symbiosis between long legged wading birds...
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SYMBIOSIS BETWEEN LONG LEGGED WADING BIRDS (Ciconiiformes) AND ALLIGATORS (Alligator mississippiensis)? TESTING THE ‘NEST PROTECTOR’
HYPOTHESIS
By
BRITTANY F. BURTNER
A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
2011
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© 2011 Brittany F. Burtner
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To my parents, for encouraging my imagination
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ACKNOWLEDGMENTS
I would like to thank my advisor and committee chair, Peter Fredrick for his
invaluable guidance and support throughout this project. I am indebted to my
committee members Rob Fletcher and Kent Vliet for their thoughtful input to this
manuscript and to my project along the way. I owe Frank Mazzotti and Brian Jeffery
great thanks for providing much needed equipment and support.
I owe great thanks to many people who helped me in the field including: Eric
Fishel, Lindsey Garner, Erin Posthumous, Melissa Schlothan, John Simon, Nick Vitale
and Chris Winchester. Furthermore, I thank my lab mates Jason Fidorra and Louise
Venne for their assistance in the field, much-welcomed suggestions, and
commiseration. Greg Smith not only provided assistance in the field, he produced a
great number of the alligator decoys and modified flamingo bird decoys for this project.
Without his assistance, I would probably still be making decoys.
This work was supported by the United States Army Corps of Engineers and an
assistantship from the School of Natural Resources and the Environment.
Finally, I thank my family and friends for unflagging love and support.
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TABLE OF CONTENTS page
ACKNOWLEDGMENTS .................................................................................................. 4
LIST OF TABLES ............................................................................................................ 7
LIST OF FIGURES .......................................................................................................... 8
ABSTRACT ..................................................................................................................... 9
CHAPTER
1 DO HERONS CHOOSE NESTING SITES BASED ON ATTRACTION TO ALLIGATORS? ....................................................................................................... 11
Introduction ............................................................................................................. 11 Methods .................................................................................................................. 15
Study Area ........................................................................................................ 15 Wading Bird – Alligator Association Survey ...................................................... 16 Decoy Experiment Design ................................................................................ 16 Detecting Responses to Decoys ...................................................................... 18 Statistical Analysis ............................................................................................ 19
Results .................................................................................................................... 19 Wading Bird-Alligator Association .................................................................... 19 Bird Response to Decoys ................................................................................. 20
Discussion .............................................................................................................. 21
2 DO WADING BIRDS BENEFIT FROM NESTING NEAR ALLIGATORS? .............. 36
Introduction ............................................................................................................. 36 Methods .................................................................................................................. 39
Predator Distribution ......................................................................................... 39 Survival rates of nestlings and fledgling herons ............................................... 41
Results .................................................................................................................... 41 Predator Distribution ......................................................................................... 41 Water Depth ..................................................................................................... 42 Color-Banded Birds .......................................................................................... 43
Discussion .............................................................................................................. 43
3 DO ALLIGATORS BENEFIT FROM ASSOCIATING WITH NESTING WADING BIRDS? ................................................................................................................... 50
Introduction ............................................................................................................. 50 Methods .................................................................................................................. 51 Results .................................................................................................................... 53 Discussion .............................................................................................................. 54
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LIST OF REFERENCES ............................................................................................... 61
BIOGRAPHICAL SKETCH ............................................................................................ 67
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LIST OF TABLES
Table page 1-1. Pairwise comparisons of numbers of birds responding to decoy treatments in
2010. .................................................................................................................. 33
1-2. Pairwise comparisons of numbers of birds responding to decoy treatments in 2011. .................................................................................................................. 33
3-1. Locations of colonies in WCA-3A where throughfall traps were placed in 2010 and 2011. ........................................................................................................... 57
3-2. The fate of nests with throughfall traps in 2010 and 2011. .................................... 59
3-3. The number, total mass, grams per nest, and percent of total mass of all food items recovered from throughfall traps beneath 20 Egretta heron nests. ........... 59
3-4. The number, total mass, grams per nest, and percent of total mass of all food items recovered from throughfall traps beneath 37 Egretta heron nests. ........... 59
3-5. The potential fresh food mass input from a 50-nest Egretta heron colony in 2010 and 2011 after 20, 40, and 60 days. .......................................................... 60
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LIST OF FIGURES
Figure page 1-1. Map of study area in south Florida, USA. .............................................................. 24
1-2. Aerial photograph of an alligator hole and small willow-dominated tree island in WCA-3A. ............................................................................................................ 25
1-3. Photograph of an alligator decoy in the field. ........................................................ 26
1-4. Photograph of a great egret decoy. ....................................................................... 27
1-5. Photograph of an altered flamingo decoy in the field. ........................................... 28
1-6. Photograph of a bird decoy array at mid-canopy height in willow trees surrounding an alligator hole. ............................................................................. 29
1-7. Comparison of alligator and Egretta heron distribution on small willow-dominated tree islands in the Everglades of Florida. .......................................... 30
1-8. Totals of maximum number of Egretta herons per treatment site that responded to each decoy treatment; 2010. ........................................................ 31
1-9. Totals of maximum number of Egretta herons per treatment site that responded to each decoy treatment; 2011. ........................................................ 32
1-10. Numbers of islands per treatment group that Egretta herons nested on; 2010. ... 34
1-11. Numbers of islands per treatment group that Egretta herons nested on; 2011. ... 35
2-1. Satellite image (adapted from Google Earth v.6.1; 2011) of predator tracking station locations in 2010 (diamonds) and 2011 (circles). .................................... 46
2-2. Stages at 3-4 gauge in WCA-3A from January 2010 until July 2011..................... 47
2-3. Cumulative percentage per day of color-banded Egretta herons resighted, dead, or not seen. ............................................................................................... 48
2-4. Raccoon scat containing feathers of young herons.. ............................................. 49
3-1. Throughfall trap set-up below an Egretta heron nest with three eggs. .................. 58
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Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science
SYMBIOSIS BETWEEN LONG LEGGED WADING BIRDS (Ciconiiformes) AND
ALLIGATORS (Alligator mississippiensis)? TESTING THE ‘NEST PROTECTOR’ HYPOTHESIS
By
Brittany F. Burtner
December 2011
Chair: Peter Frederick Major: Interdisciplinary Ecology
Wading birds (Ciconiiformes) appear to preferentially nest above alligators and
alligator habitat. Alligators could benefit nesting birds by deterring mammalian
predators. Chicks or food dropped from the bird nests could provide alligators with
food. I tested selected predictions of this hypothesis using small willow-dominated
colonies of little blue herons (Egretta caerulea), tricolored herons (Egretta tricolor), and
snowy egrets (Egretta thula) in the central Everglades as experimental units.
I experimentally manipulated apparent densities of alligators and conspecific birds
using alligator and white bird decoys to determine if wading birds were attracted to
alligators via visual cues. Egretta herons showed a strong preference for sites with both
alligator and bird decoys in 2010 (Χ2(3,N=45)=17.133, p=0.001) and 2011
(Χ2(3,N=261)=72.452, p=0.0001). Using helicopter surveys, I found that alligators and
wading birds associated significantly more often than expected by chance (p=0.007667,
Fisher’s Exact Test, N=40).
I deployed predator tracking stations and found that raccoon presence in the
Everglades was strongly dependent upon low water levels. Utilizing throughfall traps, I
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estimated that a colony of 50 pairs has the potential to drop 102 grams of food over a
60-day nesting cycle. This may be nutritionally important to alligators, particularly
during the dry season when movements may be limited and food is harder to find. My
evidence suggests that there is a mutualism between Egretta herons and alligators;
herons are attracted to nest near alligators, and alligators receive nontrivial food
benefits from nesting birds.
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CHAPTER 1 DO HERONS CHOOSE NESTING SITES BASED ON ATTRACTION TO
ALLIGATORS?
Introduction
Animals are constantly faced with situations where they need to make decisions
without complete information. Other individuals also encounter similar circumstances,
allowing one individual to observe the decision made and the consequences of the
decision for another individual. This interaction, termed social information, can be
conveyed intentionally or inadvertently by the behavior or presence of individuals.
Historically, bird habitat selection theory has focused mainly on physical and
environmental factors including habitat structure, patchiness, and food availability or
biotic factors such as predation and competition (see Jones 2001 for a review). Recent
research has begun to explore habitat selection based on social information (Danchin et
al. 2004). Social information use by an individual is a process consisting of (1) an
observable state or event, (2) the observation of that state or event, (3) a decision
manifested as an altered action, and (4) the consequences of that action (Seppanen et
al. 2007).
Initial research about social information focused on conspecific attraction and was
shown to occur across a wide variety of taxa such as Norway rats (Rattus norvegicus)
(Laland & Plotkin 1992), spotless starlings (Sturnus unicolor) (Parejo et al. 2008) and
numerous species of marine invertebrate larvae (Burke 1986). In recent years, use of
information between heterospecifics has garnered increased attention in the literature
(see Seppanen et al. 2007 for a review). Although heterospecific information has been
shown to occur within and between a number of taxa, many studies have focused on
interactions between avian species. Monkkonen et al. (1990) suggest that migrant birds
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in Finland use resident birds as indicators of safe and/ or productive breeding sites in
unpredictable circumstances. Forsman et al. (1998) found that migrant birds use
resident tit species (Parus spp.) as indicators of a profitable breeding patch.
Furthermore, Forsman et al. (2002) demonstrated that pied flycatchers (Ficedula
hypoleuca) were attracted to and accrued fitness benefits from resident titmice (Parus
spp.). Least flycatchers (Empidonax minimus) have been shown to use the vocal cues
of a competitor, the American redstart (Setophaga ruticilla), as well as those of
conspecifics when making settlement decisions (Fletcher 2007). Additionally, Fletcher
(2008) found that the addition of least flycatcher vocal cues, regardless of actual
presence of the bird, reduces small-bodied migratory bird species richness by up to
30% through the lack of local colonization. Forsman et al. (2008) found that collared
flycatchers (Ficedula albicollis) used the density of Parus spp. to assess overall habitat
quality and make offspring investment decisions.
Nest protection association, an interaction involving the transmission of social
information, has been described between bird species and between birds and other
taxa such as insects and reptiles. Protective nesting associations occur when one
species places its nest by active choice near that of another, more formidable species
that drives away predators of the first species simply by defending its own territory.
Descriptive studies of protective nesting associations are common (e.g. Myers 1929;
Moreau 1936; Durango 1949; Chisholm 1952; Grimes 1973; Uchida 1986; Richardson
& Bolen 1999); however most have not examined whether the association occurs by
active choice or if there is an actual benefit to association (Haemig 2001).
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Crocodilians are good potential bird nest protectors because they prey on bird nest
predators such as snakes and mammals (Bondavalli & Ulanowicz 1999) and
aggressively defend their own nests (Kushlan & Kushlan 1980). American alligators
(Alligator mississippiensis) have been anecdotally reported to make islands in South
Carolina “predator-secure” for nesting boat-tailed grackles (Quiscalus major) (Post &
Seals 1991; Post 1998a), least bitterns (Ixobrychus exilis) (Post 1998b), and common
moorhens (Gallinula chloropus) (Post & Seals 2000) by deterring mammalian predators
such as raccoons. Robinson (1985) suggested that black caimans (Melanosuchus
niger) and yellow-rumped caciques (Cacicus cela) may have a similar association.
Biologists at the St. Augustine Alligator Farm (St. Johns Co., Florida, USA) have noted
that wading birds began to nest in the trees overhanging the water of their Alligator
Swamp exhibit after its creation and that the birds are apparently protected from
mammalian predators (K. Vliet, personal communication). While these studies noted or
implied an association, they provided no formal test of association. In Ghana, Hudgens
(1997) found that blue-billed malimbes (Malimbus nitens) nested much closer to African
dwarf crocodile (Osteolamus tetraspis) nest sites than would be expected if they nested
at random. Hudgens demonstrated that nesting was nonrandom with respect to
crocodiles, although the costs and benefits to the birds and the reptiles were not
quantified.
Long-legged wading birds (herons, egrets, ibises, storks, spoonbills;
Ciconiiformes) nesting near alligators or alligator habitat in the Everglades of Florida
(USA) may present a good opportunity to study the potential symbiosis between the two
taxa. Anecdotal observations suggest that wading birds preferentially nest above
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alligators and alligator habitat. Birds, mammals, and snakes commonly prey on wading
bird nests (Frederick & Collopy 1989, Coulter & Bryan 1995). Although Ciconiiformes
are colonial nesters, there is almost no group or individual nest protection behavior
(Rodgers 1987). Access to the colony by only a few predators can cause significant
destruction to nest contents and facilitate nest abandonment (Rodgers 1987). Wading
birds thus appear to avoid nest predation by selecting inaccessible nesting sites, such
as islands surrounded by water and inhabited by crocodilians (Frederick & Collopy
1989).
I report here on an experimental examination of the relationship between the
presence of alligators, and nesting colonies of three species of wading birds: little blue
herons (Egretta caerulea), tricolored herons (Egretta tricolor), and snowy egrets
(Egretta thula) (collectively hereafter, Egretta herons) in the Everglades. The central
Everglades is a shallowly inundated (0 – 3 m) freshwater marsh with slightly elevated
sites called tree islands (Loveless 1959). Here, wading birds tend to nest in
aggregations of two to 100 individuals on islands dominated by willow (Salix caroliniana)
(Frederick 1995). These willow “heads” often are associated with an alligator hole; a
small pond created and maintained by American alligators (Mazzotti & Brandt 1994). In
fact, many small willow heads have probably formed as a result of the action of
alligators that dig holes during the dry season, creating slightly elevated areas
surrounding the hole by displacing sediment (Palmer & Mazzotti 2004).
I hypothesized that Egretta herons choose to associate with alligators. Not all
willow heads are occupied by herons, and not all willow heads have alligators
associated with them. I tested the prediction that alligators and wading birds would
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associate more often than expected by chance. In addition, I experimentally
manipulated apparent densities of alligators and conspecific birds using alligator and
bird decoys to determine if wading birds were attracted to alligators via visual cues. I
used maximum numbers of Egretta herons nesting as a response variable to four
different types of decoy treatments.
Methods
Study Area
Study sites were located in Water Conservation Area-3A (WCA-3A) of Dade and
Broward Counties, Florida. WCA-3A is a large (cf 400 km2) impounded area of
seasonally flooded sawgrass and wet prairie dotted with small tree islands (Figure 1-1).
Although there are numerous types of tree islands in the Everglades (Loveless 1959),
Egretta herons have been shown to nest almost exclusively on willow-dominated tree
islands (Frederick & Collopy 1988, 1989). I selected 40 small willow-head tree islands,
all of which had an alligator hole (Figure 1-2). Based on systematic annual survey
information, all sites had been unoccupied by nesting wading birds for at least 16
consecutive years (P. Frederick unpublished database). Islands ranged from
approximately 450 square meters to 5,000 square meters (mean = 1,200 square
meters). This range of island sizes is typical of Egretta heron nesting habitat in Water
Conservation Area 3A (P. Frederick unpublished database). All sites were examined on
the ground to confirm willow tree structure appropriate for wading bird nesting and
alligator hole presence. Only sites with alligator holes were chosen because the holes
are a visible indication of regular alligator presence.
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Wading Bird – Alligator Association Survey
I surveyed 40 willow-dominated tree islands using a helicopter (Bell 206B
JetRanger III) on April 27, 2011 from 0715 until 0900. The survey was timed to coincide
with the point in the small wading bird nesting cycle where most eggs had hatched, but
no young had fledged. This increased our chances of at least one adult being present
at the nest to brood young. The helicopter hovered at approximately 125 meters after
approaching the island from the east. Two observers sat on the left side of the aircraft.
One observer counted, identified, and recorded all small wading birds flushed from the
island. The other observer took photographs through the window using a Canon EOS
50D with a 28–135mm image stabilizing lens. Both observers searched for alligators or
recent tracks in the mud. Due to low water conditions in WCA-3A at the time, alligator
holes were the only areas in the marsh with standing water. Alligator tracks to and from
their holes were readily visible in the surrounding mud.
I accepted the count of birds flushed from the island plus birds seen roosting in
tree tops in the photos as the maximum count for each island and species. Alligators
were deemed present when an alligator or fresh tracks leading to the alligator hole were
visible from the air or in the photographs. I used Fisher’s Exact Test with an alpha of
0.05 to test for an association between presence-absence of alligators and wading
birds.
Decoy Experiment Design
I manipulated apparent densities of alligators and wading birds using decoys, with
tree islands as the unit of treatment. I used four different treatments: 10 alligator
decoys, 20 bird decoys, 10 alligator plus 20 bird decoys, and no decoys. Each
treatment was replicated 10 times, one per tree island. Decoy treatments were
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randomly assigned to tree island sites. Live alligators were present and free to move
throughout the study area for the duration of the study, therefore the treatments with
alligator decoys were intended to create a supernormal stimulus.
Alligator decoys were cast using polyurethane spray insulation (Foam It Green®,
Guardian Energy Technologies, Inc., Riverwoods, Illinois) from a silicone mold of a 2.3
meter harvested alligator. The mold was for the top half of the alligator, making the
decoys look as if they were floating at the surface of the water. The alligator decoys
were larger than 1.8 meters, the minimum size at which alligators begin to breed and
defend territories. The size of decoy used ensured that the decoys would represent a
dominant individual that could both occupy a gator hole and be a significant predation
threat to a raccoon. Alligator decoys were painted in realistic colors using latex paint
applied via a pneumatic spray gun (Figure 1-3).
Bird decoy arrays presented at each island were a mixture of 3 commercially
available great egret decoys (Flambeau, Inc.®, Middlefield, Ohio, Figure 1-4) and 15
modified lawn flamingo decoys (Garden Plast, Inc., Accra, Ghana). Flamingo decoys
were painted white with a pneumatic spray gun, and their heads replaced by a
polyurethane-cast piece similar to a small, white wading bird’s head and bill structure
(Figure 1-5). Modified flamingo decoys were used due to their cost-effectiveness and
proven ability to initially attract small wading birds (Dusi 1985, Crozier & Gawlik 2003).
Bird decoys were placed at mid-canopy height in willow trees surrounding the
alligator hole at each treatment site in early March 2010, 1 – 3 weeks prior to the
initiation of breeding by Egretta herons (Figure 1-6). Egretta herons were not observed
in WCA-3A before this time, so it is unlikely that the birds chose breeding locations prior
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to decoy installation. Alligator decoys were positioned in the water at the island-open-
water interface surrounding tree islands. Islands without decoys were entered and
explored in the same fashion as other treatments. All decoys were removed from the
Everglades by July 2010, after the wading bird breeding season.
I repeated the study in 2011, installing the decoys in mid-February 2011 prior to
Egretta heron arrival to WCA-3A. In anticipation of impending low water levels, I
replaced the 20 northern-most treatment sites with new ones farther south, in deeper
water areas of WCA-3A in an attempt to ensure that all sites would retain surface water
during the entire breeding season. All 40 sites were randomly re-assigned decoy
treatments. No site in 2011 had the same decoy treatment as it did in 2010.
Detecting Responses to Decoys
Treatment sites were surveyed using airboats, which provided access to all wetted
areas of WCA-3A. We approached each site as closely as possible with the airboat to
flush any nesting birds. All ciconiiform birds were identified and counted (Frederick et
al. 1996). Nests visible from the exterior of the island were also noted. Survey
frequency varied slightly from 2010 to 2011 due to manpower availability and
environmental conditions. 2010 was an abnormally wet year in the Everglades and all
sites were accessible for the entire study period. Three survey periods were conducted:
early breeding (late March – early April), mid-breeding (late April – early May), and late
breeding (late May – early June). Maximum counts of each species at each colony
during these surveys were taken as the response variable. Drought and low water
conditions in 2011 limited accessibility by airboat as the season progressed. Surveys
were conducted bi-weekly beginning the first week of March and continued until the last
week of April, when only two of the 40 sites were accessible via airboat. The helicopter
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survey (above) was used as a final survey of avian response to decoy treatments in
2011.
Statistical Analysis
I used a Chi-squared test of equal proportions to detect departures from an even
distribution of the maximum number of birds nesting at each site by treatment group for
both 2010 and 2011. Years were compared separately due to the difference in survey
timing and regularity in 2010 versus 2011. Alpha was equal to 0.05 for each analysis.
To further examine bird response, I performed post-hoc pairwise comparisons between
all treatments. A Bonferroni adjustment was applied to maintain the familywise error
rate. I divided alpha (0.05) by the number of pairwise comparisons (6), to obtain the
adjusted alpha, 0.008333. Additionally, I also used presence rather than number of
birds as a response variable to the experimental treatments. I compared the number of
islands per treatment group that birds nested on using a Chi-squared test of equal
proportions to detect departures from an even distribution of bird presence.
Results
Wading Bird-Alligator Association
Alligators and wading birds were found in association more often than expected by
chance (p=0.002176, N=40, Fisher’s Exact Test) (Figure 1-7). The two groups were
found together at 60% of islands surveyed. Alligators were found alone at 20% of the
sites. Birds were found without alligators at 2.5% of the islands. Seven sites were
occupied by neither birds nor alligators (17.5%). It is important to note that Fisher’s
Exact Test assumes that 50% of alligator holes will be unoccupied by alligators.
Campbell & Mazzotti (2004) found that in WCA 3, 44% of alligator holes (similar to
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those in my study) were unoccupied by alligators during the spring. Wading birds and
alligators were clearly associating more often than expected by chance.
Bird Response to Decoys
The distribution of nesting herons relative to decoy treatment was significantly
different from random in 2010 (Χ2(3,N=45)=17.133, p=0.001) (Figure 1-8) and 2011
(Χ2(3,N=261)=72.452, p=0.0001) (Figure 1-9). To make these two experiments more
directly comparable, I subsampled the 2011 bird response data to mimic the 3-period
sampling regime used in 2010. I found that the distribution of Egretta herons relative to
decoy treatment was still significantly different than random (Χ2(3,N=230)=58.383,
p=0.0001).
In 2010, pairwise comparisons (Table 1-1) showed that birds responded more
positively to the alligator+bird treatment than to the birds-only treatment
(Χ2(1,N=31)=7.258, p= 0.00706) or the no-decoy treatment (Χ2(1,N=28)=11.571, p=
0.000670). There was no significant difference between the numbers of birds attracted
to the alligator+bird treatment and the alligator-only treatment (Χ2(1,N=32)=6.125, p=
0.0133). Furthermore, there was no significant difference between the numbers of birds
attracted to the birds-only treatment, the alligators-only treatment, and the no decoy
treatment. In 2011, pairwise comparisons (Table 1-2) showed the alligator+bird
treatment response was higher than all other groups (p<.0001). There was no
significant difference between birds attracted to the bird decoy treatment and the no
decoy treatment (Χ2(1,N=167)= 1.012, p=0.2636). Birds were significantly less attracted
to the alligator decoy treatment than the bird decoy (Χ2(1,N=131)= 18.328, p=0.0001) or
no decoy (Χ2(1,N=118)= 10.983, p=0.007) treatments in 2011.
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The number of islands per treatment group that birds nested on was not
significantly different from random in 2010 (Χ2(3,N=40)=0.200, p=0.9776) (Figure 1-10)
or 2011 (Χ2(3,N=40)=2.000, p=0.5724) (Figure 1-11).
Discussion
My results from both the helicopter survey and the decoy experiment strongly
suggest that birds and alligators occur together in a nonrandom fashion, and that birds
are attracted to a visual display of both alligators and birds. I interpret the decoy
experiment to mean that wading birds are attracted to the visual stimulus of alligators,
but only when other wading birds are present. Wading birds were clearly not attracted
to alligators nor wading birds when presented by themselves. The combination of the
two decoy types appeared to be necessary to create an attractive environment for
nesting in novel locations.
Wading birds are often temporarily attracted to white decoys of various kinds in
feeding areas (Crozier & Gawlik 2003, Heath & Frederick 2003). Unlike many seabirds
(Atlantic puffins, Fratercula arctica, Kress & Nettleship 1988; least terns, Sterna
antillarum, Massey 1981; Kotliar & Burger 1984; Burger 1989; Arctic terns Sterna
paradisaea, Kress 1983; and common terns, Sterna hirundo, Dunlop et al. 1991;
Blokpoel et al. 1997), there is no evidence that wading birds can be induced to nest in
novel locations through the use of conspecific decoys alone. Dusi (1985) attempted to
attract little blue herons and cattle egrets (Bubulcus ibis) to a small island with
homemade decoys and recordings of each species. Although seven different species of
wading birds visited the island briefly, none nested during the 3-year study. I found no
difference in nesting response between the no decoy treatment and the bird-only
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treatment. Together with the results presented here, this suggests that white decoys
alone are insufficient to stimulate Egretta herons to nest in novel locations.
The alligator decoys alone could have been non-attractive to herons for several
reasons. I hypothesize that herons may have simply not seen the alligator decoys
because the decoys were camouflaged to the point that it was sometimes difficult for
humans to detect them. Without the long-range stimulus of the highly visible white bird
decoys, it is possible that birds never visited the islands and therefore did not assess
the alligator decoys. It is also possible that the form of the alligator alone, without the
presence of conspecifics, was not a strong enough signal to incite nesting.
The mechanisms by which protective nesting associations occur have rarely been
experimentally addressed (Haemig 2001). My study demonstrates the existence of a
strong association between herons and alligators that probably extends to other
crocodilians. The association may come from alligator attraction to birds, or bird
attraction to alligators, or both. The fact that wading birds nested without live alligators
only 2.5% of the time, and that they were attracted to novel nesting locations by the
combination of decoy types, suggests that the highly mobile birds are probably actively
choosing alligators or their recently occupied habitat, to nest in. Furthermore, alligators
are known to be highly territorial during the wading bird nesting season, and may not be
able to aggregate on wading bird colonies. This suggests, but does not prove, that
wading birds are the active choosers in this association.
My analysis used numbers of birds as a response variable rather than bird
presence on a decoy treatment site, and the result may be driven by a handful of sites
that had relatively large avian response. However, since the birds are highly social,
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aggregative nesters, presence alone may be a strenuous test of the prediction that birds
are attracted to alligators. This is especially true since the treatment site locations were
all places birds had avoided for at least 16 years prior and therefore may have had
other physical or biotic features that were less than attractive for nesting.
The decoy experiment suggested that Egretta herons choose to nest in areas
based at least partly on social information from both conspecifics (other Egretta herons)
and heterospecifics (alligators). This use of dual or multiple social information sources
may be similar to the process by which other protective nesting associations between
colonial species and nest protectors occur; such as red-breasted geese (Branta
ruficollis) nesting near peregrine falcons (Falco peregrinus), or snowy owls (Nyctea
scandiaca) (Quinn et al. 2003), blue-billed malimbes nesting near dwarf crocodiles
(Hudgens 1997) and yellow-rumped caciques nesting near black caiman (Robinson
1985). Regardless of the specific mechanism by which Egretta herons choose their
nesting habitat, my experiment showed that the birds can be attracted to novel nesting
sites with the use of a combination of alligator and white wading bird decoys. This
combination of decoys shows strong potential as a management tool for moving birds
from an undesirable to a desirable location. It is unclear what part of the recognition of
social information in nesting birds comes about via genetically programmed sign stimuli,
or as part of a learning process through experience. In either case, I suggest that
recognition of nest habitat quality through inter and intraspecific social cues may have
wide applicability to birds.
24
Figure 1-1. Map of study area in south Florida, USA with Water Conservation Area 3A
(WCA-3A), highlighted in green. Map courtesy of the U.S. Geological Survey.
25
Figure 1-2. Aerial photograph of an alligator hole and small willow-dominated tree
island in WCA-3A. The alligator hole is the darkest part of the image in the center of the photograph. Well-defined alligator tracks leading to the hole are visible in the right-hand portion of the photograph. Photo courtesy of author.
26
Figure 1-3. Photograph of an alligator decoy in the field. Photo courtesy of author.
27
Figure 1-4. Photograph of a great egret decoy overlooking an alligator hole, mounted
on a willow tree in the field. Photo courtesy of author.
28
Figure 1-5. Photograph of an altered flamingo decoy in the field. Photo courtesy of
author.
29
Figure 1-6. Photograph of a bird decoy array at mid-canopy height in willow trees
surrounding an alligator hole. Photo courtesy of author.
30
0
5
10
15
20
25
Gators + / Birds + Gators +/ Birds - Gators -/ Birds + Gators -/ Birds -
Num
ber o
f Site
s
Group presence (+)/ Absence (-)
*
Figure 1-7. Comparison of alligator and Egretta heron distribution on small willow-
dominated tree islands in the Everglades of Florida. Alligators and Egretta herons were found at the same sites more than expected (p=0.002176, N=40, Fisher’s Exact Test).
31
Figure 1-8. Totals of maximum number of Egretta herons per treatment site that
responded to each decoy treatment; 2010. Bars with different letters were significantly different, Bonferroni test.
32
Figure 1-9. Totals of maximum number of Egretta herons per treatment site that
responded to each decoy treatment; 2011. Bars with different letters were significantly different, Bonferroni test.
33
Table 1-1. Pairwise comparisons of numbers of birds responding to decoy treatments in 2010. Alpha = 0.008333.
Decoy treatment 1
Decoy treatment 2
N= Chi square value
p= Significant
Gator + Bird Control 28 11.571 0.000670 yes Gator + Bird Bird 31 7.258 0.00706 yes Gator + Bird Gator 32 6.125 0.0133 no Control Gator 14 1.143 0.285 no Bird Gator 17 0.059 0.808 no Bird Control 13 0.692 0.405 no
Table 1-2. Pairwise comparisons of numbers of birds responding to decoy treatments in
2011. Alpha = 0.008333. Decoy treatment 1
Decoy treatment 2
N= Chi square value
p= Significant
Gator + Bird Control 230 25.113 5.40E-07 yes Gator + Bird Bird 243 16.333 0.0000531 yes Gator + Bird Gator 194 64.66 <<0.00001 yes Control Gator 118 10.983 0.000920 yes Bird Gator 131 18.328 0.0000186 yes Bird Control 167 1.012 0.314 no
34
Figure 1-10. Numbers of islands per treatment group that Egretta herons nested on;
2010.
35
Figure 1-11. Numbers of islands per treatment group that Egretta herons nested on;
2011.
36
CHAPTER 2 DO WADING BIRDS BENEFIT FROM NESTING NEAR ALLIGATORS?
Introduction
Nest predation is a strong selective force in the evolution of avian nesting behavior
and life history strategies (Martin 1993), and in the evolution of coloniality (Brown et al.
1990). Early warning, group defense, and predator swamping through synchronous
breeding have often been listed as potential benefits of coloniality (Danchin & Wagner
1997). However, ciconiforms exhibit little group or individual nest protection behavior
(Rogers 1987). In addition, mammalian predators such as raccoons (Procyon lotor) are
capable of destroying a large number of nests in wading bird and seabird colonies in a
short amount of time, often leading to abandonment of the entire colony (Rodgers 1987;
Kelly et al. 1993), suggesting that swamping through synchronous breeding may be an
unlikely benefit for many kinds of colonially breeding birds.
In the central Everglades wetlands of Florida, USA, herons in the genus Egretta
tend to nest on small, slightly elevated islands dominated by willow trees (Frederick
1995). These islands are often associated with an alligator hole; an area where an
American alligator (Alligator mississippiensis) excavates and maintains a small pond
which it uses for reproduction and survival (Mazzotti & Brandt 1994). Water levels in
the central Everglades sometimes fall below the ground surface during the annual dry
season and alligator holes tend to hold water during that period (Davis et al. 2005).
Many birds nest in inaccessible island locations to avoid mammalian predators
and Egretta herons may nest on tree islands in the Everglades to do so as well. For
example, Strong et al. (1991) and Erwin et al. (1995) found that white-crowned pigeons
and great white herons, respectively, avoided nesting on islands that could harbor
37
mammalian predators in Florida Bay (Florida, USA). Others have inferred that
inaccessible locations offer decreased predation risk by comparing predation rates at
sites accessible and not-accessible to mammalian predators (Robinson 1985; Post
1990; Kelly et al. 1993). Finally, some studies have reported an increase in nest
predation when water levels in wetlands decrease significantly, apparently allowing
access to colonies by semiaquatic mammals (Rodgers 1987; Frederick & Collopy 1989;
Post & Seals 1991; Kelly 1993; Coulter & Bryan 1995).
It is unclear why nest predation by semiaquatic mammals should be responsive to
changes in water levels in shallowly inundated wetlands. Raccoons have been known
to make open-water crossings of up to 950 meters (Hartman & Eastman 1999), and
readily move among widely separated offshore islands (200 meters) to prey on nests
and eggs of waterbirds (Ellis et al. 2007). The water in many wetlands is generally
shallow (0-3 meters) (eg, Everglades, Loveless 1959, Mitsch & Gosselink 1993) and
vegetation within wetlands often provides resting substrate for swimming mammals.
This evidence suggests that even large expanses of open water are not likely to
function as a deterrent to raccoon use of wetlands and island archipelagoes.
It therefore seems likely that there are other deterrents to small mammals
traversing marshes like the Everglades. The threat of predation by crocodilians may be
a strong force. For example, researchers have long noted the occurrence of raccoons
as prey items of large (>1.8 meters total length) alligators (Giles & Childs 1949, Barr
1997, Shoop & Ruckdeschel 1990, Rice 2004). In the southeast United States,
alligators are commonly found in freshwater lakes and marshes where many kinds of
waterbirds nest, and it has been suggested that the presence of alligators prevents
38
raccoons and other predators from reaching island nesting sites (Jenni 1969; Post
1998). Ruckdeschel & Shoop (1987) found that wood stork (Mycteria americana)
nesting success was related to water depth beneath the heronry and dropped sharply
as the water level decreased. They attributed the decrease in nest success to an
increase in raccoon access and predation within the colony. Ruckdeschel & Shoop
posited that alligator presence in the swamp below the colony was a deterrent to
raccoons when water levels were high, but not when they were low. Coulter & Bryan
(1995) also reported that raccoons were able to access and prey upon nestlings in a
wood stork colony in east-central Georgia when the swamp under the nest trees
became dry. Frederick & Collopy (1989) demonstrated that other common Egretta nest
predator groups, snakes and birds, were minor to nonexistent threats in the central
Everglades. They also showed that in years where the marsh stays flooded, nest
predation is relatively low. Furthermore, Frederick & Collopy (1989) found that
raccoons were very sparsely distributed throughout the deeper marshes of the central
Everglades and that their presence on tree islands was limited to areas of the marsh
that were nearly dry.
After there is no surface water in the marsh, the threat alligators pose to raccoons
and other mammals probably decreases due to decreased mobility of the alligator and
increased mobility of the raccoon. Fleming et al. (1976) found a marked difference
between raccoon predation on alligator nests in a Louisiana marsh ranging from no
predation during a year with high water levels to 45 percent during a subsequent
drought year. Similarly, Hunt and Ogden (1991) found that high predation rates on
alligator nests by bears in a Louisiana marsh were associated with periods of low water.
39
They attributed the increased predation rates to the alligators’ diminished ability to
attack bears in low water.
Thus there is reasonable evidence that alligators may present a strong deterrent to
raccoons and other mammalian nest predators during flooded conditions. Wading birds
apparently cue in on alligator presence when deciding where to nest, and may take
advantage of the predator protection that alligators offer. I found that Egretta herons
preferred to nest on islands with alligator and wading bird decoys and that Egretta
herons and alligators were more likely to be found at the same tree island than alone
(Chapter 1).
I suggest that when there is standing water in marshes, the presence of alligators
limits the access of raccoons to the marsh and bird colonies. Conversely, I predicted
that if surface water is very low or nonexistent, the deterrent effect of alligators
diminishes considerably, and that raccoons would therefore have access to the marsh
and to wading bird colonies. To understand the relationship between water, nesting
wading birds and alligators, I examined the distribution of potential mammalian
predators in relation to water levels and nesting distributions. I also documented the
effect that raccoons can have on a wading bird colony.
Methods
Predator Distribution
I estimated the local relative abundance of potential mammalian nest predators by
attracting them to baited tracking stations during peak of the Egretta heron nesting
period from mid-April through June in 2010 and 2011. Tracking stations were 1 m by 1
m panel board finished with a smooth acrylic coating. Each station was placed on level
ground and the surrounding area was cleared of branches and debris. Stations were
40
baited with an open can of sardines packed in oil, placed within a horizontal 10-cm
section of aluminum drain pipe, open at each end. The sardines and drain pipe were
fastened to the panel board to prevent animals from removing the bait. When checked,
the panel board was dusted with blue chalk dust to make any tracks visible. Beginning
in April 2011 five tracking stations were placed in 500-meter increments on levees on
each side of the L-67C canal bordering WCA-3A (Figure 2-1). Four stations were
placed between 300 and 800 meters into the marsh away from the L-67C levee on dry
land on three willow-dominated tree islands typical of Egretta heron nesting habitat, but
which were unoccupied by nesting birds at the time. Eight tracking stations were also
placed between 7.0 km and 7.4 km away from levees on three large tree islands (cf
40,000 m2) with permanent dry ground, and on five small willow-dominated tree islands
(cf 600 m2) within 500 meters of the large islands. From 14 April until 7 June 2010, I
monitored these 23 stations for a total of 140 trap nights. I included infared game
cameras (Cuddeback®, Green Bay, WI) aimed at 6 of the stations for 26 trap nights from
1 June until 7 June. All stations were moved after a predator was detected or one week
had elapsed.
I also monitored 14 tracking stations for 186 trap nights from 21 April until 17 May
2011. I simplified the tracking station design in 2011, suspending a sardine can with
holes punched in it 1 m above the ground from a string tied to a branch. I used
Cuddeback Capture IR® game cameras to monitor all the sites. These tracking stations
were again placed on six large tree islands with permanent dry ground and on eight
nearby (within 500 meters) willow head tree islands unoccupied by, but typical nesting
41
habitat of Egretta herons within WCA-3A. I did not place tracking stations on levees in
2011.
Survival rates of nestlings and fledgling herons
To assess the impact of predation on fates of young herons in colonies, we
individually marked 9 little blue herons and 17 tricolored herons from a single nesting
colony on a 0.09 ha willow head island (N25 52.614, W80 45.318). Unique
combinations of numbered red, green and yellow coiled plastic leg bands (National
Band and Tag Co., Newport, KY) were placed on the tarsometatarsus. All banded birds
were hand-caught and banded at approximately 14 days old, after they had left the nest
but before they were able to fly (Frederick et al. 1993).
To minimize disturbance to the colony, all birds were captured, banded, and
released within one hour on 29 April, 2011. I visited the island three subsequent times,
ending on 17 May, 2011. I spent one hour during each visit with two observers
attempting to re-sight the banded birds. I ended the re-sighting efforts when the birds
became adept at flying and could leave the island during the survey attempt.
Results
Predator Distribution
In 2010, I did not detect any mid-sized or large mammals at the WCA-3A
impoundment stations during 136 trap nights (Figure 2-1). Raccoon presence was
detected on one night at 2 out of 6 stations on the mainland levee bordering the L-67C
canal that bounds the southeast edge of WCA-3A. I detected opossums (Didelphis
virginiana) (n=4) on the L-67C levee located between the marsh and the canal. The
only potential nest predator that I detected in WCA-3A was an unidentified rat species
(n=2) on two small willow tree islands. Additionally, an alligator (n=1), a Florida softshell
42
turtle (Apalone ferox) (n=1), turkey vultures (Cathartes aura) (n=4), and nine-banded
armadillos (Dasypus novemcinctus) (n=3) visited the tracking stations within WCA 3A in
2010. None of the latter are considered potential nest predators of herons.
Mammalian nest predator distribution in WCA-3A was very different in 2011.
Raccoons were found at 8 different sites over 186 trap nights (Figure 2-1). Raccoons
were detected on large islands with permanent dry ground and on small willow-
dominated islands. I also detected rice rats (Oryzomys palustris) (n=4) and a black rat
(Rattus rattus) (n=1) on both large and small tree islands. River otters (Lutra
canadensis) were detected at two small willow island sites but are not arboreal and
therefore considered implausible as nest predators. An alligator (n=1), turkey vultures
(n=4), a limpkin (Aramus guarauna) (n=1), a yellow-crowned night heron (Nyctanassa
violacea) (n=1), a black-crowned night heron (Nycticorax nycticorax) (n=1), boat-tailed
grackles (Quiscalus major) (n=2) and a common grackle (Quiscalus quiscula) (n=1) also
visited the tracking stations.
Water Depth
Water depth was greater than normal during 2010 and lower than normal during
2011. Water stages from the 3-4 gauge in central Water Conservation Area-3A were
greater than historical monthly maximum averages from January 2010 until late-October
2010 and exceeded one standard deviation higher than historical monthly maximum
averages from late-April 2010 until mid-October 2010 (Figure 2-2). Water stages were
less than historical monthly minimum averages from November 2011 through June
2011 and exceeded one standard deviation lower than historical monthly minimum
averages from mid-November 2011 through June 2011.
43
Color-Banded Birds
In the 2011 nestling survival study, I resighted 18 individuals (70% of 26 banded
birds) 6 days after their initial capture (Figure 2-3). When I returned 7 days later, I found
a large number of dead young, mostly dismembered. The majority of the remains
consisted only of the legs, allowing me to recover a number of color bands. Eight of the
dead birds had been banded 13 days prior (23% of banded birds, 37.5% of birds
resighted that day). Raccoon tracks and scat containing bird feathers (Figure 2-4)
around and on the island indicated the major source of predation. To further confirm
raccoon presence in the area, a predator tracking station was installed on the island;
raccoons visited the station. I also resighted 10 live banded herons (38% of banded
birds) during my second visit to the island. I visited the island a third time and found an
additional preyed upon young bird (9 dead total, 35% of banded birds). I resighted 4
live birds (15% of banded birds). Most of the young birds present flew into the
surrounding slough when observers entered the island on this last visit, suggesting that
most birds had become flighted by this point and were less vulnerable to mammalian
predation.
Discussion
Raccoon presence in WCA-3A appeared to be directly related to water depth.
2010 was an exceptionally wet year in the Everglades and water stages were greater
than historical monthly maximum averages and exceeded one standard deviation higher
than historical monthly maximum averages from April until October. Raccoons were
present in low occurrences on the levees across the canal from WCA-3A but were
never detected in the marsh, including on large islands that have year-round dry
ground. Distances across the canal surrounding WCA-3A and between tree islands or
44
dry ground within WCA-3A (50 m – 500 m) are significantly less than those which
raccoons have been shown to swim across in order to prey upon bird nests in alligator
free environments (Hartman & Eastman 1999). Thus, it seems unlikely that surface
water in the marsh by itself is sufficient to keep raccoons from heron nesting sites.
I believe that the absence of raccoons in 2010 was due to the threat of predation
posed by swimming alligators in WCA-3A. This argument is further evidenced by the
drastic shift in raccoon presence in WCA-3A during the extremely dry conditions of
2011. Unlike 2010, almost the entire marsh in WCA-3A was devoid of surface water in
2011 at the end of the Egretta heron nesting season (mid-May until mid-June). A
helicopter survey at this time indicated that alligators were almost entirely confined to
small pools and deeper canals (Chapter 1). The lack of water therefore dramatically
decreased the alligators’ mobility and distribution, and thus the predation threat that
they posed to raccoons (Willey et al. 2004). Raccoons were found to be present in both
WCA-3A on large islands with permanent dry ground and on small willow-head islands
that usually have no dry ground, up to 13 kilometers from the edge of WCA-3A.
Raccoons were detected in 2011 on a large island that was raccoon free 2010. The
island where I had a large proportion of our color-banded birds preyed upon by
raccoons was at least 11.5 kilometers from permanent dry ground at the edge of WCA-
3A, the nearest location they had been detected in 2010. In mid-May 2011, raccoons
preyed upon 9 of the young herons that I color-banded. Similar to patterns seen by
other researchers (Ruckdeschel & Shoop 1987, Frederick & Collopy 1989, Coulter &
Bryan 1995), the predation was associated with a low water period. These observations
45
suggest that raccoons are highly attracted to wading bird colonies, and capable of
locating widely dispersed colonies even at long distances.
In addition to this study, the relationship between crocodilian movement, water
depth and mammalian presence has been suggested by Ruckdeschel & Shoop (1987)
and Frederick & Collopy (1989). It has also been noted that raccoons readily entered
an inundated impounded marsh without alligators in North Carolina and preyed upon
hundreds of muskrat nests (Wilson 1953). Furthermore, Urban (1970) found that 87
percent of the home ranges of raccoons in a managed wetland near Lake Erie were
inundated marsh (including 25 percent open water).
Together with these studies, my results suggest that Egretta heron nests could be
more vulnerable to raccoon predation in parts of their range where alligators are not
present. This results in the prediction that outside of the range of alligators, colonial
birds are more reliant on large distance from mainland, nesting tree structure, or other
potential mammalian predators to reduce access by mammals. Indeed, wading birds in
the northern United States nest exclusively on islands in the middle of large bays
(Parsons et al. 2001, Parsons 2003, Paton et al. 2005) rather than in shallower
wetlands.
46
Figure 2-1. Satellite image (adapted from Google Earth v.6.1; 2011) of predator
tracking station locations in 2010 (diamonds) and 2011 (circles) in the southeast corner of WCA-3A. Dark green areas are tree islands. Raccoon presence is denoted with a yellow icon (diamond, 2010; circle 2011). Canals are highlighted in red. Location of the satellite image highlighted in green on map of southern Florida (courtesy of the U.S. Geological Survey) in lower right corner of figure.
47
Figure 2-2. Stages at 3-4 gauge in WCA-3A from January 2010 until July 2011. Daily stage is shown as a solid line. Period of record monthly mean minimums (squares), maximums (x’s), minimums minus 1 standard deviation (triangles), and maximums plus 1 standard deviation (asterisks) are shown for reference to long term trends.
48
Figure 2-3. Cumulative percentage per day of color-banded Egretta herons resighted,
dead, or not seen over three 1-hour observation periods at a single colony in the Florida Everglades.
49
Figure 2-4. Raccoon scat containing feathers of young herons. Photo courtesy of
author.
50
CHAPTER 3
DO ALLIGATORS BENEFIT FROM ASSOCIATING WITH NESTING WADING BIRDS?
Introduction
Alligators (Alligator mississippiensis) are top predators in wetland systems
throughout their range in the southeastern United States. As such, they consume a
wide variety of prey items. Small alligators mainly consume molluscs, crustaceans, and
insects (Barr 1997). Large alligators tend to eat fish, mammals, birds, reptiles, and
amphibians (Barr 1997). However, alligators are opportunistic feeders and readily
consume any available prey (Delany and Abercrombie 1986, Barr 1997). Alligators
have been reported as opportunistic scavengers below wading bird nesting colonies
and as active predators of mammals that prey upon wading bird eggs and young. Dusi
and Dusi (1968) noted that alligators inhabiting the marsh below a bird colony in
Alabama ate nestlings that fell into the water in addition to restricting other predators
from moving through the trees. Jenni (1969) also noted several instances of young
birds from an island colony in Florida falling out of nests being preyed upon by
alligators. Although they are top carnivores in the Everglades ecosystem, alligators in
the region are known to be seasonally food-limited (Jacobsen and Kushlan 1989,
Mazzotti and Brandt 1994, Fujisaki et al. 2009). They grow more slowly, take longer to
reach maturity, and are smaller at maturity than in other regions of their range
(Jacobsen and Kushlan 1989, Mazzotti and Brandt 1994). Barr (1997) found that birds
were the most important prey item by mass for adult and subadult alligators in the
Everglades, although they were found in few samples. This indicates that capturing a
bird could be nutritionally important to the food-limited alligators of the Everglades. Barr
also found evidence of alligator predation on raccoons (Procyon lotor) and several rat
51
species (Florida water rat, Neofiber alleni; rice rat, Oryzomys palustris; cotton rat,
Sigmodon hispidus), all of which are potential wading bird nest predators. In addition to
consuming birds and their nest predators, all size classes of alligators also eat crayfish
(Procambrus sp.) and grass shrimp (Palaemonetes intermedius, Barr 1997), two prey
groups that adult Egretta herons commonly regurgitate for young at the nest, and that
may spill into the water below (B.Burtner, personal observation).
Wading bird nesting aggregations are associated with alligator holes significantly
more often than chance (Chapter 1), probably as a result of active choice by birds.
Based on the reported eating habits of alligators, I hypothesized that alligators
benefitted from associating with wading birds through direct food inputs (dropped eggs,
chicks, grass shrimp, crayfish) from the colony. Because Everglades alligators may be
food limited, I hypothesized that the amount of food input from the colony could be
nutritionally significant to the alligators. I examined this question by measuring potential
food inputs from birds, and estimated the nutritional content that might be used by
alligators. Alligators could also benefit from associating with wading birds through
opportunistic predation of wading birds themselves as well as wading bird nest
predators. However, quantifying raccoons being eaten by alligators was beyond the
scope of this study.
Methods
I measured potential food for alligators from wading bird nests by measuring
dropped regurgitant, nestlings, and other protein below nests. I placed throughfall traps
below 57 Egretta heron nests in 4 colonies in 2010 and 2011 (Table 3-1) to quantify the
amount of food an alligator could derive from a typical colony. Throughfall traps were
constructed from heavy-duty black plastic garbage bags modified into the shape of a
52
funnel, mounted on a 75 cm diameter frame of chicken wire. The traps had a window
screen drain inserted into the bottom to allow for water drainage. Traps were centered
directly below nests and secured with string affixed to surrounding branches (Figure 3-
1). Since young herons and egrets spend at least half the time from hatching to
independence (30 – 40 days, Werschkul 1979, Erwin et al. 1996 a,b, Frederick 1997) in
the colony but away from the nest, I also measured throughfall at 20 additional traps
placed at random locations about the colony.
To minimize bird disturbance and risk of colony abandonment, I installed traps
during a 1-hour maximum visit to each colony when the majority of nests in the colony
were in incubation phase with full clutches (3-4 eggs/ nest). In 2010, 40 traps were
divided between 2 colony islands in WCA-3A. One island had 12 traps below nests and
12 traps in random locations within the nesting area. The second island had 8 traps
below nests and 8 traps in random locations. In 2011, all 40 traps (20 under nests, 20
random locations) were initially placed on one of the two islands used in 2010.
However, nestlings were repeatedly preyed upon by a great-horned owl (Bubo
virginianus) and the colony slowly abandoned during a two week period. I moved the
traps to a second colony where unhatched nests were found and installed 24 traps (12
under nests, 12 random).
I returned to the colony every 4-7 days to monitor the traps and nest contents. In
2011, 5 nests failed after the first week of study. I moved the traps below those nests to
other nests with full clutches of unhatched eggs. During each visit I recorded number of
eggs and chicks present and their species. I removed all debris from the trap and
measured fresh weight to the nearest gram and counted all food items that could be
53
consumed by alligators. All nests were followed until failure or young had left the nest
structure.
Results
In 2010 I found grass shrimp, crayfish, whole Egretta eggs, and an Egretta chick in
the throughfall traps. The number of collection days from a single nest ranged from 7 -
35 days. Of the 20 nests I followed in 2010, 10% (2 nests) failed before hatching (Table
3-2). Of the 18 nests that produced chicks, 72.2% (13 nests) fledged at least one chick.
All of the food items were found in traps below nests and no items were found in the
traps placed in random locations about the colony. The total wet mass of all food items
collected from 20 nests over 440 collection days in 2010 was 299 grams (Table 3-3).
Total shrimp input was 39 grams (39 shrimp). Total crayfish mass was 53 grams (31
crayfish). Total egg mass was 73 grams (3 eggs). Total chick mass was 134 grams (1
little blue heron chick). Average throughfall per nest was 14.95 grams (standard
deviation: 31.23 grams) and per nest per day was 0.034 grams (standard deviation:
1.59 g/nest/day). When extrapolated to a typical sized colony of Egretta herons in the
Everglades (50 nests) average input per nest, per day was 1.70 grams per day
(standard deviation: 79.35 grams). After 20, 40, and 60 days, a 50-nest colony’s
average input would be 33.98, 67.95, and 101.93 grams of food, respectively (Table 3-
5).
In 2011, I found far fewer food items in throughfall traps than in 2010. However,
there were a large number of nests that had been preyed upon and abandoned, and a
high proportion that failed before chicks hatched. Of the 37 nests that I followed, 67.6%
(25 nests) failed prior to hatching. Ten (83.3%) of the 12 nests that produced chicks
54
were preyed upon or failed less than a week after hatching. The remaining two nests
were preyed upon or failed less than two weeks after hatching.
I found heron eggs, shrimp, and half of a fish in the throughfall traps in 2011. Total
food mass collected was 144 grams over 695 collection days (Table 3-4). Eggs made
up the vast majority of the food input (142 grams, 6 eggs). The two shrimp weighed 1
gram. The half of a fish also weighed 1 gram. The shrimp and fish were found in traps
below nests with chicks. The eggs were all found in traps below predated or failed
nests. Average throughfall per nest was 3.89 grams (standard deviation: 10.48 grams)
and per nest per day was 0.0056 grams (standard deviation: 1.34 g/nest/day). When
extrapolated to a typical sized colony of Egretta herons in the Everglades (50 nests)
average input per nest, per day was 0.28 grams per day (standard deviation: 66.78
grams). After 20, 40, and 60 days, a 50-nest colony’s average input would be 5.60,
11.20, and 16.80 grams of food, respectively (Table 3-5).
Discussion
The amount of food dropped from the nests we monitored in 2010 suggests that a
typical Egretta heron colony of 50 nests has the potential to provide nontrivial food
benefits to an alligator foraging below. The amount of heron eggs, shrimp, crayfish and
occasional chick falling from the colony could certainly contribute to the overall body
condition of the alligator. Additionally, the colony is a reliable food source positioned
directly above the alligator hole where the alligator resides. Rather than expending
energy to forage, the alligator only has to sit and wait for food to fall from the trees.
However, as evidenced by the mass nest failures and predation by raccoons in 2011, it
is possible that an Egretta heron colony could provide very little in the way of food to the
alligator.
55
Although Egretta heron chicks leave the nest as early as two weeks after hatching,
they remain at the colony and are fed by their parents until approximately 50-58 days
post-hatching. In addition to regurgitated food being dropped when it is transferred from
adult to chick, the young begin to learn to forage in and around the alligator hole
(Werschkul 1979, Rodgers and Nesbitt 1979). It seems likely that alligators would also
be able to prey upon these naïve, poorly flighted birds. Adult Egretta herons do not
usually forage in alligator holes.
The hydrological conditions that I noted during my study represent two extremes of
normal Everglades weather and hydrology. Water levels in the Everglades were higher
than usual in 2010 and the typical dry season decline in marsh water depth never
occurred. Alligators living in the Everglades rely on the seasonal marsh dry-down to
concentrate aquatic prey (Rehage & Trexler 2006). High water levels in the marsh
during 2010 could have made the food input from bird colonies a more important food
source for the alligators as their aquatic prey were probably more dispersed than a
typical year. In contrast in 2011, the marsh was so dry by May that it limited the
alligators’ movements, allowing raccoons to infiltrate the colony we were monitoring.
Raccoons decimated the colony, and may have deprived alligators of the additional food
benefits nesting birds provide. However, alligators could have benefitted from raccoon
presence in the marsh by opportunistically preyed upon them.
Wading birds are apparently attracted to alligators (Chapter 1) and probably reap
benefits from the association through protection from raccoons (Chapter 2). In this
chapter, I have shown that alligators appear to receive food benefits from the
association as well. Together, these lines of evidence suggest that the association
56
between the two taxa may be driven by benefits to both. I would therefore term the
relationship between nesting Egretta herons and alligators in the Everglades a
mutualism.
57
Table 3-1. Locations of colonies in WCA-3A where throughfall traps were placed in 2010 and 2011.
Colony location Number of traps Year N25 52.105 W80 48.398 8 2010 N26 7.624 W80 44.715 12 2010 N25 52.105 W80 48.398 20 2011 N25 52.614 W80 45.318 12 2011
58
A B Figure 3-1. Throughfall trap set-up below an Egretta heron nest with three eggs. A)
view from the side, B) view from above. Photo courtesy of author.
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Table 3-2. The fate of nests with throughfall traps in 2010 and 2011. Percent values are the percent of total number of nests at that stage. Numbers of nests at each stage are in parentheses.
2010 2011 Nests failed pre-hatching 10.0% (2) 67.6% (25) Nests failed pre-fledging 27.8% (5) 100% (12) Nests fledged at least 1 chick 72.2% (13) 0% (0)
Table 3-3. The number, total mass, grams per nest, and percent of total mass of all food items recovered from throughfall traps beneath 20 Egretta heron nests for 440 collection days in 2010.
Food item Number of items
Total mass Grams per nest Percent of total mass
Shrimp 39 39 1.95 13.04 Crayfish 31 53 2.65 17.73 Egg 3 73 2.65 24.41 Chick 1 134 6.7 44.82 Total 74 299 14.95 100
Table 3-4. The number, total mass, grams per nest, and percent of total mass of all food items recovered from throughfall traps beneath 37 Egretta heron nests for 695 collection days in 2011.
Food item Number of items
Total mass Grams per nest Percent of total mass
Shrimp 2 1 0.03 0.69 Fish 1 1 0.03 0.69 Egg 6 142 3.84 98.61 Total 9 144 3.89 100
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Table 3-5. The potential fresh food mass input from a 50-nest Egretta heron colony in 2010 and 2011 after 20, 40, and 60 days.
Number of Days 2010 (grams) 2011 (grams) 20 33.98 5.60 40 67.95 11.20 60 101.93 16.80
61
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BIOGRAPHICAL SKETCH
Brittany Burtner was born and raised in Keedysville, a rural community in western
Maryland. She graduated with high honors from the University of Maryland – College
Park in 2006 with a B.S. degree in biology and a concentration in behavior, ecology,
and evolution. After a three-year interlude studying oyster restoration in the
Chesapeake Bay at the University of Maryland’s Horn Point Lab, she returned to her
behavioral ecology roots for her M.S. thesis at the University of Florida. She completed
her M.S. degree in December 2011.