Guadagno Scholarship, a Hegy-Woolford grant and a Wildlife Graduate Students Society

grant.

vi

TABLE OF CONTENTS

Page

ABSTRACT……………………………………………………………………………...iii

ACKNOWLEDGEMENTS……………………………………………………………….v

LIST OF TABLES……………………………………………………………………...viii

LIST OF FIGURES………………………………………………………………………x

CHAPTER ONE: EFFECTS OF LIVESTOCK GRAZING ON FORAGING ECOLOGY

OF WESTERN WOOD-PEWEES

Introduction……………………………………………………………………..1

Methods…………………………………………………………………………6

Study Area…………………………………………………………….6

Foraging Observations………………………………………………..7

Aerial Arthropod Sampling…………..……………………………….9

Territory Mapping…...…………………………………………..…..10

Foraging Habitat Selection…………………………………………..11

Statistical Analyses…………………………………………………..13

Results…………………………………………………………………………16

Discussion……………………………………………………………………...19

Literature Cited………………………………………………………………...24

vii

TABLE OF CONTENTS (continued)

CHAPTER TWO: INTERSEXUAL DIFFERENCES IN THE FORAGING ECOLOGY

OF WESTERN WOOD-PEWEES

Introduction……………………………………………………………………42

Methods………………………………………………………………………..46

Study Area…………………………………………………………...46

Foraging Observations………………………………………………46

Statistical Analyses…………………………………………………..49

Results…………………………………………………………………………52

Discussion……………………………………………………………………...55

Literature Cited………………………………………………………………...60

viii

LIST OF TABLES

Table Page

Table 1. Number of kite traps and mean (SE) aerial arthropod biomass (mg/2m2/21days)

captured by taxonomic order, year and grazing status. Traps were located on edges of

montane meadows in the southern Sierra Nevada Mountains (Fresno County, California,

USA) during June-July 2007 and 2008...………………………….…….………….……36

Table 2. Effects of livestock grazing on Western Wood-Pewee territory density in

montane meadows. Meadows were located in the southern Sierra Nevada Mountains

(Fresno County, California, USA) and surveyed during the 2007 and 2008 breeding

seasons………...…………………………………………………………………………37

Table 3. Mean (SE) patch-scale vegetation variables measured on Western Wood-pewee

territories in 4 grazed and 3 ungrazed montane meadows in the southern Sierra Nevada

Mountains (Fresno County, California) during summer 2008……………..…………….38

Table 4. Summary of tree-scale model selection results of paired logistic regression

analysis comparing substrate characteristics of Western Wood-Pewee foraging locations

and random locations around montane meadows in the southern Sierra Nevada

Mountains (Fresno County, California, USA) during the 2008 breeding season…….….39

Table 5. Means (SE) of tree (n = 85) and patch (n = 67) scale variables measured at

Western Wood-pewee foraging locations and paired available locations on montane

meadow edges in the southern Sierra Nevada Mountains (Fresno County, California,

USA) during the 2008 breeding season……..…………………..……………………….40

Table 6. Summary of patch-scale model selection results of paired logistic regression

analysis comparing substrate characteristics of Western Wood-Pewee foraging locations

and random locations around montane meadows in the southern Sierra Nevada

Mountains (Fresno County, California, USA) during the 2008 breeding season….….…41

Table 7. Comparison of foraging behavior and perch site characteristics of male (n = 77)

and female (n = 75) Western Wood-pewees foraging in montane meadows in the

southern Sierra Nevada during the 2007 and 2008 breeding season…………………….71

ix

LIST OF TABLES (continued)

Table 8. Foraging tree species selection of male and female Western Wood-Pewees

breeding in montane meadows in the southern Sierra Nevada Mountains (Fresno County,

California, USA) during summer 2007 and 2008. Lodgepole and ponderosa pines were

combined into one category (Pines). Number and proportion of used and available trees

are shown and 95% confidence intervals around proportion used (pi)………….…...…..72

x

LIST OF FIGURES

Figure Page

Figure 1. Location of seven meadows in the Sierra Nevada Mountains where the study

was conducted during 2007 and 2008 (Fresno County, California, USA). Ely, Sulphur,

Stevenson, Lost and Bear Meadows were located on Southern California Edison land.

Markwood and Swanson Meadows were on US Forest Service land. Ely, Sulphur and

Stevenson Meadows were excluded from livestock grazing since 1985; Swanson, Lost

and Bear Meadows have been continuously grazed and Markwood Meadow has been

rested from grazing since 2006…………………………………………………………….35

Figure 2. Foraging attack rate (mean ± SE) according to nesting stage of male and

female Western Wood-Pewees breeding around montane meadows in the southern Sierra

Nevada Mountains (Fresno County, California, USA) during summer 2007 and 2008....67

Figure 3. Intersexual differences in foraging height in relation to substrate height for

Western Wood-Pewees breeding around montane meadows in the southern Sierra Nevada

Mountains (Fresno County, California, USA) during summer 2007 and 2008…..……...68

Figure 4. Distance (mean ± SE) from foraging point to nest location according to nest

stage for male and female Western Wood-Pewees breeding on edges of montane

meadows in the southern Sierra Nevada Mountains (Fresno County, California) during

summer 2007 and 2008…………………………………………………………………..69

Figure 5. Nest height (m) in relation to foraging height (m) for male and female Western

Wood-Pewees breeding on edges of montane meadows in the southern Sierra Nevada

Mountains (Fresno County, California, USA) during summer 2007 and

2008………………………………………………………………………………………70

1

CHAPTER ONE: EFFECTS LIVESTOCK GRAZING ON THE FORAGING

ECOLOGY OF WESTERN WOOD-PEWEES

INTRODUCTION

Montane meadows throughout the Sierra Nevada Mountains are vitally important to

many wildlife species and represent one of the most critical habitats in the region for

breeding birds (DeSante 1995, Graber 1996, Siegel and DeSante 1999). In addition to

the species that nest in montane meadows, many songbird species breed in higher

densities around meadows and meadows serve as important staging areas for fall

migrants (DeSante 1995). Bird communities in montane meadows are threatened by a

variety of activities including livestock grazing, road building, dam building, exotic

plants and recreation (Kattelmann and Embury 1996). In particular, livestock grazing

may cause changes in plant and animal species composition, disruptions of ecosystem

functions and alteration of ecosystem structure (Fleischner 1994, Ohmart 1994) and has

been identified as one of the biggest threats to the productivity and persistence of birds in

the Sierra Nevada Mountains (DeSante 1995).

Forests and meadows of the Sierra Nevada Mountains (hereafter Sierra) have

been grazed by sheep and, more recently, cattle since the early part of the twentieth

century (Beesley 1996). Long term effects from livestock grazing in meadows have

resulted in declining water tables, changes in species diversity and increases in

percentage bare soil with corresponding decreases in herbaceous cover (Menke et al.

1996). In forests surrounding meadows, grazing has generally changed the structure and

2

complexity of the herbaceous, shrub and tree layer (Dobkin 1994). Grazing can also

reduce the biomass of understory grasses and sedges thereby preventing the spread of

low-intensity fire and encouraging conifer seedling establishment (Belsky and

Blumenthal 1997, Saab and Powell 2005). In much of the western United States,

livestock grazing, in combination with fire suppression, has resulted in forests with a high

density of small trees relative to pre-European settlement (Saab et al. 1995, Belsky and

Blumenthal 1997, Finch et al. 1997). Although these changes may be apparent

throughout western forests, cattle tend to congregate around meadows perhaps because of

the availability of water, gentle slopes, and quality of forage and consequently may

accentuate these changes in areas adjacent to meadows (Roath and Krueger 1982,

Kauffman and Krueger 1984).

Cattle grazing can adversely affect songbird species that forage and nest in the

shrub or herbaceous layer by decreasing height, density and complexity of herbaceous

growth (DeSante 1995, Saab et al. 1995, Ammon and Stacey 1997) and may increase nest

predation rates (Ammon and Stacey 1997). Grazing has also been linked to higher rates

of brood parasitism (Verner and Ritter 1983). Shrub nesting species, such as the Yellow

Warbler (Dendroica petechia), have shown marked increases following cattle removal in

riparian areas (Taylor and Littlefield 1986, Krueper et al. 2003). It is perhaps to be

expected that ground or shrub nesting species would be adversely affected by livestock

grazing because of direct impacts on availability of nesting sites. Surprisingly, some

species that nest or forage in the canopy in riparian areas have also increased dramatically

following cattle removal or occurred in higher densities in areas from which cattle were

3

excluded for several years (Tewksbury et al. 2002, Krueper et al. 2003, Earnst et al.

2005). The reasons for increases of bird species that nest and forage in the canopy in the

absence of livestock grazing are less obvious but may be due to changes in prey

availability or availability of foraging or nesting sites.

Western Wood-Pewees (Contopus sordidulus – hereafter “pewees”) are

neotropical migrant songbirds (Family Tyrannidae) that typically breed in open canopy

forests, riparian areas and forests that have been burned or mechanically thinned. They

occupy a variety of forested habitats but appear to require exposed perches in the

understory and upper canopy (Bemis and Rising 1999). Pewees are aerial foragers,

sallying from exposed perches to catch insects on the wing. High densities of pewees

occur along the edges of mid-elevation montane meadows in the southern Sierra Nevada

Mountains and in associated riparian habitats (Siegel and Wilkerson 2005). Breeding

bird survey trends indicate that pewees are declining in the Sierra Nevada Mountains (P =

0.02) and a few other areas, but not consistently throughout their range (Sauer et al.

2008). Possible causes affecting the decline of pewees and other migratory Contopus

flycatchers may include habitat alteration and destruction of their breeding and wintering

grounds and loss or alteration of staging areas during migration (Altman and Sallabanks

2000). Pewees are likely affected by these changes but the patchiness of their range-wide

decline suggests additional factors on their breeding grounds.

Avian researchers studying livestock grazing in western riparian habitats have

reported mixed responses from pewees. In several river systems across their range,

pewee densities were lower in grazed areas (Tewksbury et al. 2002). Pewee densities

4

dramatically increased following cattle removal in a riparian river system that may have

been related to increases in prey densities in lower vegetation layers (Krueper et al.

2003). Pewees were also entirely absent from grazed aspen woodlands in parts of the

Great Basin but were present on nearby ungrazed plots (Page et al. 1978). Aerial

insectivores, including Western Wood-Pewees, increased following cattle removal from

riparian woodlands in southeastern Oregon (Earnst et al. 2005). In contrast, pewees

showed no response to cattle grazing in cottonwood riparian areas (Saab 1998) or had

increased densities in a heavily grazed plot vs. a lightly grazed plot although densities in

ungrazed areas were not measured (Mosconi and Hutto 1982). Thus, pewee response to

livestock grazing may depend on the location and intensity of livestock grazing.

Western Wood-pewees depend on a constant supply of invertebrate prey,

specifically flying insects, during the breeding season. Pewees forage and nest in the

forest canopy and understory and generally do not use the herbaceous layer in forests and

meadows. It has been hypothesized that removal of the herbaceous layer and changes in

vegetation complexity and density due to livestock grazing may result in reduced food

resources for canopy foragers (DeSante 1995, Siegel and DeSante 1999, Krueper et al.

2003, Earnst et al. 2005). The potential relationship between livestock removal and

increased prey availability has not been investigated and deserves attention. It is

unknown whether canopy foragers benefit from cattle exclusion due to increased prey

densities, changes in foraging and nesting habitat or both.

The goal of this project was to study patterns of food abundance, foraging rates

and foraging habitat selection of Western Wood-pewees in montane meadows under

5

different livestock grazing practices. My objectives were to (1) determine if ungrazed

meadows provide a more abundant or higher quality food source than grazed meadows,

(2) document differences in territory densities between grazed and ungrazed meadows

and (3) examine foraging habitat selection of male and female pewees on grazed and

ungrazed meadows. I hypothesized that pewees would respond negatively to presence of

livestock and predicted that foraging rates, aerial insect biomass and territory densities

would be higher in ungrazed meadows than actively grazed meadows. I examined third-

order within-home range foraging habitat selection (Johnson 1980) on both tree and patch

scales to investigate whether structural and floristic changes due to cattle grazing could

affect pewees’ positive response to cattle removal. I predicted that pewees would avoid

attributes associated with cattle grazing such as high canopy cover and low diversity in

tree sizes (Saab et al. 1995, Finch et al. 1997).

6

METHODS

STUDY AREA

I conducted my study in wet montane meadows on the western slope of the southern

Sierra Nevada Mountains in Fresno County (elevational range: 1682 m – 1828 m; Figure

1). Seven meadows were included; 3 grazed (Swanson, Lost and Bear Meadows), and 4

ungrazed (Markwood Meadow was rested from grazing in 2006; Ely, Sulphur and

Stevenson Meadows have been ungrazed since 1985). Meadows averaged 15 ± 10 ha

(range 4 – 29 ha). Markwood Meadow was located on Sierra National Forest land and

has been unaffected by fire and widespread logging for at least the past 30 years. Five

other meadows (Ely, Sulphur, Stevenson, Lost and Bear Meadows) were located on land

owned by Southern California Edison. Both Sierra National Forest and Southern

California Edison have multiple-use management goals including recreation,

woodcutting, livestock grazing, wildlife conservation, fuels reduction and restoring

forests to historic conditions using prescribed fire and silviculture techniques (U.S. Forest

Service 1991, Southern California Edison 2001). Swanson Meadow has split ownership

between Sierra National Forest and Southern California Edison with a barbed-wire fence

denoting the boundary. Pewee observations were focused on the Sierra National Forest

portion of the meadow. Ely, Sulphur and Stevenson Meadows and the forests extensively

surrounding these meadows have been fenced and excluded from grazing since 1985. In

addition, the forest surrounding Sulphur Meadow has experienced several prescribed

burns in the past 20 years. The forest surrounding Lost and Bear Meadows have

experienced some spring prescribed burn but were not as intensively managed as Sulphur

7

Meadow. Cattle were the only domestic livestock using grazed meadows (Lost, Bear and

Swanson Meadows) and were present daily during my study with groups averaging 10 -

20 cow-calf pairs (pers. obs.). Cattle were moved into the allotments in early June and

were removed by late September (High Sierra Ranger District 2007, personal

communication, P.O. Box 559, Prather, CA, 93651).

Montane hardwood-conifer and Sierra mixed-conifer forests surrounded the

meadows and common tree species included: ponderosa pine (Pinus ponderosa),

California black oak (Quercus kelloggii), incense cedar (Calocedrus decurrens), white fir

(Abies concolor), sugar pine (Pinus lambertiana) and lodgepole pine (Pinus contorta)

(Mayer and Laudenslayer 1988). The shrub layer was dominated by greenleaf manzanita

(Arctostaphylos patula), whitethorn ceanothus (Ceanothus cordulatus), snowberry

(Symphoricarpos acutus), and Sierra gooseberry (Ribes roezlii). Riparian tree species,

including willow (Salix sp.), red alder (Alnus rubra) and quaking aspen (Populus

tremuloides), occurred around meadow edges and in some areas, throughout the meadow.

The meadows themselves were characterized by a high diversity of herbaceous cover

including forbs, grasses, rushes (Juncus sp.) and sedges (Carex sp.).

FORAGING OBSERVATIONS

Western Wood-pewees were observed between mid-May and early August in both 2007

and 2008. I surveyed Sulphur, Ely, Markwood and parts of Swanson and Lost Meadows

in 2007. I extended my surveys in 2008 to include all of Lost and Swanson Meadows

and Stevenson and Bear Meadows. The majority of foraging observations of pewees

were obtained in 2008 (62%); I obtained only six foraging observation on grazed

8

meadows in 2007. I recorded foraging behavior between sunrise and sunset

(approximately 0600 and 2000). Only one observation was conducted on a single

individual per day and at least 4 days passed before a territory was visited again. The

order in which sites were visited and direction walked during visits to each meadow were

rotated to reduce potential bias.

I quantified foraging rate using foraging attack rate (Hutto 1990, Lovette and

Holmes 1995, Kilgo 2005, Lyons 2005). I observed an individual as it foraged and

recorded each attack and non-foraging behavior on a portable digital recorder. An attack

was defined as a sally toward an aerial prey item, whether it was successful or not. I

rarely observed what the bird captured and assumed that pewees swallowed insects

immediately. An observation ended when I lost the bird from sight for more than 5

minutes or after 20 minutes. To calculate attack rate, I summed the number of attacks

during the observation, and divided by total time spent foraging. Time spent in non-

foraging behaviors, including preening, prey handling, nest building, feeding nestlings,

and interacting with other birds or predators were subtracted from the observation period.

To reduce potential biases due to observability (Morrison 1984), I used the

location of the second foraging sally for all habitat measurements even if the bird

returned to the same perch. The location of the tree where the foraging sally originated

was recorded using a handheld global positioning system (GPS) device and the tree and

the site of the initial prey attack were flagged to ensure that observations were not

collected from the location in subsequent visits. To avoid sampling the same individual, I

walked at least 150 m (the average radius pewee territory, K. Purcell, unpublished data)

9

before I obtained another foraging observation. The only exception was if I positively

identified the sex of the individual and observed an individual of the opposite sex.

AERIAL ARTHROPOD SAMPLING

Kite traps (Modified Malaise traps; Meehan 2002) were used to sample arthropods

available to Western Wood-Pewees. Kite traps effectively sample flying arthropods

including many of the insect orders, such as Coleoptera, Diptera, Hymenoptera and adult

Lepidoptera, that pewees and other Contopus flycatchers consume (Beal 1912, Beaver

and Baldwin 1975, Otvos and Stark 1985, Meehan 2002, Meehan and George 2003).

Kite traps consisted of four, 0.5 x 1.0 m, mesh interception panels (9 x 8 mesh/cm)

radiating at 90 degree angles from the central vertical seam. Attached horizontally to the

top and bottom of each mesh interception panel was a 0.7 x 0.7 m square of clear, 6 mil

plastic, coated on the upper side (bottom sheet) or lower side (upper sheet) with Tangle-

Trap adhesive (BioQuip Products, Gardena, California). Arthropods were captured in the

adhesive applied to the top and bottom panels.

I placed traps at 2-4 randomly chosen sites per meadow that were within 50 m

of identified foraging locations. Traps were hung 5-10 m high from trees along the edge

of the meadow at least 300 m apart and 100 m from active pewee nests. I generally hung

at least one trap near perennial water (stream or pond) and at least one trap away from a

running water source. I hung traps for 21 days during late June and early July (date

range: 24 June – 22 July) alternating trap placement at grazed and ungrazed meadows.

This time corresponded with late-incubation and nestling period for locally breeding

pewees (K. Purcell, unpublished data). Due to changes in grazing status and time

10

constraints, grazed meadows only had 2 traps in 2007 and 6 traps in 2008 while ungrazed

meadows had 11 traps in 2007 and 8 traps in 2008. After kite traps were taken down, I

covered the plastic sheets with plastic wrap and stored them in a refrigerator or freezer

until they were inspected the following winter.

To assess arthropod species and biomass, insects were identified to taxonomic

order and measured to the nearest millimeter using a stereo dissecting microscope and a

ruler. To calculate arthropod biomass, length was used to estimate weight employing the

mass versus weight regression of Rogers et al. (1976). I identified and measured insects

larger than 3 mm because pewees are found to primarily prey on insects larger than this

(mean insect length = 7.5 mm; Beaver and Baldwin 1975). Arthropod biomass was

expressed as mg/2m2/21days.

TERRITORY MAPPING

I mapped each territory that included either a mated pair or an unmated male along the

perimeter of each meadow to obtain a measure of territory density (Verner 1985).

Territories were mapped on Ely, Sulphur and Markwood Meadows in both 2007 and

2008. Lost, Bear, Swanson and Stevenson Meadows were mapped only during 2008. I

visited each meadow an average of 12 times and walked the length of it during each visit

while also recording foraging observations. Larger meadows (Markwood, Lost,

Swanson) were visited more frequently to ensure I covered the entire meadow edge at the

same rate in which I visited the smaller meadows. During each visit, I used meadow

maps to record the location of singing males, territorial displays between neighboring

males and active nests to distinguish territory boundaries. In addition, foraging

11

observations aided in revealing which nest pewees were attending in areas of dense

territories.

To determine territory density, I used aerial photos and ArcGIS 9.1 to digitize

meadow edges as a polygon layer and calculated the perimeter of the polygon for

Sulphur, Stevenson and Swanson Meadows. I used the Sierra Nevada Montane Meadow

Vegetation polygon coverage (U.S. Forest Service 2001) to calculate perimeter length for

Markwood, Lost, Bear and Ely Meadows. Territory density was expressed as pewee

pairs per kilometer of meadow length (pairs/km).

FORAGING HABITAT SELECTION

I used a hierarchical approach to measure foraging habitat selection (Orians and

Wittenberger 1991). The trees pewees foraged from may have been less important than

the space where they catch their prey, and the structural and vegetation characteristics

surrounding that air space (Brandy 2001). To resolve this difference in foraging perch vs.

site of insect capture, I measured habitat attributes at two different scales. The first scale,

which I term the tree scale, focused on the perch tree and included variables used by

Brandy (2001) to describe foraging site characteristics of the Olive-sided Flycatcher

(Contopus cooperi) which forages in a very similar manner to Western Wood-Pewees. I

recorded tree species, diameter breast height (dbh), and tree height using a digital

hypsometer and estimated percentage of the foliage that was living.

The site of initial prey capture was the center of a larger plot where I assessed

patch-scale characteristics. Plots were circular with a radius of 11.3 m (0.04 ha) with two

perpendicular transects radiating out in four cardinal directions. Within the 0.04 ha

12

circle, I counted and measured the dbh of all stems by species (including snags) more

than 10 cm in dbh. I also counted all saplings less than 10 cm dbh and more than 1.3 m

in height and identified them to species. I used a point-intercept system at 0.5 m intervals

along the north, south, east and west radii for a total of 80 points and calculated

percentage cover as the proportion of the 80 points where shrub or herbaceous cover was

present. I measured distance from plot center to meadow edge and distance to pond or

running water with a laser rangefinder. I recorded edge type as a maximum of two of the

following: timber/meadow, timber/understory, timber/road or timber/creek. I measured

canopy cover above 3 m using a Moosehorn cover scope (Cook et al. 1995) by taking

readings at 5 and 10 m intervals along each radius and in the center of the plot. I

averaged these 9 measurements to determine mean plot canopy cover. Only one patch-

scale plot was measured per male or female on each territory. Patch-scale measurements

were not done at occupied locations if plots of two individuals overlapped.

I measured the same variables on paired random sites to assess pewee habitat

selection within their home range (Jones 2001). From the center of each foraging site, I

chose a random distance (25-75 m) and azimuth (0-360 degrees) and used a laser

rangefinder to locate the random plot. The random plot had to be within 60 m of the

meadow edge, include a foraging substrate more than 1.3 m tall within 10 m of plot

center and fall within either of the two identified edge types of the used plot. If these

assumptions were not met, I chose another azimuth. Tree scale measurements were taken

on the foraging substrate closest to the center of the random plot.

13

STATISTICAL ANALYSES

I examined the effect of grazing, year and sex on foraging attack rate (attacks/min.) using

factorial analysis of variance (ANOVA) and included all two-way interactions. I

examined the effect of grazing and year on total insect biomass (mg/2m2/21days), and

biomass of eight different taxonomic orders using two-way ANOVA. Taxonomic orders

included insects that pewees foraged on in forested habitats in the southern Rocky

Mountains (Beaver and Baldwin 1975): Coleoptera, Diptera, Hemiptera, Homoptera,

Hymenoptera, Lepidoptera, Neuroptera and Trichoptera.

I used two-way ANOVA to examine the effect of livestock grazing, year and

their interaction on territory density (number of territories per kilometer of meadow

edge). Because cattle were removed from Markwood Meadow 21 years after the other

ungrazed meadows, I analyzed it separately to account for long-term vs. short-term

effects of cattle removal (Krueper et al. 2003). I grouped territory densities using 3

levels: grazed, ungrazed and Markwood Meadow. Post-hoc Tukey multiple comparisons

tests were used to determine significant differences in territory densities between the 3

levels. To investigate structural differences between meadows, I used Wilcox rank sum

tests to compare patch-scale foraging habitat selection variables between grazed and

ungrazed meadows. Markwood Meadow was included as a grazed meadow for this

analysis of habitat variables because structural changes in the canopy and understory,

such as tree density, canopy cover and shrub cover, tend to occur slowly after livestock

removal (Kattelmann and Embury 1996, Krueper et al. 2003).

14

I conducted a 1-1 matched pairs logistic regression analysis (Hosmer and

Lemeshow 1989) to examine differences between used foraging locations and paired

available locations for both tree- and patch-scales using PROC LOGISTIC in SAS

statistical software (SAS Institute 1999). I included 3 variables in the tree-scale analysis:

tree height, tree dbh, and percentage live foliage. Tree height and tree dbh were highly

correlated (rs = 0.89) and subsequently not included in the same candidate models. Ten

variables were included in the patch-scale analysis: edge distance, canopy cover, shrub

cover, herbaceous cover, tree density, sapling density, hardwood density, snag density,

mean dbh and variance in dbh. Tree species and snags included in density measurements

were > 10 cm dbh.

Male and female pewees appeared to use foraging habitat differently (see

Chapter 2). Before building habitat models, I calculated the difference between used and

available habitat measurements and used MANOVA to examine the effect of sex on the

differences between used and available habitat measurements for both tree- and patch-

scale variables. If the effect of sex was not significant, I combined data on both sexes

and used a model selection approach to compare competing models for tree- and patch-

scale habitat selection (Burnham and Anderson 2002). I selected 5 tree-scale and 11

patch-scale candidate models. I chose a priori models based on results of habitat

selection studies on pewees and other Contopus flycatchers and personal observation. I

kept the models simple by limiting tree-scale models to a maximum of two variables and

patch-scale models to a maximum of four variables except for the global model. To

determine the best model, I examined Aikaike’s information criterion corrected for small

15

sample sizes (AICc ) and AICc weights (wi). Models with values of ∆AICc ≤ 2.0 were

considered competitive. If model selection statistics indicated that one model was not

strongly supported over all other models (wi > 0.90), I used a multi-model approach to

estimate coefficients and unbiased standard errors (Burnham and Anderson 2002). I used

odds ratios and maximum likelihood estimates of the coefficients to interpret how tree-

and patch-scale habitat variables influenced pewee habitat selection. All statistical tests

were conducted at a 0.05 level of significance (α < 0.05) and results are reported as mean

± SE. Parametric tests were only used when data was distributed normally. Data

analysis was completed using R (R Development Core Team 2008) unless otherwise

noted.

16

RESULTS

I obtained 144 foraging observations over the two years (55 in 2007 and 89 in 2008).

Foraging rates varied between sexes (F1, 140 = 92.6, P < 0.0001) but not between years

(F1, 140 = 2.1, P = 0.15) or by grazing status (F1, 140 = 0.1, P = 0.73). Females foraged an

average of 2.83 ± 1.77 (n = 69) times per minute while males foraged an average of 1.09

± 0.057 times per minute (n = 75).

Kite traps sampled 5992 aerial arthropods representing 16 orders. Total aerial

arthropod biomass averaged 938 ± 484 mg/2m2/21days at grazed sites (n = 2) and 1140 ±

291 mg/2m2/21days at ungrazed sites (n = 11) in 2007, and 469 ± 123 mg/2m

2/21days at

grazed sites (n = 6) and 467 ± 100 mg/2m2/21days at ungrazed sites (n = 8) in 2008

(Table 1). Two-way ANOVA indicated that total aerial arthropod biomass was similar

between grazed and ungrazed meadows (F1,23 = 0.05, P = 0.83) but was higher in 2007

than 2008 (F1,23 = 5.9, P = 0.02). Dipteran, hemipteran, hymenopteran and lepidopteran

biomass was significantly higher in 2007 than 2008 (Table 1).

Territory densities varied between different meadow types (F1,6 = 11.2, P =

0.02; Table 2) but not between years (F1,6 = 0.04, P = 0.84). Mean territory density in

ungrazed meadows (4.4 ± 0.3 pairs/km) was higher than in Markwood Meadow (2.1 ±

0.1 pairs/km) and grazed meadows (2.1 ± 0.1 pairs/km; Tukey post-hoc tests: all p-values

< 0.05). Sulphur Meadow had the highest density of 5.0 pairs/km in 2008 while Swanson

Meadow had the lowest density with 1.4 pairs/km in 2008. Wilcox rank sum tests

indicated that pewee territories around grazed meadows had higher tree densities (W =

1651, P = 0.03; Table 3) and lower mean dbh of trees (W = 2685, P = 0.01).

17

A total of 84 pairs of used and available tree-scale foraging observations were

completed during 2008. Even though male and female pewees foraged in different

vertical strata and chose different sized trees (see Chapter 2), the used tree and patch

observations for males and females were more similar than available tree (F1.62 = 2.6, P =

0.06) or patch observations (F1.62 = 0.8, P = 0.61). Sample sizes were reduced for these

tests because not all variables could be measured at each observation (e.g.,tree dbh could

not be measured at log observations). Thus I combined male and female observations in

the model selection procedure. AICc values and Aikaike weights indicated that tree dbh

and percentage live foliage were the variables included in the top model (wi = 0.52; Table

4). However, the second model was equally competitive (wi = 0.48) and included tree

height and percentage live foliage indicating that all three variables were important for

pewee tree-scale habitat selection (Table 5). Maximum likelihood and odds ratio

estimates suggested that pewees were more likely to forage from trees with a greater dbh

than locally available (β = 0.036, odds ratio = 1.037, 95% CI = 1.016 – 1.057) and

preferred trees that had more dead branches and less live foliage (β = -0.0317, odds ratio

= 0.969, 95% CI = 0.945-0.993). Pewees also selected taller trees (β = 0.104, odds ratio

= 1.109, 95% CI = 1.046 – 1.176).

Patch-scale habitat measurements were completed at 67 pairs of used and

available foraging points in 2008. AICc weights and ∆AICc values indicated that the top

model differentiating used from available locations included distance to meadow edge,

dbh variance and mean canopy cover (wi = 0.99; Table 6). Coefficients indicated that

pewees selected foraging patches closer to the meadow edge (β = -0.017, odds ratio =

18

0.931, 95% CI = 0.884 – 0.981; Table 5), with a higher variance in tree dbh (β = 0.005,

odds ratio = 1.005, 95% CI = 1.002 – 1.008) and lower canopy cover (β = -0.073, odds

ratio = 0.930, 95% CI = 0.885 – 0.977) than locally available.

19

DISCUSSION

My primary objective was to examine whether differences in densities of Western Wood-

Pewees between grazed and ungrazed meadows could be explained by effects of grazing

on food availability or their foraging ecology. Although pewee density was significantly

higher along the edges of ungrazed than grazed meadows, I found no difference in

foraging rate or aerial insect biomass between grazed and ungrazed meadows. Thus,

cattle grazing does not appear to limit food available to pewees nor does it appear to

affect their foraging rate in mid-elevation southern Sierra montane meadows. While

foraging rate can be an accurate measurement of foraging habitat quality (Meehan and

George 2003, Lyons 2005), conclusions relating to the effects of cattle grazing on aerial

insect biomass should be made with reservations because of the variation associated with

insect sampling (Hutto 1990, Smith and Rotemberry 1990). The number of traps and the

duration of sampling were limited in this study and insect populations can fluctuate

widely through time (Blancher and Robertson 1987). Variation among individual

meadows, such as changes in elevation, local climate, duration of flooding and plant

composition, was reflected in large standard errors for insect biomass. However, factors

affecting arthropod productivity in montane meadows are likely quite different from

those in other ecosystems and thus conclusions from this study may not be applicable to

other areas that pewees inhabit. For instance, herbaceous cover increased dramatically

following cattle removal in an arid riparian environment (Krueper et al. 2003), which

could lead to increases in food availability to aerial foraging insectivores (Meehan and

George 2003). In my study, herbaceous cover did not differ between grazed and

20

ungrazed meadows but cattle grazing reduced the height of the herbaceous layer in some

parts of the meadows (personal observation). The effects of grazing in mid-elevation

meadows and surrounding forests may be less pronounced than in lower elevation arid

riparian areas and could depend on grazing intensity. Increased sampling and further

study could reveal important differences in insect communities.

Even though insect biomass and foraging rates were similar between meadow

types, ungrazed meadows had consistently higher pewee densities than grazed meadows.

One possible explanation for these results is that pewees are limited by suitable foraging

habitat surrounding meadow edges because of the long-term synergistic effects of cattle

grazing and fire suppression. Forests surrounding grazed meadows had smaller trees and

higher tree densities than forests surrounding ungrazed meadows. Livestock grazing has

been shown to increase tree densities and result in thick stands of small-diameter trees

(Belsky and Blumenthal 1997, Finch et al. 1997). Pewees foraged in areas with lower

canopy cover and higher variation in tree dbh than available areas within their territory.

High tree density is often associated with high canopy cover and, combined with smaller

tree sizes, could mean reduced openings in the understory and canopy for pewees to

forage in. Pewees also selected large, dying trees to forage from. Because forests

surrounding grazed meadows contained trees of smaller dbh than ungrazed meadows,

pewee territory density may be lower simply because there are not enough large, dying

trees that are suitable to forage from.

Female pewees foraged lower in the canopy than males (see Chapter 2) and

therefore may be disproportionately affected by higher tree densities. High tree densities

21

would likely reduce openings within the forest thereby limiting foraging sites for female

pewees. Nearly half of female foraging observations took place in the meadow rather

than in the forested edges (females = 40%, males = 19%) suggesting that females may

move out of forest edges to forage in more open locations within meadows. In grazed

meadows, 43% of female and 24% of male observations took place in the meadow

compared to 37% of female and 13% of male observations in ungrazed meadows. Males

and females in grazed meadows may switch to foraging in the meadow because openings

suitable for foraging were limited in both the understory and the canopy.

One possibility that could explain differences in territory density is that in

grazed meadows, pewees may be placing their territories only in areas with suitable

foraging habitat and that the amount of this habitat is more limited than in ungrazed

meadows. My study does not address this question because I did not evaluate pewee

habitat selection at home range scale (second order habitat selection; Johnson 1980).

Variables in my study were measured within or directly adjacent to active territories.

Because sample sizes for tree- and patch-scale observations differed, I could not combine

the two scales into the same analysis to evaluate scale effects (Orians and Wittenberger

1991). Mean differences between occupied and random tree-scale variables were greater

than patch-scale variables, indicating stronger tree selection than patch selection. Male

pewees, in particular, appeared to be choosing legacy features within their territories

including large trees and snags with numerous dead branches or dying tops.

Snags, sugar pines and hardwoods were important to pewees (see Chapter 2).

In addition, pewees used coarse woody debris including logs and dead branches

22

frequently as foraging substrates. Twenty-nine percent of male foraging observations

were from snags while females used snags 15% of the time and logs 22% of the time.

Pewees foraged from dead branches 94% of the time, presumably because dead branches

offer fewer obstructions to searching for prey than vegetated branches. Thus pewees may

select snags simply because branches were not vegetated. Most logs that were used as

foraging substrates had fallen into meadows and birds were observed foraging from

retained vertical dead branches. Large sugar pines on the study sites had long horizontal

dead or dying branches that may have provided suitable bare perches for foraging. In

contrast, pewees avoided shade tolerant species such as white fir and incense cedar.

Increases in densities of firs and cedars in western forests have been associated with

livestock grazing, fire suppression and logging (Belsky and Blumenthal 1997, van

Wagtendonk and Fites-Kaufman 2006). Pewees, specifically females, may not forage in

areas that are primarily white fir or incense cedar stands.

Western Wood-Pewees appear to require complex vertical and horizontal forest

structure, as indicated by higher variation in tree sizes, in areas where they forage. Dense

forests, especially those with high tree densities of small trees or high canopy cover, may

not provide this condition. Personal observation of meadow edges that pewees did not

inhabit revealed dense forest with little to no openings for foraging and high densities of

shade-tolerant tree species such as white fir and incense cedar that pewees avoided

foraging from (see Chapter 2). Forest structural heterogeneity and pine-dominated stands

are often linked to pre-fire suppression forest conditions (McKelvey et al. 1996). Pewees

may be closely associated with this habitat structure rather than the dense stands of

23

shade-tolerant trees that currently prevail in the Sierra Nevada (McKelvey et al. 1996).

Current Southern California Edison and local Sierra National Forest

management practices to reduce fuels and restore forests to uneven-aged systems using

mechanical thinning and prescribed fire (Southern California Edison 2001, U.S. Forest

Service 2008) could benefit Western Wood-Pewees if legacy features and a multi-layered

canopy and understory are retained on the landscape. These practices generally reduce

tree densities and canopy cover and promote diversity in tree classes (U.S. Forest Service

2008). The forest surrounding Sulphur Meadow, which had the highest density of

breeding pewee pairs, has experienced limited selective logging that focused on removing

white fir and incense cedar and a minimum 1-3 prescribed burns since 1980 (Southern

California Edison 1999). This resulted in snag and downed woody debris creation,

decreased tree densities and increased shrub and herbaceous growth (personal

observation). Livestock grazing is also currently limited in areas near montane meadows

in the Sierra Nevada, including Markwood Meadow, that lie within potential areas for

breeding Willow Flycatchers (Empidonax traillii; U.S. Forest Service 2004), a species

negatively affected by cattle browsing (Graber 1996). While land managers in the Sierra

Nevada are taking action to restore meadows mainly through livestock removal,

alleviating the effects of long-term grazing in conifer forests surrounding meadows may

be needed to improve the foraging habitat of Western Wood-Pewees.

24

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35

Figure 1. Location of seven meadows in the Sierra Nevada Mountains where the study

was conducted during 2007 and 2008 (Fresno County, California, USA). Ely, Sulphur,

Stevenson, Lost and Bear Meadows were located on Southern California Edison land.

Markwood and Swanson Meadows were on US Forest Service land. Ely, Sulphur and

Stevenson Meadows were excluded from livestock grazing since 1985; Swanson, Lost

and Bear Meadows have been continuously grazed and Markwood Meadow has been

rested from grazing since 2006.

36

Table 1. Number of kite traps and mean (SE) aerial arthropod biomass

(mg/2m2/21days) captured by taxonomic order, year and grazing status. Traps

were located on edges of montane meadows in the southern Sierra Nevada

Mountains (Fresno County, California, USA) during June-July 2007 and 2008.

Order 2007 2008

Grazed Ungrazed Grazed Ungrazed

No. traps 2 11 6 8

Arachnida 0 (0) 2 (1) 2 (1) 2 (1)

Coleoptera 109 (63) 308 (184) 110 (31) 155 (36)

Diptera1 329 (97) 300 (58) 209 (70) 114 (23)

Ephemeroptera 0 (0) 7 (3) 1 (1) 4 (2)

Hemiptera1 43 (40) 27 (4) 7 (4) 14 (3)

Homoptera 7 (7) 19 (4) 14 (8) 11 (4)

Hymenoptera1 165 (98) 113 (19) 32 (12) 34 (8)

Isoptera 15 (7) 22 (9) 14 (8) 18 (8)

Lepidoptera1 136 (104) 225 (64) 34 (28) 31 (13)

Mecoptera 0 (0) 2 (2) 0 (0) 0 (0)

Neuroptera 23 (23) 37 (10) 16 (7) 38 (25)

Odonota 39 (39) 20 (12) 2 (2) 0 (0)

Orthoptera 0 (0) 1 (1) 0 (0) 0 (0)

Plecoptera 7 (4) 22 (7) 2 (1) 17 (10)

Thysanaura 0 (0) 3 (2) 0 (0) 2 (2)

Trichoptera 29 (11) 31 (6) 27 (9) 29 (9)

Unknown 39 (39) 1 (1) 0 (0) 0 (0)

TOTAL 938 (484) 1140 (291) 469 (123) 467 (100)

1indicates year term significant (P < 0.05) in two-way ANOVA.

37

Table 2. Effects of livestock grazing on Western Wood-Pewee

territory density in montane meadows. Meadows were located

in the southern Sierra Nevada Mountains (Fresno County,

California, USA) and surveyed during the 2007 and 2008

breeding seasons.

Ungrazed1

Area

(ha)

Length

(km) Pairs Year

Territories

per km

Sulphur 8.7 2.8 11 2007 4.0

14 2008 5.0

Ely 11.2 1.9 9 2007 4.6

7 2008 3.6

Stevenson 6.8 1.8 8 2008 4.5

Markwood 25.7 4.4 9 2007 2.0

10 2008 2.3

Grazed

Swanson 29.4 4.4 6 2008 1.4

Lost 16.1 3.9 13 2008 3.4

Bear 4.5 1.5 5 2008 3.4

1 Markwood Meadow was rested from grazing in 2006 and all

other ungrazed meadows were rested in 1985.

38

Table 3. Mean (SE) patch-scale vegetation variables measured on

Western Wood-pewee territories in 4 grazed and 3 ungrazed montane

meadows in the southern Sierra Nevada Mountains (Fresno County,

California) during summer 2008.

Grazed Ungrazed

Variable n = 82 plots n = 52 plots

Canopy cover (%) 31 (2.7) 30 (2.9)

Shrub Cover (%) 12 (1.9) 14 (2.5)

Herbaceous Cover (%) 52 (3.6) 55 (4.9)

Tree Density (0.04 ha)1 13 (1.0) 10 (1.3)

Hardwood density (0.04 ha) 1.2 (0.3) 0.4 (0.1)

Snag Density (0.04 ha) 1.1 (0.2) 1.4 (0.3)

Mean dbh (cm)1 29 (1.5) 38 (3.0)

Variance dbh (cm) 480 (92) 553 (90) 1 P < 0.05; Wilcox rank sum tests between grazed and ungrazed

meadows.

39

Table 4. Summary of tree-scale model selection results of paired logistic

regression analysis comparing substrate characteristics of Western Wood-Pewee

foraging locations and random locations around montane meadows in the

southern Sierra Nevada Mountains (Fresno County, California, USA) during the

2008 breeding season.

Model k AICc ∆AICc wi

Tree dbh, percentage live foliage 2 42.46 0 0.52

Tree height, percentage live foliage 2 42.61 0.93 0.48

Tree dbh 1 51.88 9.48 0.00

Tree height 1 55.23 12.84 0.00

Percentage live foliage 1 65.61 23.21 0.00

40

Table 5. Means (SE) of tree (n = 85) and patch (n = 67) scale

variables measured at Western Wood-pewee foraging locations and

paired available locations on montane meadow edges in the

southern Sierra Nevada Mountains (Fresno County, California,

USA) during the 2008 breeding season.

Mean (SE)

Variable

Used

Location

Available

Location

Tree Scale

Tree DBH (cm) 77.6 (5.4) 31.3 (2.4)

Tree height (m) 27.3 (1.8) 12.1 (1.1)

Percentage live foliage (%) 57.8 (4.7) 86.4 (2.8)

Patch Scale

Edge distance (m) 4.6 (2.1) 10.8 (2.4)

Stream distance (m) 30.4 (3.9) 30.9 (3.4)

Canopy cover (%) 28.1 (2.8) 32.5 (2.7)

Shrub cover (%) 11.2 (1.9) 14.5 (2.4)

Herbaceous cover (%) 54.9 (4.3) 51.3 (4.0)

Tree density (0.04 ha) 10.5 (1.0) 13.0 (1.3)

Hardwood density (0.04 ha) 0.8 (0.2) 1.0 (0.3)

Snag density (0.04 ha) 1.2 (0.3) 1.3 (0.3)

Sapling density (0.04 ha) 14.7 (2.1) 18.5 (2.3)

Mean DBH (cm) 35.1 (2.5) 29.9 (1.7)

DBH variance 775.1 (119.7) 274.9 (36.1)

41

Table 6. Summary of patch-scale model selection results of paired

logistic regression analysis comparing substrate characteristics of

Western Wood-Pewee foraging locations and random locations around

montane meadows in the southern Sierra Nevada Mountains (Fresno

County, California, USA) during the 2008 breeding season.

Model1 k AICc ∆AICc wi

Dbh variance, canopy cover, edge distance 3 50.21 0.00 0.99

Full Model 10 60.39 10.19 0.01

Dbh variance, canopy cover 2 62.59 12.38 0.00

Dbh variance, edge distance 2 63.25 13.05 0.00

Dbh variance, canopy cover, hardwood

density 3 63.93 13.73 0.00

Dbh variance, canopy cover, sapling density 3 64.69 14.48 0.00

Canopy cover, mean dbh, dbh variance,

herbaceous cover 4 66.21 16.01 0.00

Dbh variance, tree density 2 70.89 20.68 0.00

Dbh variance, shrub cover 2 75.49 25.28 0.00

Dbh variance, tree density, mean dbh, sapling

density 4 74.96 24.75 0.00

Dbh variance 1 77.12 26.91 0.00

42

CHAPTER TWO: INTERSEXUAL DIFFERENCES IN THE FORAGING

ECOLOGY OF WESTERN WOOD-PEWEES

INTRODUCTION

Sex-specific differences in foraging behavior during the breeding season have been

documented for many songbird species, most notably parulids (Family Parulidae: Morse

1968, Alatalo and Alatalo 1979, Morrison 1982, Franzreb 1983, Holmes 1986, Hanowski

and Niemi 1990, Petit et al. 1990, Sodhi and Paszkowski 1995, Kelly and Wood 1996,

Keane and Morrison 1999). Intersexual differences in foraging behavior may be a result

of (1) partitioning of resources due to intraspecific competition that may be expressed as

sexual dimorphism (Rand 1952, Selander 1966, Bell 1982) or (2) constraints associated

with reproductive behavior, such as females foraging close to nests and males foraging

close to song perches (Morse 1968, Morrison 1982, Franzreb 1983, Holmes 1986). To

test these ideas, authors generally have examined foraging behavior and foraging location

in relation to morphology (Bell 1982) and nest and songpost locations (Petit et al. 1990,

Kelly and Wood 1996, Keane and Morrison 1999). In parulids, males foraged higher and

closer to their song perch than females (Morse 1968, Alatalo and Alatalo 1979, Morrison

1982, Franzreb 1983, Holmes 1986, Hanowski and Niemi 1990, Keane 1991, Kelly and

Wood 1996) while female foraging height was generally correlated with nest height

(Holmes 1986, Petit et al. 1990). Tree species, foraging maneuvers, and substrate type

also differed between sexes of some warblers (Morrison 1982, Franzreb 1983, Petit et al.

43

1990, Keane and Morrison 1999). When sexual dimorphism was not present, most

authors concluded that intersexual foraging differences were a result of constraints

associated with reproductive behavior (Morse 1968).

Most Parulid species are sexually dichromatic but for species where the sexes

look similar (monochromatic), male and female foraging observations have generally

been pooled when examining foraging ecology (Franzreb 1983, Airola and Barrett 1985,

Szaro et al. 1990, Hartung and Brawn 2005). Few researchers have documented sexual

differences in foraging behavior of monochromatic species, due to difficulties associated

with sexual identification during foraging observations. In one case where sexual

differences of monochromatic species were studied, color-banded populations of

Mountain Chickadee (Poecile gambeli) and Chestnut-backed Chickadees (Poecile

rufescens) did not exhibit intersexual difference in foraging height but males and females

of both species foraged in different tree species and utilized different foraging maneuvers

(Brennan et al. 2000). Males and females in grassland species such as the Clay-colored

Sparrow (Spizella pallid) and Henslow’s Sparrow (Ammodramus henslowii) foraged in

different areas within their territory and females generally foraged closer to the nest site

(Robins 1971, Knapton 1981). Intersexual foraging differences in other monochromatic

species are unknown.

Some songbird studies that use foraging behavior as an approach to examine

how birds respond to ecological factors have focused on one sex to control for sexual

variation (Lovette and Holmes 1995, Meehan and George 2003, Kilgo 2005). Only in one

case was sex addressed as an explanatory variable in the analysis (Lyons 2005). Others

44

did not differentiate between sexes (Hartung and Brawn 2005, Rodewald and Brittingham

2007). Foraging studies that do not take into account sexual variation may lead to

erroneous conclusions about habitat use with results not representative for either sex

(Hanowski and Niemi 1990). If parameters are not correctly identified, then habitat

managers may not have accurate information about the resources birds use to ensure their

viability over time.

The Western Wood-Pewee (Contopus sordidulus – hereafter “pewee”) is a

sexually monochromatic neotropical migrant songbird that typically occurs in open

canopy western forests and riparian areas. They occupy a variety of habitats within their

breeding range that provide exposed perches both in the understory and high in the

canopy (Bemis and Rising 1999). Pewees are aerial foragers, feed primarily on insects,

and have a low search/pursuit ratio indicating that they passively locate their prey from a

perch (Eckhardt 1979). Previous studies of foraging behavior of this species have not

differentiated males from females (Beaver and Baldwin 1975, Verbeek 1975, Eckhardt

1979, Szaro et al. 1990). Intersexual foraging differences for New World flycatchers

(Family Tyrannidae) have been documented for only a few species, all of which are

sexually dichromatic (Teather 1992). Many flycatcher species are monochromatic and

sexes are difficult to distinguish in the field. Thus it is unknown whether tyrannids and

other flycatchers exhibit intersexual differences in foraging ecology.

During two breeding seasons in the southern Sierra Nevada, I used song and

nesting behavior to document differences in foraging behavior between male and female

pewees. My objectives were to (1) quantify foraging rate for male and female pewees in

45

relation to changes in breeding stage, (2) describe intersexual differences in habitat use

and tree species selection and, (3) compare distances between foraging locations and nest

locations for both sexes. This would allow me to examine predictions of the reproductive

constraints hypothesis (Morse 1968). If female pewees are constrained by reproductive

duties then females should forage closer to the nests than males and female foraging

height should be correlated with nest height.

46

METHODS

STUDY AREA

I conducted my study in wet montane meadows on the western slope of the southern

Sierra Nevada Mountains in Fresno County (elevational range: 1828 m – 1682 m; Figure

1). Seven meadows were included: 3 grazed by cattle (Swanson, Lost and Bear

Meadows), and 4 ungrazed (Markwood Meadow was grazed until 2006, Ely, Sulphur and

Stevenson Meadows have been ungrazed since 1985). Meadows averaged 15 ± 10 ha

(range 4 – 29 ha) and were located on either Sierra National Forest or Southern California

Edison land. Montane hardwood-conifer and Sierra mixed-conifer forests surrounded

meadows. Common tree species included ponderosa pine (Pinus ponderosa), California

black oak (Quercus kelloggii), incense cedar (Calocedrus decurrens), white fir (Abies

concolor), sugar pine (Pinus lambertiana) and lodgepole pine (Pinus contorta) (Mayer

and Laudenslayer 1988). The shrub layer was dominated by greenleaf manzanita

(Arctostaphylos patula), whitethorn ceanothus (Ceanothus cordulatus), snowberry

(Symphoricarpos acutus), and Sierra gooseberry (Ribes roezlii). Riparian tree species,

including willow (Salix sp.), red alder (Alnus rubra) and quaking aspen (Populus

tremuloides), occurred around meadow edges and in some areas, throughout the meadow.

The meadows themselves were characterized by a high diversity of herbaceous cover

including forbs, grasses, rushes (Juncus sp.) and sedges (Carex sp.).

FORAGING OBSERVATIONS

Western Wood-Pewees were observed from mid-May to early August in 2007 and 2008.

I surveyed Sulphur, Ely, Markwood Meadows and parts of Swanson and Lost Meadows

47

in 2007. I extended my surveys in 2008 to include all of Lost and Swanson Meadows,

and Stevenson and Bear Meadows. I recorded foraging behavior of pewees between

sunrise and sunset (approximately 0600 and 2000) and systematically searched meadow

edges and located breeding pairs and unmated males that defended a territory. Only one

observation was conducted on a single individual per day and at least 4 days passed

before that territory was visited again. The order in which meadows were visited and

direction walked was rotated to reduce bias for particular birds within sites.

I quantified foraging rate using foraging attack rate (Hutto 1990, Lovette and

Holmes 1995, Kilgo 2005, Lyons 2005). I observed a bird as it foraged and recorded

each attack and the duration of non-foraging behaviors (to the nearest second) on a

portable digital recorder. An attack was defined as a sally toward an aerial prey item,

whether it was successful or not. I rarely observed what the bird captured and assumed

that pewees swallowed insects immediately. An observation ended when I lost the bird

from sight for more than 5 minutes or after 20 minutes. To calculate attack rate, I

summed the number of attacks during the observation, and divided by total time spent

foraging. Time spent in non-foraging behaviors, including preening, prey handling, nest

building, feeding nestlings, and interacting with other birds or predators were subtracted

from the observation period.

To reduce potential biases due to observability (Morrison 1984), I used the

location of the second foraging sally for all habitat measurements even if the bird

returned to the same perch. I recorded the date, tree species or substrate, perch status

(dead or alive), horizontal strata (inner, middle, outer), and the presence or absence of

48

limbs less than 1 m above and below the perch. I estimated distance to prey capture and

measured height of capture, height of bird in tree and tree height with a hypsometer to the

nearest 0.1 m and measured tree diameter (cm) at breast height (dbh). The location of the

tree where the foraging bout originated was flagged and recorded using a global

positioning device (GPS) to ensure that observations were not collected from the location

in subsequent visits. To avoid sampling the same individual, I walked at least 150 m (the

average radius pewee territory, K. Purcell, unpublished data). The only exception was if

I positively identified the sex of the individual and observed an individual of the opposite

sex.

Sex was identified through two methods. Males were identified by their dawn

song, Peee-pip-pip or by their Pee-er songs (Bemis and Rising 1999). The dawn song is

only sung for prolonged periods before sunrise but males sing the Pee-er song throughout

the day. Females were identified only if I observed them incubating or if I successfully

identified the male on the same territory as the female. I generally used a combination of

nesting and song behavior to identify both sexes. To verify these sex-identification

methods, I captured birds in 2008 using mist nets and song playback. I sexed birds using

the presence or absence of a brood patch. I used brood patch as the defining

characteristic because the cloacal protuberance is poorly developed in male Western

Wood-Pewees (Pyle 1997). The only female captured had a well-developed brood patch.

I placed colorbands on 1 female and 7 male pewees in three meadows and observed their

behavior for longer periods throughout the summer to verify that only males gave the

Pee-er song and only females incubated eggs.

49

While conducting foraging observations, I mapped territory boundaries and

opportunistically searched for nests. At each nest I recorded the UTM coordinates with a

handheld global positioning device (GPS). I measured height of nest, recorded the tree

species and determined the breeding stage by checking the contents with a mirror pole or

observing the nest for 15 minutes (Martin and Geupel 1993). Breeding stage

classifications were limited to building, laying, incubation, feeding nestlings or feeding

fledglings. If I observed one or both of the pair foraging nearby and could confirm

breeding stage, I compared GPS locations of foraging birds with nest locations using the

Euclidean Distance formula:

where ∆x = change in distance between the easting coordinates, ∆y = change in distance

between the northing coordinates and ∆z = change in distance between the nest height

and foraging height. All measurements were in meters (m).

STATISTICAL ANALYSES

I tested for differences in foraging attack rate (attacks/min.) between males and females

in both years using two-way analysis of variance (ANOVA). I used a reduced dataset

where I included foraging observations with confirmed breeding status to compare

foraging rates between sexes and nesting stages (building, incubation, nestling and

fledgling) using two-way ANOVA (Dobbs and Martin 1998). I then tested for

differences between breeding stages for each sex using Tukey’s Honest Significant

Differences test. Differences in foraging behavior between male and female pewees were

50

compared by looking at the effects of sex and year in a multivariate analysis of variance

(MANOVA) with 3 dependent variables (attack height, perch height and distance to

capture). If year was not significant, I pooled data from both years and used two-sample

t-tests to examine differences in perch height, attack height, capture distance, tree height

and tree dbh. I used contingency table analyses to assess relationships between sex and

perch status (dead/alive), presence of branches less than 1 m above perch, presence of

branches less than 1m below perch and whether the bird foraged from the top of the tree

or not.

I used a chi-square goodness-of-fit analysis to evaluate tree species selection

separately for males and females. Tree availability was collected only during 2008 but

tree use observations were from 2007 and 2008. I pooled hardwood tree species and

combined ponderosa and lodgepole pine observations to fulfill assumptions for statistical

tests and to generalize species availability across meadows. Tree species availability

varied between meadows (e.g., aspen was not in all meadows) but the following

categories were present in all meadows: white fir, incense cedar, sugar pine, hardwoods,

snags and ponderosa/lodgepole pines. I used a Bonferroni correction to calculate 95%

confidence intervals around proportion of tree species used and examined whether

proportion of tree species available fell within that confidence interval (Neu et al. 1974).

Euclidean distances between nest locations and foraging locations were

compared between sex, breeding stage (building, incubation, nestling) and their

interaction using two-way ANOVA. Distances were square-root transformed to improve

normality. All statistical tests were conducted at a 0.05 level of significance (α < 0.05)

51

and results are reported as mean ± SE. Data analysis was completed using R (R

Development Core Team 2008).

52

RESULTS

I completed 152 foraging observations (2007: male n = 33, female n = 30; 2008: male n =

44 female n = 45). Pewees were observed sallying out from perches to capture aerial

insects except for one instance where a female was seen hopping along a tree branch and

fanning her wings to flush an insect (“flush pursuit”; Remsen and Robinson 1990:154).

Female foraging rates were higher than male rates (F1, 139 = 92.0, P < 0.001) and there

was no difference in foraging rates between years (F1, 139 = 2.1, P = 0.15) nor was there

an interaction between year and sex (F1, 139 = 0.8, P = 0.38). Foraging rates differed

among breeding stages and there was a significant interaction with sex (F4, 76 = 5.2, P <

0.001, interaction: F3, 76 = 3.5, P = 0.02; Figure 2). Female foraging rates during

incubation were consistently higher than during the building, nestling or fledgling stage

(Tukey HSD test: all p-values < 0.05). Male foraging rates remained consistent

throughout the breeding season and was significantly lower than female foraging rate

during incubation, nestling and fledgling periods (Tukey HSD tests: all p-values > 0.05).

Sample size constraints precluded inclusion of foraging rate during laying observations

for either sex.

Foraging behavior differed significantly between males and females (F1, 147 =

19.6, P < 0.0001), but not between years (F1, 147 = 0.1, P = 0.95), nor was there a

significant interaction (F1, 147 = 1.3, P = 0.28). Two-sample t-tests indicated that males

were perched higher in the canopy and attacked prey higher than females, flew further

distances to capture prey and used trees that were taller and larger than females (Figure 3,

Table 7). Both sexes foraged mostly from dead perches (94% of observations, n = 152).

53

Frequency analyses indicated that there was no relationship between sex and foraging at

the tops of substrates (males = 35%, females = 31%, χ2

1 = 0.16, P = 0.69), foraging from

perches with branches less than 1 m above (both sexes = 36%, χ2

1 = 0.1, P = 0.90),

foraging from perches with branches less than 1 m below (males = 58%, females = 43%,

χ2

1 = 3.2, P = 0.07) or foraging in a particular horizontal strata (χ2

3 = 4.0, P = 0.26).

Male pewees foraged from trees or snags 93% of the time (n = 77). Females foraged

from logs or fence posts 31% of the time (n = 74) with the remaining observations from

trees or snags. Both male and female pewees exhibited tree species selection (male χ2

5 =

137.8, female χ2

5 = 54.3, both P-values < 0.0001; Table 8). Confidence intervals

indicated that males selected snags and sugar pines, avoided incense cedar, and used

white fir, hardwoods and ponderosa/lodgepole pines in proportion to their availability.

Female pewees selected snags and hardwood species, avoided white fir and incense cedar

and used all pine species in proportion to their availability.

I located 42 nests during the study (13 in 2007, 29 in 2008). All nests were

located close to the meadow edge. Nest height averaged 9.7 ± 1.1 m (range 2.2 – 29.5

m). Pewees nested in a variety of tree species; 25 were located in incense cedar, 4 in

black oak, 4 in aspen, 3 in ponderosa pine, 2 each in white fir and sugar pine and 1 in a

willow. Another successful nest was located in the fork of dead branches from a fallen

log. At Markwood meadow, two females nested on a territory held by one male.

Because I could not confirm which nest the male was attending at the time of his foraging

observation, I did not include him in the analysis but I included both females. Nests were

located and breeding stage confirmed in conjunction with 51 foraging observations (18

54

male and 32 female). Both sexes foraged at a similar distance from the nest (F1, 45 = 1.8,

P = 0.19; Figure 4). Breeding stage had no effect on foraging distance from the nest for

either sex (F1, 45 = 0.8, P = 0.47) nor was there a significant interaction between sex and

breeding stage (F1, 45 = 2.3, P = 0.11). Males foraged on average 25.1 ± 3.3 m from the

nest and females foraged on average 23.0 ± 3.0 m. Nest height and foraging height were

not correlated for either male or female pewees (Spearman rank correlation, P > 0.86 for

both sexes; Figure 5).

55

DISCUSSION

Male and female pewees differed in their foraging behavior and use of foraging

substrates. Average female foraging attack rate was more than double the average male

attack rate. This pattern remained consistent throughout the breeding season but was

most pronounced during incubation. Because of the demands associated with incubation,

it is expected that females would forage more rapidly than males during this period

(Morse 1968). As the breeding season progressed, female foraging rate declined during

the nestling and fledgling stages but remained consistently higher than male foraging

rate. Several hypotheses may explain why male attack rate was much lower than female

attack rate. First, males may be using a foraging strategy that allows them to forage and

sit in locations where they can observe intruding males and females to either defend their

territory from males or secure extra-pair copulations from females. Female pewees may

focus their time budget solely on foraging and may not expend as much energy on

vigilance. While there are few studies comparing male and female foraging rates in

songbirds, attack rates were similar between sexes in Mountain Bluebirds (Sialia

currucoides) but when confronted with a greater workload, females expanded their

repertoire of foraging behaviors (e.g., hover-gleaning; Power 1980). Lyons (2005) found

that female Prothonotary Warblers (Protonotaria citrea) spent more time foraging than

males but their attack rates were similar. I could not examine differences in time

allocation between sexes because pewees are “sit and wait” foragers and search behavior

is the same whether they are looking for prey or for potential nest predators and

conspecifics.

56

A second hypothesis that may explain the differences in foraging rates is that

females may focus on smaller, more abundant insects in the understory to maximize their

food intake while males may be capturing fewer, larger, less abundant insects in the

canopy to maximize time spent on territorial defense. I rarely identified what pewees

captured except for lepidopterans, which both sexes caught later in the summer. Further

study is needed to identify size and type of prey that both sexes are capturing, possibly by

examining fecal contents (Durst et al. 2008). If males and female specialize on different

prey types, this may be reflected in differences in wing or tail length as these metrics may

influence aerial maneuverability (Bell 1982). Male pewees have longer wings than

females (Pyle 1997). Eckhardt (1979) also documented longer wing sizes in male pewees

that may have resulted from the sexual demands of flight but could not identify reasons

why these differences exist. My results revealed that distance to insect capture was

greater for males than females perhaps reflecting a flight advantage for males to forage

on larger, more mobile prey such as biomass-rich lepidopterans (Beaver and Baldwin

1979).

Sexes foraged at different heights: females spent more time in the understory in

smaller and shorter trees and males foraged in the canopy from taller and larger trees.

Even though females foraged from smaller trees than males, these trees were taller and

larger than available trees indicating that both sexes prefered larger trees (see Chapter 1).

Effects from livestock grazing in Sierra Nevada montane meadows may

disproportionately affect females because they spend more time foraging in the

understory (see Chapter 1). Understory foragers and nesters show fairly consistent

57

negative responses to livestock grazing (Tewksbury et al. 2002, Krueper et al. 2003, Saab

et al. 1995). Even though legacy features such as snags and large conifers may be present

and provide suitable foraging locations for males, increased tree densities in forests

surrounding grazed meadows (see Chapter 1) may make foraging in the understory

unsuitable for female pewees. Pewees require exposed perches and small openings to

search for and capture aerial insects. In grazed meadows, 43% of female and 24% of

male observations took place in the meadow rather than in forested edges compared to

37% of female and 13% of male observations in ungrazed meadows. Suitable foraging

openings for both sexes may have been limited in edges surrounding grazed meadows

because of higher tree densities. In addition to increased tree densities, cattle grazing

may also decrease recruitment of hardwood species such as willows and aspen (Page

1978, Schulz and Leininger 1990) preferred by females.

Both sexes foraged at a similar distance from their nest throughout the breeding

season. On average, males foraged higher than nest height while females foraged lower

than nest height. Nest sites may have been an intermediate location for both sexes. It is

likely that differences in foraging height and location are a result of both reproductive

duties (e.g., male vigilance) and possibly partitioning of resources. In Texas, breeding

male Scissor-tailed Flycatchers (Tyrannus forficatus) perched higher than females

although mean differences were small and substrate height and nest height were not

measured (Teather 1992). Intersexual foraging differences in height have been attributed

to females foraging closer to their nest in Dendroica warblers (Morse 1968, Franzreb

1983) or where height differences weren’t apparent, female foraging height was

58

correlated to nest height (Holmes 1986). Western Wood-Pewees do not closely follow

this pattern; even though males generally foraged from their songpost location, nest

height and foraging height were not correlated for females. Male pewees may perch in

the canopy to facilitate territory defense while females may occupy the understory or

meadow habitat to take advantage of different foraging opportunities (Holmes 1986). In

Prothonotary Warblers (Protonotaria citrea), intersexual foraging differences attributed

primarily to reproductive duties and secondarily to resource competition were dependent

on breeding stage (Petit et al. 1990). Bell (1982) described foraging niche differences in

Frill-necked Flycatchers (Arses telescopthalmus) as intersexual competition resulting in

sexual dimorphism. While there is evidence to support the reproductive duties

hypothesis (Morse 1968), to make a definitive conclusion regarding the resource

competition hypothesis (Rand 1952), it would be necessary to determine what males and

females are eating.

Foraging studies that do not examine both sexes throughout the breeding season

may miss critical ecological patterns. Because females of most songbirds have greater

time constraints, foraging attack rate of females during the incubation period may provide

a more sensitive measure of foraging habitat quality than other measures. By focusing on

female attack rate, researchers may be able to detect greater differences in habitat quality.

Identifying resources important to both sexes of a bird species can also help

conservationists gain greater knowledge about habitat attributes to protect and restore.

The broad scope of this study was to identify the ecological factors that could cause

lower pewee densities in meadows grazed by cattle. If I had not differentiated between

59

sexes, I may have missed critical ecological factors important to female pewees that may

be affected by cattle grazing.

60

LITERATURE CITED

Airola, D.A. and R.H. Barrett. 1985. Foraging and habitat relationships of insect

gleaning birds in a Sierra Nevada mixed-conifer forest. Condor 87:205-216.

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67

0

1

2

3

4

5

6

Building Incubating Nestling Fledgling

Nesting Stage

Foraging attack rate (attacks/min.)

Male

Female

Figure 2. Foraging attack rate (mean ± SE) according to nesting stage of male and female

Western Wood-Pewees breeding around montane meadows in the southern Sierra Nevada

Mountains (Fresno County, California, USA) during summer 2007 and 2008.

68

Figure 3. Intersexual differences in foraging height in relation to substrate height for

Western Wood-Pewees breeding around montane meadows in the southern Sierra Nevada

Mountains (Fresno County, California, USA) during summer 2007 and 2008.

69

0

5

10

15

20

25

30

35

40

45

Building Incubating Nestling

Nest stage

Distance from foraging point to nest (m)

Male

Female

Figure 4. Distance (mean ± SE) from foraging point to nest location according to nest

stage for male and female Western Wood-Pewees breeding on edges of montane

meadows in the southern Sierra Nevada Mountains (Fresno County, California) during

summer 2007 and 2008.

70

Figure 5. Nest height (m) in relation to foraging height (m) for male and female Western

Wood-Pewees breeding on edges of montane meadows in the southern Sierra Nevada

Mountains (Fresno County, California, USA) during summer 2007 and 2008.

71

Table 7. Comparison of foraging behavior and perch site characteristics of male (n =

77) and female (n = 75) Western Wood-pewees foraging in montane meadows in the

southern Sierra Nevada Mountains during the 2007 and 2008 breeding season.

Mean (SE)

Variable Male Female t-stat p-value

Height in tree (m) 17.1 (1.2) 6.7 (0.8) 7.35 <0.0001

Attack height (m) 14.7 (1.2) 5.6 (0.7) 6.58 <0.0001

Capture Distance (m) 7.8 (0.7) 4.4 (0.4) 4.69 <0.0001

Tree Height (m) 28.0 (1.7) 18.1 (1.8) 4.02 <0.0001

Tree dbh (cm) 88.1 (5.0) 70.1(5.5) 2.28 0.02

72

Table 8. Foraging tree species selection of male and female Western Wood-Pewees breeding in

montane meadows in the southern Sierra Nevada Mountains (Fresno County, California, USA)

during summer 2007 and 2008. Lodgepole and ponderosa pines were combined into a one

category (Pines). Number and proportion of used and available trees are shown and 95%

confidence intervals around proportion used.

Male*

Number

of Used

Trees

Proportion

of Used

Trees (pi)

Number

of

Available

Trees

Proportion

of Available

Trees

95% Confidence Interval

(pi)

White Fir 9 0.13 17 0.21 0.22 ≤ p1 ≤ 0.03

Cedar1 21 0.29 38 0.46 0.42 ≤ p2 ≤ 0.16

Snags2 22 0.31 5 0.06 0.44 ≤ p3 ≤ 0.18

Sugar Pine2 8 0.11 1 0.01 0.20 ≤ p4 ≤ 0.02

Pines 9 0.13 17 0.21 0.22 ≤ p5 ≤ 0.03

Hardwoods 3 0.04 4 0.05 0.10 ≤ p6 ≤ 0.00

Total 72 82

Female*

White Fir1 6 0.10 17 0.21 0.20 ≤ p1 ≤ 0.01

Cedar1 16 0.28 38 0.46 0.42 ≤ p2 ≤ 0.14

Snags2 11 0.19 5 0.06 0.31 ≤ p3 ≤ 0.07

Sugar Pine 3 0.05 1 0.01 0.12 ≤ p4 ≤ 0.00

Pines 11 0.19 17 0.21 0.31 ≤ p5 ≤ 0.07

Hardwoods2 11 0.19 4 0.05 0.31 ≤ p6 ≤ 0.07

Total 58 82

*Male X2

5 = 137.8, P < 0.0001

Female Χ2

5 = 54.3, P < 0.0001 1Used less than available

2Used more than available