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22001155 SOUTHEASTERN NATURALIST 1V4o(1l.) :1142,3 N–1o3. 61
Tree Swallow Frugivory in Winter
Natalia C. Piland1,* and David W. Winkler2
Abstract - This study assesses, through the first systematic field observations of winter
foraging of Tachycineta bicolor (Tree Swallow), whether swallow foraging on the fruits
of Morella cerifera (Southern Wax Myrtle) is correlated to air temperature. We observed
Tree Swallows in central Florida for 53 days between 3 November 2011 and 14 January
2012. Tree Swallows foraged on Southern Wax Myrtle more often on colder days, producing
a statistically significant negative relationship between maximum daily temperature
and foraging on Southern Wax Myrtle. Our results also indicated that Tree Swallows ate
Southern Wax Myrtle fruit over a broad range of temperatures at which flying insects are
also available.
Introduction
Tachycineta bicolor (Vieillot) (Tree Swallow) is the only 1 of 9 species in its
genus that stays north of the Tropic of Cancer (23°N) during the winter (Winkler et
al. 2011). This behavior may be due to its ability to digest the waxy fruits of Morella
cerifera (L.) Small (Southern Wax Myrtle) and Morella caroliniensis (Mill.)
(Eastern Bayberry), similar to the dietary adaptation of Setophaga coronata (L.)
(Yellow-rumped Warbler) (e.g., Bent 1942, Bernhardt et al. 2009, Kilham 1980,
McCarty 1997, Parrish 1997, Place and Stiles 1992). Most sources suggest that Tree
Swallow frugivory is a last-resort strategy when cold weather makes it impossible
for insects to fly (e.g., Chapman 1955, Turner and Rose 1989), yet until now, there
have been no field studies to verify the Tree Swallows’ use of these fruits on its
wintering grounds.
During the breeding season, Tree Swallows are obligate aerial insectivores
(Sibley 2000). However, during early migration in August, Tree Swallows have
been observed in large groups feeding on Eastern Bayberry and Southern Wax
Myrtle fruits (C. Gates, Salmon Creek Tree Swallow Project, NY, pers. comm.;
Winkler et al. 2011), signaling a potentially important relationship, beyond serving
as emergency food, between these species and Tree Swallow nutrition. Morella
spp. are found along the eastern seaboard: Eastern Bayberry’s distribution extends
from Newfoundland to the Mid-Atlantic, and Southern Wax Myrtle occurs from
New Jersey and along the coast of the Gulf of Mexico to a western limit in Aransas
Bay, TX (USDA 2002a). Both Morella species co-occur from New Jersey to the
Mid-Atlantic (Fig. 1). They are naturally found along brackish pond edges with
moderately moist soils as well as in newly cleared areas, and are commonly used in
1Committee on Evolutionary Biology, University of Chicago, 1025 East 57th Street, Culver
Hall 402, Chicago, IL 60637. 2Department of Ecology and Evolutionary Biology, Corson
Hall, Cornell University, Ithaca, NY 14850. *Corresponding author - npiland@gmail.com.
Manuscript Editor: Frank Moore
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landscaping and land management around human settlements (Austin 2004, Cuda
et al. 2006, Kalmbacher et al. 1993, Tomlinson and Fawcett 1980).
The range of Southern Wax Myrtle coincides with the southeastern US range
of Tree Swallows in winter (Fig. 1), suggesting that there might be a relationship
between foraging on Southern Wax Myrtle and the swallows’ ability to stay in the
continental US during the winter. This relationship may have formed as a response
to reduced availability of insect prey during times of cold temperatures. When the
temperature is ≤18.5 °C (Winkler et al. 2013), insects do not fly and their aerial
availability plummets (Hess et al. 2008, Luo 2011, Lysyk 2010, Winkler et al.
2013), making Southern Wax Myrtle fruits relatively more attractive to Tree Swallows.
Therefore, if foraging on Southern Wax Myrtle is dependent on reduced insect
availability, then there should be a correlation between temperature and Tree Swallow
frugivory on Southern Wax Myrtle berries.
Field-site Description
The field-site included southern Hillsborough County and all of Sarasota and
Manatee counties in Florida (Fig. 2). We chose these counties as the study area because
of observed radar presence of Tree Swallow roosts (see http://radar.cs.umass.
edu/roost-label/). Tree Swallows were widespread in this area in all habitat types
through the entire winter except in densely populated urban areas where they were
less commonly observed. Southern Wax Myrtle is abundant in all habitat types,
particularly in housing communities and on roadsides, but is least abundant in
protected natural areas (USDA 1994). Habitats in the study area included wetlands
Figure 1. Ranges of Morella spp. (IMS Health, Inc. 2014) and Tree Swallow observations reported
on eBird.org for November–January, all years; base image provided by eBird (www.
ebird.org) and created 18 November 2014.
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such as marshes and cypress domes, southeastern conifer forests, and dry prairies
(grasses and palmettos).
Methods
Data collection
We collected field data by foot and car for a total of 53 days between 3 November
2011 and 14 January 2012. We located foraging Tree Swallows by driving on
roads in open, less densely populated parts of the study area—gated communities,
golf courses, mines, farms, construction sites, state parks—for a minimum of 2 h
each day (Fig. 2). We chose this threshold because in our preliminary observations
Figure 2. Study area (overall study area delineated in black—includes Sarasota and Manatee
counties with a small portion of southern Hillsborough County). Each observation is
grouped by a different shape/pattern combination and represents the key areas: Cockroach
Bay (striped circle), Four Corners (dotted circle), Bradenton (dark diamond), Wauchula
(striped diamond), Lakewood-Fruitville (light circle), SR-72 (dotted diamond), and Laurel/
Taylor Ranch (dark circle).
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(winter 2010 and the week preceding the formal study), two hours appeared to be a
sufficient amount of time in the field to detect whether or not Tree Swallows were
foraging on Southern Wax Myrtle on that day. Any field-time shorter than two hours
introduced the probability of false negative data.
We identified observation locations by the road/trail taken, and for each observation,
we noted start and stop times, weather conditions (cloud cover, cloud type,
and precipitation), mode of transportation, and whether any Tree Swallows were
present during the observation. If Tree Swallows were present during the observation,
we estimated the number of Tree Swallows and documented their foraging
behaviors (feeding on insects, Southern Wax Myrtle, or nothing). At the end of the
day, we accessed data from the weather station nearest to the place and time of each
observation (http://www.wunderground.com)—including Venice High School,
Gulf Gate East, and Ruskin FL US—and noted temperature, maximum daily temperature,
and minimum overnight temperature of the night before .
To control for location bias, we made observations on at least 1 new road per
day. We made our observations from roost-ascent time (~20 min before sunrise) to
3 PM (to allow sufficient time for observation before roost-descent time at ~30 min
after sunset). To control for temporal bias, we alternated starting times of observations
between morning (6 AM–10 AM), midday (10 AM–2 PM), and afternoon
(2 PM–6 PM). N.C. Piland recorded data for all observations.
Volunteers (contacted through the Sarasota Audubon Society, local birding
list-serves, and door-to-door visits in zones of high Southern Wax Myrtle density)
reported additional field observations. There were 2 tiers of volunteer effort. The
first tier was one in which the volunteer went out on a regular basis to collect field
observations using the same methodology as Piland. Volunteers reported back every
time they were out in the field regardless of whether or not they saw Tree Swallow
activity. There were 3 volunteers in this tier. The second tier was comprised of
volunteers who submitted reports only when they saw large aggregations of Tree
Swallows. The information in these reports included an estimation of the number of
individuals in the aggregation and whether or not the Tree Swallows were feeding
on Southern Wax Myrtle. There were 16 volunteers at this tier. Observations made
by volunteers in both tiers account for less than 9% of all obs ervations.
Descriptive analysis
We defined one replicate in the descriptive analysis as all observations made
during a day, and categorized each sampling day as a non-myrtle-foraging day or a
myrtle-foraging day; n = 53. We classified days when no Tree Swallows were observed
feeding on Southern Wax Myrtle fruit as non-myrtle-foraging days and days
when there was at least 1 observation of Tree Swallows foraging on Southern Wax
Myrtle fruits as myrtle-foraging days. Tree Swallows fed on insects every day.
This type of data aggregation is appropriate for 2 reasons: (1) there is no way to
know what an individual bird was doing, given that no birds were tagged during the
course of the study, making it essential to answer the study question at a population
level, and (2) each day in a Florida winter can be reasonably treated as an independent
replicate (due to its relative temperature stability). Treating each observation
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as an individual replicate would introduce dependence bias given that within 1h,
birds could have fed on a Southern Wax Myrtle bush in one place, and later in another
without the observer being able to identify the individual bird. Furthermore,
Tree Swallows fed in flocks, meaning that each Tree Swallow did not represent an
independent sample of feeding choice (c.f. Witmer 1996). We considered all Tree
Swallows in the study area as one overwintering population because of the high density
of roosts found, observations that individual Tree Swallows alternate roost usage
during stopover or overwintering months (Laughlin et al. 2014), and observations
on migratory behavior reporting that individual Tree Swallows migrated slowly during
the day, suggesting a less-than-direct migration (Laughlin et al. 2013, Winkler
2006). Although there is a small chance that each day is not 100% independent due
to insect life cycles and weather patterns, we believe that the likelihood is negligible,
given the size of the study area, the population’s probable home range, and the relative
stability of Florida weather, all of which support the assumption that one day’s
cold weather would not influence the next day’s insect availability. To address this
possibility, we examined the maximum temperature for a replicate and the minimum
overnight temperature the night before the replicate. Once aggregated, we used the
data to create two boxplots—maximum daily temperature vs. levels of Tree Swallow
foraging on Southern Wax Myrtle, and minimum overnight temperature vs. levels of
Tree Swallow foraging on Southern Wax Myrtle.
Generalized linear mixed model
In further analyses, we ran 5 generalized linear mixed models assuming a
Gaussian distribution by Laplace approximation (Bates 2010). We used these
models to identify which of the explanatory factors or fixed effects—maximum
daily temperature, minimum overnight temperature, individual effort by Piland,
and total effort—had a stronger effect on whether or not the observed Tree Swallows
foraged on Southern Wax Myrtle. Individual effort and total effort were
treated as 2 different fixed effects because tier 2 volunteers reported observations
only when Tree Swallows were observed foraging on Southern Wax Myrtle, thus
introducing a methodological bias. Therefore, total effort includes all observers in
their configuration as a potential effect, whereas individual effort only takes into
account Piland’s observations, given that they represent over 90% of the individual
observations. If the volunteer observations were explanatory for the response, the
individual effort would have a higher P-value than the total effort. We considered
spatial (key areas, defined as a qualitative conjunction of observations grouped
around access roads; see Fig. 2) and temporal (date) random ef fects.
A full model that had fixed effects for maximum daily temperature, minimum
overnight temperature, individual effort, and total effort was simplified by successive
removal of the effect with the highest probability until all fixed effects had
P-values less than 0.10. A model with P-values of less than 0.10 for all fixed effects indicated that
all fixed effects included in that model were predictive. To assess differences between
each of the simplified models, we compared their AIC values and conducted
ANOVAs for each relative to the full model. All analyses were completed using R
software (version 2.14.1).
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Results
Descriptive analysis
The mean maximum temperature for non-myrtle foraging days was 25.2 °C, and
the mean maximum temperature for myrtle-foraging days was 23.5 °C. The largest
difference in this measure between days with myrtle foraging and those without can
be seen in the boxplot by comparing the space between the median and the upper
quartile (Fig. 3).
In contrast, there was no apparent difference in the distributions and medians of
the minimum overnight temperatures for myrtle-foraging days verses non-myrtle
foraging days (Fig. 4). The mean overnight temperature on the days before nonmyrtle
foraging days was 12.9 °C and the mean overnight temperature on the days
before myrtle-foraging days was 11.5 °C.
Generalized linear mixed model
The full generalized linear mixed model was simplified by first taking out total
effort (P = 0.56 at removal), then individual effort (P = 0.48 at removal), and finally
Figure 3. Box-plot summarizing maximum daily temperature of days when Tree Swallows
were not observed foraging on Southern Wax Myrtle (0) versus when they were (1). The boxes
represent the middle 50% of the data, with the thicker black line representing the median daily
temperature. The lines above and below each box represent the range of temperatures.
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minimum overnight temperature (P = 0.52 at removal), leaving a final model with
maximum daily temperature as the only fixed effect (see Table 1 for AIC values for
all models). Although ANOVAs showed that every model reduction was statistically
significant (P < 0.05), the simplification to the model with maximum daily
temperature as the sole fixed effect (Fig. 5) had the highest statistical significance
(P = 0.005).
Discussion
The results from our study suggest that Tree Swallow frugivory has an inverse
relationship with temperature: the lower the temperature, the higher the probability
that Tree Swallows will be foraging on Southern Wax Myrtle fruits. However, given
the cold-snap threshold suggested by Winkler et al. (2013) of 18.5 ºC in New York,
and the fact that we observed swallows feeding on insects on all days including
Figure 4. Box-plot summarizing minimum overnight temperature of days when Tree Swallows
were not observed foraging on Southern Wax Myrtle (0) versus when they were (1).
The boxes represent the middle 50% of the data with the thicker black line representing
the median daily temperature. The lines above and below each box represent the range of
temperatures, and the dots represent statistical outliers.
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Figure 5. Final GLMM model with maximum daily temperature as th e only fixed effect.
Table 1. Akaike information criterion (AIC) and P-value under ANOVA analysis against a no-fixedeffects
model for all models run; * AIC indicates the model with the best information fit; ** ANOVA
indicates the model most statistically different from the no-fixed-effects model.
Model AIC P
4 fixed effects:
Response ~ individual effort + total effort + max temp + 322.3 0.06301
min overnight temp + (1|key area) + (1|date)
3 fixed effects:
Response ~ individual effort + max temp + min overnight temp + 320.6 0.03497
(1|key area) + (1|date)
2 fixed effects:
Response ~ max temp + min overnight temp + (1|key area) + 319.1 0.01719
(1|date)
1 fixed effect:
Response ~ max temp + (1|key area) + (1|date) 317.5* 0.00548**
No fixed effects:
Response ~ 1 + (1|key area) + (1|date) 323.2 N/A
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those during which the birds also ate fruits, Tree Swallow frugivory was not a lastresort
foraging strategy in the absence of insects, but rather a complement to insect
foraging. This result is contrary to the literature on Tree Swallows (e.g., Chapman
1955, Sibley 2000, Turner and Rose 1989), and suggests that the Swallows eat
Southern Wax Myrtle fruits regularly in addition to insects (Beal 1918, Bent 1942),
though more so at lower temperatures when insect abundance is l ikely reduced.
Our results support the conclusion that Tree Swallows may remain omnivorous
throughout the non-breeding season despite insect availability. During the breeding
season, Tree Swallows eat mainly insects in the orders Diptera, Homoptera,
Hemiptera and Odonata, the members of which are high-protein food sources
(McCarty and Winkler 1999, Quinney and Ankney 1985). Strongly frugivorous
birds tend to prefer sugar-rich fruits; omnivorous birds do not seem to value lipidrich
fruits over others but eat them in conjunction with animal prey and/or sugary
fruits (Martin et al. 1961, Wheelwright 1986, White and Stiles 1990, Witmer
1996). Tree Swallows have not been observed feeding on any fruits other than
those of Morella spp., leading to the assumption that for this species lipid-rich
fruits compliment the nutrition that insects provide them. Morella pensylvanica
(Mirb.) Kartesz (Northern Bayberries) are composed of 50.3% ± 1.4% dry weight
of fat, 3.0% ± 0.0% dry weight of protein, 41.3% ± 0.2% dry weight of carbohydrates,
and 3.4% ± 1.3% dry weight of ash, and have an energy density of
28.7 ± 0.5 kJ/dry weight (Smith et al. 2007). Although they differ in preferred
soil type—Eastern Bayberry (species proposed to encompass both Northern and
Southern Bayberry; c.f. Wilbur 2002) is found on dunes, old fields, and dry hills,
and Southern Wax Myrtle generally prefers damper, sandier soils (Austin 2004,
Place and Stiles 1992)—both species can grow successfully in almost all soil
types and are widely abundant within their ranges (Gilman and Watson 1994;
USDA 2002a, b). Fruits of congeneric species can be quite disparate in nutritional
content (Witmer 1996), but Eastern Bayberry’s similarity to Southern Wax
Myrtle in morphology and ecological function as bird sustenance suggest that it
is reasonable to use the nutritional content of the fruits of the congeneric Eastern
Bayberry when considering our study system given that similar information
is lacking for Southern Wax Myrtle. These species can be discerned by differences
in their distribution and leaf shape (Wilbur 1994). An important research
direction to improve understanding of the role of fruit in the Tree Swallow’s diet
would be a characterization of the nutritional content of Southern Wax Myrtle;
these findings might also validate our assumption of compositional similarity.
The nutritional composition of Eastern Bayberry fruit, including its low protein
content, suggests that Tree Swallows in Florida could not support themselves on
Southern Wax Myrtle berries alone (Smith et al. 2007). As such, the fruits’ benefits
must lie in the high-energy content they provide and their consumption by this
species may suggest that although Tree Swallows are often slow diurnal migrating
birds (Winkler 2006), the population in central Florida may still be moving between
roosts more than expected and may require more energy than during the breeding
season. Using the equations presented in Smith et al. 2007, and assuming a 20-g
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body weight for an adult Tree Swallow based on mist-nest-capture data (Karasov
1990, Koteja 1991, Winkler et al. 2011), we estimate that the basal daily energy
requirement (DER) of a Tree Swallow is ~106.75 kJ/day. This calculation was derived
from an analysis of literature for “free-living, non-reproductive passerines”
and does not necessarily account for the increased movement observed at roosts and
during frugivory or the cost of thermoregulation at night. Thermoregulation at night
for birds weighing ~20 g can be significant and can also imply non-restful nocturnal
behavior (c.f. Wojciechowski and Pinshow 2009). A few fruits a day might complement
an insect-rich diet in order to meet DER. Yet, we believe Southern Wax Myrtle
fruits alone would not sustain swallows and allow them to meet all their energetic
needs because they would need to consume over 5 g of the fruits’ wax assuming
similar nutritional content to Eastern Bayberry fruits and a 66.4% assimilation efficiency
(Morella spp. fruit volume from Fordham 1983, Place and Stiles 1992,
Smith et al. 2007).
The bird with the strongest association to Southern Wax Myrtle fruits is the
Yellow-rumped Warbler — one of the only warblers to stay in the continental US
during the winter (e.g., Brewer 1840, Hausman 1927, Martin et al. 1951, Parrish
1997, Place and Stiles 1992). The warbler’s relationship with Southern Wax Myrtle
fruits has been studied, and findings suggest that the fruits provide much-needed
energy in conjunction with other fruits and insects (Place and Stiles 1992). Physiological
adaptations thought to have developed to digest these high-melting-point
waxes are an elevated luminal bile-salt concentration in the gall-bladder, and an
apparent retrograde intestinal reflux to the gizzard (Place and Stiles 1992). Yellowrumped
Warblers eat other fruits in addition to Morella spp. including those of
Toxicodendron radicans (L.) Kuntze (Poison Ivy), Parthenocissus spp. (Virginia
creeper), and Rhus spp. (sumac) (Place and Stiles 1992). This feeding pattern suggests
that Southern Wax Myrtle fruits alone are not a sufficient source of sustenance
for Yellow-Rumped Warblers, and assuming the same is true for Tree Swallows,
could support the hypothesis that Tree Swallows are omnivorous throughout the
winter, regardless of temperature and its effect on insect availability. Other birds
that have been found to eat Southern Wax Myrtle fruit (albeit rarely) are Catharus
guttatus (Pallas) (Hermit Thrush) (Strong et al. 2005), Picoides pubescens (L.)
(Downy Woodpecker), Baeolophus bicolor L. (Tufted Titmouse), and Setophaga
pinus L. (Pine Warbler) (Borgmann et al. 2004).
Resource availability is an important predictor for distribution and omnivory
in birds (Kwit et al. 2004, McClanahan and Wolfe 1992, Speirs 1953, Strong et al.
2005,Witmer 1996). The distribution of the Tree Swallow in the continental US during
the non-reproductive season appears to mirror the distribution of Southern Wax
Myrtle and Eastern Bayberry (Fig. 1). More research is needed to help us better
understand the implications of this relationship for the species’ ecology and evolution.
It would be particularly useful to compare Tree Swallows that migrate further
south and the populations that stay in the US. If fruit is essential for some of the
populations, it may be interesting to determine if the populations are the same year
after year or if there is variation in the specific individuals that migrate onwards and
perhaps do not consume fruits. If it is found that Tree Swallows that migrate south
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also feed on Southern Wax Myrtle fruits before and after they leave the US, it could
imply omnivory in the entire species.
In general, further research is needed to understand Tree Swallow foraging
ecology and natural history during the non-reproductive season. Fecal studies are
an effective way to investigate diet, particularly in omnivorous birds (Strong et
al. 2005), and large numbers of fecal samples can sometimes be obtained around
sites where birds forage on Southern Wax Myrtle (N.C. Piland, unpubl. observ.).
A potential study could mark Southern Wax Myrtle fruits in order to monitor Tree
Swallow populations to estimate digestion time. Possible marking techniques
could be genetic—if the plants differ enough in particular single-nucleotide polymorphisms
or microsatellite signatures, they could be identified through the fecal
samples. Alternatively, fruits could be tagged with distinctive fluorescent dusts,
but this method would require significant fieldwork preceding field observations,
which may be difficult given the abundance of both Southern Wax Myrtle
and Tree Swallows. Either approach would have to deal with the great difficulty
of mist-netting Tree Swallows during the non-reproductive season and not insignificant
assumptions about the relatedness regarding the identity of individuals
observed foraging on certain wax myrtles and the individuals caught, and the time
elapsed between the two events, but could still provide important information
about prevalence of wax myrtle foraging within a group of birds. Additionally,
any study that obtained direct measurements of insect abundance in conjunction
with foraging observations would increase our understanding of the relationship
between insect- and fruit-foraging by Tree Swallows. Even repeating our study
would be beneficial because the winter during which we conducted it was one of
the warmest recorded in Florida (Duffy and Fried 2012), and a time series of data
would give better indications of the temperature dependency that Tree Swallow
frugivory may have, and how that may be affected by future weather events and,
in the longer term, climate change. Furthermore, the behaviors demonstrated during
Tree Swallow frugivory—tight aggregations of large number of individuals
similar to roost descent/ascent behavior (c.f. Winkler 2006)—have never been
formally studied and could inform research about frugivory by determining the
percentage of aggregations that actually feed on the fruits. Finally, changing
land-use practices in Florida including the commercial development of large
ranch estates in the Sarasota area (e.g., Metrostudy News 2014) could become
an important factor in changing the distribution of Tree Swallows. To date, these
properties have provided Typha spp. (cattails) (valuable for swallow roosting)
in areas that are unmanaged and Southern Wax Myrtle in those that are managed
(Cuda et al. 2006; N.C. Piland, pers. observ.).
In conclusion, Tree Swallows are eating Southern Wax Myrtle fruits at higher
temperatures than published literature regarding Tree Swallows would suggest.
We found that frugivory and maximum daily temperature were inversely related
during the winter season of 2011–2012, but the precise relationship between these
variables and insect abundance has not yet been ascertained. The role of Southern
Wax Myrtle fruits in Tree Swallow distribution across time and space is poorly
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understood, and we hope this study is a first step in an exciting new research direction
within the study of the natural history of an otherwise we ll-known bird.
Acknowledgments
A special thanks goes to the people who supported and contributed to this research
from its planning stages to its execution and its analysis: Jeanne Dubi, Maria Stager, Sue
Guarasci, Sandy Cooper, Barry Rossheim, Belinda Perry, Nancy Edmonson, Adam Ross,
Janny Wurtz, and Andrew Laughlin. An additional thanks goes to the Wildlife Conservation
Society for its support and to Mark Witmer, Chris Gates, Patricia Mendoza, and Ilana
Malekan for valuable comments and insights during the writing of this manuscript. Without
the much-appreciated help of Sarasota County, the Florida Department of Environmental
Protection, the Ecology and Evolutionary Department at Cornell University, and the Office
of Undergraduate Biology at Cornell University, this study would have been particularly
limited. This research was supported by funding from Cornell University, by a research
grant from the Sarasota Audubon Society, and by a student research grant from Golondrinas
de las Americas through an NSF PIRE grant (OISE—0730180).
Literature Cited
Austin, D.F. 2004. Florida Ethnobotany. CRC Press, Boca Raton, FL 909 pp.
Bates, D.M. 2010. Lme4: Mixed-Effects Modeling with R. Springer, New York, NY. Available
online at http://lme4.r-forge.r-project.org/book/. Accessed April 2012.
Beal, F.E.L. 1918. Food habits of the swallows, a family of valuable native birds. US Department
of Agriculture No. 619. Washington, DC. 28 pp.
Bent, A.C. 1942. Tree Swallow (Tachycineta bicolor). Smithsonian Institution. United
States National Museum Bulletin 179:384–400. Available online at http://www.birdsbybent.
com/ch81-90/treeswallow.html. Accessed July 201.
Bernhardt, G.E., J.Z. Patton, L.A. Kutschbach-Brohl, and R.A. Dolbeer. 2009. Management
of bayberry in relation to Tree-swallow strikes at John F. Kennedy International Airport,
New York. Human–Wildlife Conflicts 3(2):237–241.
Brewer, T.M. 1840. Wilson’s American Ornithology. Otis, Broaders, and Company, Boston,
MA. 1399 pp.
Borgmann, K.L., S.F. Pearson, D.J. Levey, and C.H. Greenberg. 2004. Wintering Yellowrumped
Warblers (Dendroica coronata) track manipulated abundance of Myrica cerifera
fruits. The Auk 121(1):74.
Chapman, L.B. 1955. Studies of a Tree Swallow colony. Third paper. Bird-banding
26(2):45–70.
Cuda, J.P., V. Manrique, and J.C. Medal. 2006. Interagency Brazilian Peppertree (Schinus
terebinthifolius) management plan for Florida. Florida Exotic Pest Plant Council. Fort
Lauderdale, FL.
Duffy, P., and B. Fried. 2012. US Winter 2011–2012 is fourth warmest in recorded history.
The White House Office of Science and Technology Policy. Available online at http://
www.whitehouse.gov/blog/2012/03/26/us-winter-2011-2012-fourth-warmest-recordedhistory.
Accessed January 2013.
Fordham, A.J. 1983. Of birds and bayberries: Seed dispersal and propagation of three
Myrica species. Arnoldia. 43(4):20–23.
Gilman, E.F., and D.G. Watson. 1994. Myrica cerifera, Southern Waxmyrtle.” United States
Department of Agriculture. Available online at http://hort.ifas.ufl.edu/database/documents/
pdf/tree_fact_sheets/myrcera.pdf. Accessed November 2014.
Southeastern Naturalist
135
N.C. Piland and D.W. Winkler
2015 Vol. 14, No. 1
Hausman, L.A. 1927. On the winter food of the Tree Swallow (Iridoprocne bicolor) and the
Myrtle Warbler (Dendroica coronata). The American Naturalist 61(675):379.
Hess, P.J., C.G. Zenger, and R.A. Schmidt. 2008. Weather-related Tree Swallow mortality
and reduced nesting effort. Northeastern Naturalist 15(4):630–631.
IMS Health, Inc. 2014. PollenLibrary.com. Available online at http://pollenlibrary.com.
Accessed 2 December 2014
Kalmbacher, R.S., J.E. Eger, and A.J. Rowland-Bamford. 1993. Response of Southern Wax
Myrtle (Myrica Cerifera) to herbicides in Florida. Weed Technology 7(1):84–91.
Karasov, W.H. 1990. Digestion in birds: Chemical and physiological determinants and
ecological implications. Studies in Avian Biology 13:391–415.
Kilham, L. 1980. Assemblages of Tree Swallows as information centers. Florida Field
Naturalist 8(1):26–28.
Koteja, P. 1991. On the relation between basal and field metabolic rates in birds and mammals.
Functional Ecology 5:56–64.
Kwit, C., D.J. Levey, C.H. Greenberg, S.F. Pearson, J.P. McCarty, and S. Sargent. 2004.
Cold temperature increases winter fruit-removal rate of a bird-dispersed shrub. Oecologia
139(1):30–31.
Laughlin, A., C. Taylor, D.W. Bradley, D. LeClair, R.G. Clark, R.D. Dawson, P.O. Dunn,
A. Horn, M. Leonard, D.R. Sheldon, D. Shutler, L.A. Whittingham, D.W. Winkler, and
D.R. Norris. 2013. Integrating information from geolocators, weather radar, and citizen
science to uncover a key stopover area of an aerial insectivore. The Auk 130(2):230–239.
Laughlin, A.J., D.R. Sheldon, D.W. Winkler, and C.M. Taylor. 2014. Behavioral drivers
of communal roosting in a songbird: a combined theoretical and empirical approach.
Behavioral Ecology 25(4):734-743.
Luo, M.K. 2011. Climate change and temperature effects on the breeding success of Tree
Swallows (Tachycineta Bicolor). Honors Thesis. Cornell University, Ithaca, NY.
Lysyk, T.J. 2011. Species abundance and seasonal activity of mosquitoes on cattle facilities
in Southern Alberta, Canada. Journal of Medical Entomology 47(1):32–42.
Martin, T.E., H.S. Zim, and A.L. Nelson. 1961. American Wildlife and Plants, a Guide to
Wildlife Food Habits. Dover Publications, New York, NY. 718 pp.
McCarty, J.P. 1997. Aquatic community characteristics influence the foraging patterns of
Tree Swallows. The Condor 99:213–217.
McCarty, J.P., and D.W. Winkler. 1999. Foraging ecology and diet selectivity of Tree Swallows
feeding nestlings. The Condor 101(2):246–254.
McClanahan, T.R., and R.W. Wolfe. 1992. Accelerating forest succession in a fragmented
landscape: The role of birds and perches. Conservation Biology 7(2):279–28 8.
Metrostudy News. 2014. Central Florida housing market Metrostudy 1Q14 Survey Results:
Starts 2014 strong; Rising prices will drive demand in suburban markets. Available
online at http://www.metrostudyreport.com/central-florida-market/central-florida-housing-
market-metrostudy-1q14-survey-results-starts-2014-strong-rising-prices-will-drive-
demand-in-suburban-markets. Accessed July 2014.
Parrish, J.D. 1997. Patterns of frugivory and energetic condition in Nearctic landbirds during
autumn migration. The Condor 99(3):681–97.
Place, A.R., and E.W. Stiles. 1992. Living off the wax of the land: Bayberries and Yellow-
Rumped Warblers. The Auk 109(2):334–345.
Quinney, T.E., and C.D. Ankney. 1985. Prey-size selection by Tree Swallows. The Auk.
102(2):245–250.
Sibley, D. 2000. The Sibley Guide to Birds. Alfred A. Knopf, New York, NY. 544 pp.
Southeastern Naturalist
N.C. Piland and D.W. Winkler
2015 Vol. 14, No. 1
136
Smith, S.B., K.H. McPherson, J.M. Backer, B.J. Pierce, D.W. Podlesak, and S.R. McWilliams.
2007. Fruit quality and consumption by songbirds during autumn migration.
Wilson Journal of Ornithology 119(3):419–428.
Speirs, J.M. 1953. Winter distribution of robins east of the Rocky Mountains. Wilson Bulletin
65:175–183.
Strong, C.M., D.R. Brown, and P.C. Stouffer. 2005. Frugivory by wintering Hermit Thrush
in Louisiana. Southeastern Naturalist 4(4):627–638.
Tomlinson, P.B., and P. Fawcett. 1980. The Biology of Trees Native to Tropical Florida.
Harvard University, Cambridge, MA. 480 pp.
Turner, A.K., and C. Rose. 1989. A Handbook to the Swallows and Martins of the World.
Christopher Helm, Lindon, UK. 258 pp.
USDA. 2002a. Plant Fact Sheet: Bayberry. Available online at http://plants.usda.gov/factsheet/
pdf/fs_mope6.pdf. Accessed November 2014
USDA. 2002b. Plant Fact Sheet: Dwarf Wax Myrtle. USDA NRCS. Available online at
https://plants.usda.gov/factsheet/pdf/fs_mypu.pdf. Accessed November 2014
Wheelwright, N.T. 1986. The diet of American Robins: An analysis of US Biolgoical Survey
records. The Auk 103:710–725.
White, D.W., and E.W. Stiles. 1990. Co-occurrences of foods in stomachs and feces of fruiteating
birds. Condor 92:291–303.
Wilbur, R.L. 1994. The Myricacaea of the United States and Canada: Genera, subgenera,
and series. SIDA 16(1):93–107.
Wilbur, R.L. 2002. The identity and history of Myrica caroliniensis (Myricaceae). Rhodora
104(917):31–41.
Winkler, D.W. 2006. Roosts and migrations of swallows. Hornero 21(2):85–97.
Winkler, D.W., K.K. Hallinger, D.R. Ardia, R.J. Robertson, B.J. Stutchbury, and R.R.
Cohen. 2011. Tree Swallow (Tachycineta bicolor). Number 11, In A. Poole (Ed.). The
Birds of North America Online. Cornell Lab of Ornithology, Ithaca, NY. Available online
at http://bna.birds.cornell.edu.proxy.library.cornell.edu/bna/species/011. Accessed
April 2012.
Winkler, D.W., M.K. Luo, and E. Rakhimberdiev. 2013. Temperature effects on food supply
and chick mortality in Tree Swallows (Tachycineta bicolor). Oecologia 173:129–138.
Witmer, M.C. 1996. Annual diet of Cedar Waxwings based on US Biological Survey records
(1885–1950) compared to diet of American Robins: Contrast in dietary patterns
and natural history. The Auk 113(2):414–430.
Wojciechowski, M.S., and B. Pinshow. 2009. Heterothermy in small, migrating passerine
birds during stopover: Use of hypothermia at rest accelerates fuel accumulation. The
Journal of Experimental Biology 212:3068–3075.