Feeder position preferences in response to potential and artificial predators in Hummingbirds Su Mei Lai Department of Ecology and Evolutionary Biology, University of Colorado at Boulder. ABSTRACT When risks are present , animal s consider the cost s and benefit s of foraging . This study investigates the preferred feeder pos itions when potential risks are involved in feeding hummingbirds. Two experiments were conducted in this study in order to have a better understanding of animal risk assessment . The f irst experiment examined whether there was a preference for high or low feeders in hummingbirds . Results showed that hummingbirds had a preference for higher feeders. Results also sho wed that hummingbirds have a greater preference to feed from feeders that are closer to trees (outer) than feeders f arther away (inner ) from trees. The second experiment showed no significant difference in the number of visits of hummingbirds when artificial predators were present versus when they were absent. This indicates that h ummingbirds prefer to feed on feeder s wi th the minimal potential risk, w hich suggests that hummingbirds are able to perceive risk . However, risk presented in this study mi ght not be significant enough to have an impact . This study provides an understand ing of foragi ng behavior in hummingbirds and insights for future studies. RESUMEN Cuando hay un riesgo presente, los animales consideran el costo y beneficio de forrajear. Este estudio investiga la posiciÃ³n de comederos preferida cuando hay riesgos potenciales para colibrÃes. Dos experimentos se condujeron en este estudio para tener un mayor entendimiento de la evaluaciÃ³n por animales. El primer experimento examinÃ³ si hay una preferencia por comederos altos o bajos. Los resultados muestran que los colibrÃes tienen preferencia por comederos altos. Los resultados tambiÃ©n muestran que los colibrÃes tienen una mayor preferencia para utilizar los comederos ubicados cerca de lo s Ã¡rboles y no los que estÃ¡n lejos de los mismos. El segundo experimento no muestra ninguna diferencia en el nÃºmero de visitas a los comederos cuando depredadores falsos fueron expuestos versus la ausencia de los mismos. Esto indica que los colibrÃes pre fieren alimentarse con un mÃnimo riesgo potencial, lo que sugiere que los colibrÃes son capaces de percibir riesgo. Sin embargo, el riesgo presentado en este estudio no es lo suficientemente significativo para tener un impacto. Este estudio provee un ent endimiento del comportamiento de forrajeo en colibrÃes y puntos de vista para futuros estudios. I NTRODUCTION In 1966, MacArthur, Pianka and Emlen f irst proposed the o p timal foraging theory . Their study examined the foraging behavior in animal s , and they found that organisms hav e the tendency to forage in such a way that can provide the most energy with the least amount effort expended over time . However, when risk s are involved, risks have the potential to complicate the predictions of th e optimal foragin g theory. T herefore , an animal must assess th e risk and weigh it against the energy consump tion. A study of in hierarchy dominant white throated sparrows ( Zonotrichia albicollis ) demonstrated the trade off between energy in take and predation risk . Given that d ominant birds had a better access to preferred feeding site , this study shows that dominant birds often feed near shelter
more often than the less dominant individuals . W hen the white throated sparrows were presented with patches of food with varied distance from cover , t he white throated sparrows maximizing the ene rgy in take per unit time, but i nstead foo d that was the closest to s helter was depleted the fastest. This suggests that cover provides effective protection from predators and that risk can intervene with the optimal foraging theory ( Schneider 1984). Hummingbirds are the smallest bird species with the fastest metabolism on the planet. They have to eat their own weight in food every day to compensa te for the high energetic cost of their metabolic rate (Long 1997). Thus, high energy demand lead s to hummingbirds having a rapid and extremely sensitive response to energy manipulation. S ince hummingbirds have low energy reserves, it is crucial for hummin gbirds to be proficient at balancing energy budget and decision making when foraging (Ti ebout 1990). Although hummingbirds can feed on many flowers, they will usually pick flowers with the highest sugar conte nt and most nectar (Long 1997). These reasons make hummingbirds good subjects for this study. This study investigates the feeder position preference based on the potential risks that might exist in feeding hummingbirds . Some of these potential risk s include fe eding closer to the ground, which make s hummingbirds more vulnerable to terrestrial danger. T hus , to minimize danger , h ummingbirds are most likely to select feeders higher above the ground . Also, in order to obtain the most energy intake per un it time spent , h ummingbirds are most like ly to choose feeders that are close r to trees, since trees provide perching sites that hummingbirds are able to use to conserve energy between feedings . Lastly, hummingbird s are most likely to reduce the number of visits to feeders when artificial predators are present. METHODS Study site This study was conducted at the edge of forest at the Biological Station in Monteverde, Costa Rica at elevation 1550m. Two experiments were conducted, and data were collected from 3 November to 18 November 2010. Observations were made in continuous three hour period between 6:30 a.m. to 11:00 a.m. Hummingbirds were attracted to the study site using artificial feeders , which were setup 2 days prior to the experiment , and all feeders contained 1:5 water; sugar concentration (20%) solution . Twenty percent sugar content was used because it is the average sugar concentration found in hummingbird flowers (Baker 1975). A visit is defined when a hummingbird drink from a feeder. Experiment 1 : This experiment examined the h ummingbird responses to potential terrestrial predators . Two levels of feeders were set up between 2 trees. The top feeders were 2 meters abo ve ground and the bottom feeders were 1 meter above ground. Eight feeders total , 4 on each level and each feeder were 1.5 meters apart. Feeders from top and bottom level were in an alternating fashion (F ig. 1). Feeders closest to the trees were classified as t he outer feeders, and the middle feeders were classified as the inner feeders . Five days of observations were obtained for this experiment.
Experiment 2 : This experiment examined h ummingbird responses to artificial predators. The top feeders were removed and 4 feeders 1 meter above the ground were presented with artifi cial predators. Hummingbirds have many different predators : egg and nestling predators such as toucans, jays, squirrels, and snakes. These predators are responsible for over 59% of the mortality rate in early stages of hummingbirds (Long, 1997). As an adult, hummingbird s have predators such as raptors, snakes, frogs, praying mantises, and orb webbing spiders . However, these predators only play a minor role in the adult hummingbird mortality (Fogden 2005). Six days of observed were made in this experiment. The first 3 days of observation were made with the absence of predators were used as control. On the fourth day, 2 rubber artificial snakes : boa constrictor (Boidae) and a rattlesnake (Viperidae) were presented with next to the outer feeders (see Fig . 2 ). On the fi ft h day, feeders were presented with artificia l wooden egg predators: Blue crowned motmot (Momotidae), Chestnut mandibled toucan ( Ramphasti dae ) and non egg predator resplendent quetzal (Trogonidae ) . On the sixth day, a ll feeders were presented with artificial predator s expect for feeder 3D. F eeder 3D and r esplendent quetza l (Trogonidae) were used to test if hummingbirds were able to perceive no predators feeders as a refuge within the p resence of artificial predators. FIGURE 1: First e xperiment desi gn, top level feeders were 2 meters above ground, and bottom level feeders were 1 meter above ground. 4 feeders at each level and each feeder were 5 ft part.
Figure 2. Second experiment design. Here shows the position s of the artificial snakes outer feeders. Xs are the represent ations of the artificial birds . Blue crowned motmot (Momotidae) and artificial snake were presented at the 1D feeder. 2D feeder was presented with non egg predator r esplendent quetzal (Trogonidae ). 3D feeder had no artif icial predator, and 4D feed was presented with Chestnut mandibled toucan (Ramphastidae ) and snake. R ESULTS Eight species of hummingbirds were observed in this experiment. Green Hermit ( Phaethornis guy ), Green Violet ear ( Colibri thalassinus ), Green crowned Brilliant ( Heliodoxa jacula ), Stripe tailed ( Eupherusa eximia ), Coppery headed Emerald ( Elvira cupreiceps ), Purple throated ( Archilochus colubris ), Ruby throated ( Archilochus colubris ) and Violet Saberwing hummingbird ( Campylopterus hemileucurus) . The Gre en crowned Brilliant hummingbird was the most abundant species, c omprising 54% of the total 887 observations . 373 hummingbirds were observed in experiment 1, and 514 in experiment 2. In experiment 1, result showed a significant diff erence between the two levels. The 4 t op feeders had an average of 53 visits over 5 days of observation , and 4 bottom feeders had an average of 23 visits over 5 days of observation ( t= 4.7, d.f. =4, p= 0.005 ) (Fig. 4). While the n umber of visits va ried bet ween each day, result s sho wed hummingbird favored top feeders . When comparing the position preference between inner and outer feeders, hummingbirds showed a significant preference for outer feeders over inner feeders. Th e outer feeders had average number o f visits of 49, and inner feeders had an average number of visits of 29 ( t= 3.12, d.f. =4, p= 0.02; Fig. 5 ). However, hummingbirds did not seem to be affected by the presence of the artificial predators in experiment 2 (Fig. 6 ). There was no sta tistical significant difference between the con trol
group , feeders with no predators while predators were present and when artificial predators were near by. Figure 3. The observed hummingbird species and abundance. Figure 4 : Here show the M ean ( + SD) number of visits 4 feeders/level/day over 5 days of observations.
Figure 5 . Here show the comparison in M ean ( + SD) number of visits between inner feeders, and outer feeders . Each contained 2 feeders /day over 5 days of observations. Figure 6 . No significant differences in th e number of visits between control, no and presence of artificial snake. D ISCUSSION Results in experiment 1 showed that hummingbirds favored feeders higher above ground than close to the ground. This could be influenced by the potential risk o f facing terrestrial carnivores whe n hummingbirds forage close to the ground . T errestrial predators such as c oati s (Nasua narica) were common ly seen at the study site . It was
observed that some coatis were feeding on the lower feeders during the stud y; this could be one of the reasons for hummingbirds to be wary of terr estrial danger (Lima 1991) . When comparing the number of visits betw een outer and inner feeders, humm ingbirds favored outer feeders more than inner feeders. This supports the prediction that hummingbirds prefer to feed from feeders closer to tree cover. Althou gh trees could harbor predators like sn akes , but the study site was performed at the edge of the forest and the view was rather open, h ummingbirds had a better view that allow them to be less vigilance for predators. Also, s ince outer feeders were close to trees, t ree covers allow birds to use as perch sites to conserve energy (Schneider 1984). While the presence of artificial predator in experiment 2 failed to support the original hypothesis, but the change in hummingbir d behaviors were detected . Alertness increased in the hummingbirds in the presence of predators . I t was apparent that hummingbirds spent more time investigating by hovering over, circling, and vocaliz in g when predators were present at the first hour. Even though feeding time at the feeders was not recorded in this study, it is encouraged for future investigations . Favoritism on feeders that were higher above ground and closer to tree c over supports the risk assessment and the optimal foraging theory. When options are available, hummingbirds preferred to feed from a low risk location . Even though hummingbirds showed a greater preference to feed from the top feeders in experiment 1. But when top feeders we re removed in experiment 2, hummingbirds continue to feed from the bottom feeders instea d of eliminating feeding completely . This suggests that hummingbirds reconsidered the risk factors could be presented when feed closer to the ground and that the risks were probably not high enough to abandon feeding. I n this case, it was suggested that the ability of hummingbirds to asses s risk was not great enough to deter feeding. Since f eeders contained unlimited amount of resources and were easily accessible, feedin g from the feeders can maximize the energy intake per unit time spend. More energy can be conserved by staying close to fe eders versus spend ing energy on visiting flower to flower to get minimum nectar rewards (Long 1997). The high demand of h ummingbirds means that they are obligated to be efficient at foraging and decision making . T his suggests e nergetically efficient behavi or is more important than the potential for risk in this study (Valone 1991). While the amount of risk animals p erceive is difficult to study, this study helps to understand the importance of risk assessment for foraging hummingbirds . This study calls for fu ture studies to investigate the effect of different artificial predators and with recorded feeding time for each visit. A CKNOWLEDGMENTS I would like to thank my advisor Pablo Allen for his assistance in developing my project concept, advising, and wit h the questions for the paper. Raquel MartÃnez for helping me to identify the hummingbird species. Moncho CalderÃ³n for baby sitting us and showing me the tools that I needed for this project. Anjali Kuma, and Alan Masters for being the coolest professors. The CIEE program, the biologi cal station, the Vega family for welcoming me into their ve made and shared my past 3 months of my life with.
LITERATURE CITED Baker, H.G. 1975. Suga r concentrations in nectars from hummingbird flowers. Biotropica 7: 37 41. Digatkin, L.A. 2009. Principles of Animal Behavior. Second ed. S.S. Norton & Company, Inc, New York, pp. 338 350. Flowers, T. 1998. Trade offs between caloric gain and anti predator defense in hummingbirds (Trochilidae). Tropical Ecology and Conservation, CIEE . Spring: pp. 150 162 Fogden, M. and P. Fogden. 2005. Hummingbirds of Costa Rica . Distrbudores Zona Tropical Publisher, South America. pp. 22 26. Lima, S.T. 1991. Energy, predat or, and the behavior of feeding hummingbirds. Evolutionary Ecology . 220 230 Lima, S.T. and T. Valone, at el. 1985. Foraging efficiency predation risk trade off in the grey squirrel. Animal behavior 33:155 165. Long, K. 1997. Hummingbirds: A wildlife handbo ok . Johnson. Boulder: 114 120 Miller, R. S. and Gass, C. L. 1985. Survivorship in hummingbirds: Is predation important? The Auk . American Ornithologists' Union, California, pp. 175 178. Schneider, K. J. 1984. Dominance, predation, and optimal foraging whi te throated sparrow flocks. Ecology 65:1820 1827. Tiebout, H. 1991. Daytime energy management by tropical hummingbirds: response to foraging constraint. Ecology 72: 839 851. Valone , T. 1991. Information for patch assessment: a field investigation with black chinned hummingbirds. Behavior Ecology . 211 222.
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Lai, Su Mei
Preferencias en la posicin de los comederos en respuesta a los depredadores potenciales y artificiales en los colibres
Feeder position preferences in response to potential and artificial predators in hummingbirds
When risks are present, animals consider the costs and benefits of foraging. This study investigates the preferred feeder positions when potential risks are involved in feeding hummingbirds. Two experiments were conducted in this study in order to have a better understanding of animal risk assessment. The first experiment examined whether there was a preference for high or low feeders in hummingbirds. Results showed that hummingbirds had a preference for higher feeders. Results also showed that hummingbirds have a greater preference to feed from feeders that are closer to trees (outer) than feeders farther away (inner) from trees. The second experiment showed no significant difference in the number of visits of hummingbirds when artificial predators were present versus when they were absent. This indicates that hummingbirds prefer to feed on feeders with the minimal potential risk, which suggests that hummingbirds are able to perceive risk. However, risk presented in this study might not be significant enough to have an
impact. This study provides an understanding of foraging behavior in hummingbirds and insights for future studies.
Cuando hay un riesgo presente, los animales consideran el costo y el beneficio de forrajear. Este estudio investiga la posicin preferida de los comederos cuando hay riesgos potenciales para los colibres. Se realizaron dos experimentos en este estudio con el fin de tener una mejor comprensin de la evaluacin de los animales de riesgo. El primer experimento examin si exista una preferencia por los comederos altos o bajos. Los resultados muestran que los colibres tienen preferencia por los comederos altos. Los resultados tambin muestran que los colibres tienen una mayor preferencia en utilizar los comederos ubicados cerca de los rboles y no los que estn lejos de los mismos. El segundo experimento no muestra ninguna diferencia en el nmero de visitas a los comederos cuando los depredadores falsos fueron expuestos versus la ausencia de los mismos. Esto indica que los colibres prefieren alimentarse con un mnimo de riesgo potencial, lo que sugiere que los colibres son capaces de percibir el riesgo. Sin embargo, el riesgo presentado en este estudio no es lo suficientemente significativo para tener un impacto. Este estudio provee un entendimiento del comportamiento de forrajeo en los colibres y los puntos de vista para futuros estudios.
Text in English.
Monteverde Biological Station (Costa Rica)
Estacin Biolgica de Monteverde (Costa Rica)
Tropical Ecology Fall 2010
Ecologa Tropical Otoo 2010
t Monteverde Institute : Tropical Ecology