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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.

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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.

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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

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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

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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.