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Tree Swallow Frugivory in Winter
Natalia C. Piland and David W. Winkler

Southeastern Naturalist, Volume 14, Issue 1 (2015): 123–136

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Southeastern Naturalist 123 N.C. Piland and D.W. Winkler 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 Southeastern Naturalist N.C. Piland and D.W. Winkler 2015 Vol. 14, No. 1 124 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. Southeastern Naturalist 125 N.C. Piland and D.W. Winkler 2015 Vol. 14, No. 1 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). Southeastern Naturalist N.C. Piland and D.W. Winkler 2015 Vol. 14, No. 1 126 (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 Southeastern Naturalist 127 N.C. Piland and D.W. Winkler 2015 Vol. 14, No. 1 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). Southeastern Naturalist N.C. Piland and D.W. Winkler 2015 Vol. 14, No. 1 128 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. Southeastern Naturalist 129 N.C. Piland and D.W. Winkler 2015 Vol. 14, No. 1 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. Southeastern Naturalist N.C. Piland and D.W. Winkler 2015 Vol. 14, No. 1 130 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 Southeastern Naturalist 131 N.C. Piland and D.W. Winkler 2015 Vol. 14, No. 1 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 Southeastern Naturalist N.C. Piland and D.W. Winkler 2015 Vol. 14, No. 1 132 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 Southeastern Naturalist 133 N.C. Piland and D.W. Winkler 2015 Vol. 14, No. 1 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 Southeastern Naturalist N.C. Piland and D.W. Winkler 2015 Vol. 14, No. 1 134 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. 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