Interspecific Competition and Social Hierarchies in Frugivorous

Journal of Young Investigators RESEARCH ARTICLE

Brittany M. Thornton1, Jessie L. Knowlton23

Dominance hierarchies have long been observed in nature across many different species. These hierarchies often arise via agonistic - neotropical birds at feeding stations baited with bananas in the Wilson Botanical Garden of the Las Cruces Biological Station in Costa Rica. At each feeding station, we used a video camera to record all birds that came to the station, the amount of time each spent on the platform, and all displacement behaviors, including which species was the aggressor and which was displaced. An analysis of 390min of - ing stations. This dominance hierarchy largely correlated with species’ body weights, with heavier species being more dominant. More dominant species were also found to spend a greater amount of time on the platforms than subordinate birds. These results suggest that well as predict which species might be most threatened if resources become scarce.

INTRODUCTION Dominance hierarchies can either be transient, forming at a - patchy resource or in times of resource scarcity (Rowell, 1966), or tent winner of agonistic interactions (Drews, 1993). Differences develop early and last over an individual’s lifetime (Chase, 1982). in dominance levels among individuals are common both within These hierarchies are often formed by interference competition, and between species (French & Smith, 2005). These differences in whereby one individual will use physical aggression or agonistic displays to intimidate a competitor away from a resource (French in the community (Chase, 1982). Within species, these dominance & Smith, 2005). Agonistic displays are predicted to be used more - as food and mates (Hawley, 1999). Between species, these hier- physical altercation, in part due to the time and energy saved by - not engaging in combat (Bernstein & Ehardt, 1985). Larger individuals are often the winners in interference competition, and thus dominance hierarchies can contribute to the maintenance of high species diversity in an ecosystem by allowing individuals to coex- linear fashion (French & Smith, 2005). Tropical frugivorous birds ist, even if they have overlapping niches (Tilman, 2004). Domi- provide excellent model organisms with which to study dominance nance hierarchies are important not only at the species and com- hierarchies in species with overlapping niches because this guild munity levels, but at the individual level as well. The position of of birds is very high in species richness. Many frugivorous bird an individual in a dominance hierarchy is hugely important, as it species have undergone convergent evolution and thus share simi- can determine access to resources such as food, mates, shelter and breeding sites (Morse, 1974; Daily & Ehrlich, 1994). Costa Rica is home to many tropical fruit specialists as well as birds that function as opportunistic omnivores (Pratt, 1984). In 1 resource accessibility at feeding stations located around the Wil- son Botanical Garden at Las Cruces Biological Station in Costa 2 School of Forest Resources & Environmental Science, Michigan Rica. The Wilson Botanical Garden provided an attractive study site due to the large number of bird species that frequent the area. 3 Math and Sciences Department, Kapi’olani Community College, of social status on foraging behavior and community structure of tropical frugivorous birds in natural versus human-dominated hab- itats in this same location. They observed birds feeding at fruiting *To whom correspondence should be addressed: trees during the dry season and found that species with larger bill [email protected] able to gain greater access to resources. These more dominant spe-

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cies were also able to forage further from primary forest than sub- cut from the invasive bamboo forest in the area. On the top of each ordinate species (Daily & Ehrlich, 1994). bamboo pole we nailed a 25 x 20cm plywood platform and as- Our study differs from that of Daily and Ehrlich (1994) be- signed each station an individual name or number. Bananas were cause we observed an overlapping but unique suite of frugivorous - bird species interacting at controlled feeding stations during the servations began, to condition birds in the area to come to the new - ings with those of Daily and Ehrlich (1994) to determine if a dif- eaten from the bananas between the hours of 5:30 a.m. to 12:30 ferent suite of species during a different time of year at novel food p.m. Bananas were placed out about every 2hr to account for the sources would show the same pattern of larger individuals being bananas that were removed by squirrels. more dominant in the feeding hierarchy. We set out to test if even - ferences would yield differences in dominance levels, since these species have overlapping niches and yet are able to coexist.. We speculated that larger species would spend more time on the feeding platforms than smaller species, and larger species would dis- place smaller species more often, thus creating a linear dominance - archies, however this study is unique because we used a controlled setting to determine how dominance hierarchies are structured within a species-rich community of neotropical frugivorous birds with overlapping niches.

METHODS Study Area Our study was conducted at the Las Cruces Biological Station, located in Coto Brus County in southern Costa Rica (8° 47’ 7’’ N, 82° 57’ 32’’ W). The station encompasses over 300ha of premon- tane wet forest habitat ranging in elevation from 1,000 to 1,400m. More than 400 species of birds can be found in this environment, - tablished feeding station baited with bananas and other fruit and Figure 1. The Wilson Botanical Garden (light green) in Las Cruces creating a radius of new feeding stations around it, we were able Biological Station, Costa Rica. Each circle represents a distance of 25m to observe social dominance in a controlled setting. We placed from the established feeding station (red). The additional feeding sta- feeding stations within the Wilson Botanical Garden, a large 16ha green represents primary forest. Station. Behavioral Observations Sampling Design Establishing Feeding Stations behavior at each feeding station between 5:30 a.m. and 12:30 p.m. A popular bird feeding station, located near the dining area of the from 12 July to 26 July 2013. We randomly selected the order in Las Cruces Biological Station within the Wilson Botanical Gar- which we observed the stations, and each station was observed for den, has been in use for roughly 15 years as a place for local neo- one 30min period using visual observations or a Bell and Howell tropical bird species to perch and eat fruit (Z. Zahawi, personal DNV900HD camera recorder (Bell and Howell, Durham, NC). Total observation time was 390min. During observations, we re- starting point, we plotted circles around it at 25m intervals using corded each species that came to the station, the amount of time ArcMap (Version 9.3.1.; Environmental Systems Resource Insti- each bird spent on the station, and all displacement events, includ- tute, 2009). We then placed an additional 12 feeding stations to ing which species was the displacer and which was the displacee. the east and west of the established station at intervals of roughly We considered a bird to be the displacer when it caused another 25m, based on logistical constraints (Figure 1). Platforms were bird to leave the feeding platform, either by non-physical intimi- placed 1m from trees with low branches to provide escape vegeta- dation or physical contact. The bird that left was the displacee. We tion. The stations were constructed out of bamboo poles (1 - 1.5m) used tallies of these observations to determine which species were

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Figure 2. Adjusted dominance scores (dominance score/adjusted dominance score) of 13 neotropical bird species, based on 232 interactions observed at feeding stations located around the Wilson Botanical Garden. Dominance scores were calculated as (# of times species displaced another species) – (# of times species was displaced). We adjusted the dominance scores for each species by the number of times that species was observed, so as not to over weigh common species.

species by consulting A Guide to the Birds of Costa Rica (Stiles & on the feeding platforms by each species. It should be noted that although we treated each individual sighting as independent, the individuals could have violated this assumption.

RESULTS Social Dominance Hierarchy We observed 13 bird species and 232 interactions during 150min of video recordings and 240min of direct observation of the 13 Figure 3. Linear regression of adjusted dominance scores (dominance banana feeding stations. We found strong evidence for an interspe- score/adjusted dominance score) vs. weight (g) of 13 neotropical bird X2 = 35.71; species. p < .001; df = 12; Figure 2). Blue-crowned motmots (Momotus momota) had the highest dominance score, while yellow-crowned higher on the dominance hierarchy (see analysis below). euphonias (Euphonia luteicapilla) had the lowest (Figure 2). Statistical Analyses Blue-gray tanagers (Thraupis episcopus) had a dominance score To determine the adjusted dominance score of each species we used the following equation: . The dominance score of a species was determined by subtracting the Dominance status generally correlated with body weight among number of times individuals of the species were displaced from the species (linear regression r2 = 0.587, Figure 3). number of times an individual displaced a bird of another species. Time at Feeding Platforms - of each species, we used a contingency table to show the frequen- tween species, although the two largest species, the blue-crowned cy distribution. In order to determine whether there was a relation- motmot and clay-colored thrush (Turdus grayi), spent more time ship between the adjusted dominance scores and species’ weights, on the platforms than the other species (H = 21.34; df = 12; p = we ran a linear regression. We obtained the average weight of each 0.046; Figure 4).

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Figure 4. Boxplots of time (min) spent on feeding platforms for each of 13 bird species, based on 390min of observation of 13 feeding platforms in the Wilson Botanical Garden, in Las Cruces Biological Station, Costa Rica.

DISCUSSION Feeding Station Activity time spent on the feeding stations. However, the top two domi- The greatest bird activity occurred at the established feeding sta- nant species, the blue-crowned motmot and clay-colored thrush, tion in the nucleus of our study site, and thus a majority of the ob- spent more time on the stations than the other species. Daily and servations were recorded there. The interactions between bird spe- the probability of a species being interrupted while foraging and dominance hierarchy, since the feeding station has been in use for its social dominance status. Our feeding stations were lower and 15 years. The temporary bamboo feeding stations we constructed more in the open than the trees where Daily and Ehrlich (1994) - and remember the locations of the individual stations. Silver- nerable to predators. This difference in location of the behavioral throated tanagers (Tangara icterocephala) were often observed at observations may explain why all the species we observed showed the new feeding stations earlier than most other species, and their relatively short times spent on the platforms, as well as why we presence at the feeding stations may have cued other species into did not obtain the same results as Daily and Ehrlich (1994). This the presence of a novel food source. result is interesting because it suggests that dominance hierarchies have prevented active competition between some species. Indeed, Dominance Interactions mostly single individuals were observed at these stations, with few During periods of limited resource availability, the cost of dis- interactions (competitive or otherwise) with other birds. There placement or defensive behaviors may be deemed necessary by were very few displacement events observed at these temporary some individuals (Case & Gilpin, 1974). However, some studies feeding stations. Toward the end of the study, blue-crowned mot- suggest that species high on the dominance hierarchy may display mots were observed with increased frequency at these bamboo costly interference behaviors regardless of food stress (Rowell, stations compared to the established feeding station, possibly pre- larger, because the cost of being aggressive is less of an expense initiate. for those species. Our hypothesis was supported, and we found Time at Feeding Platforms the most dominant species were the blue-crowned motmot, clay- Sal- feeding or waiting at the feeding stations in order to be more vigi- tator atriceps), while the least dominant were the tellow-crowned lant and less vulnerable to competing species and predators. We Euphonia laniirostris), golden-

hooded tanager (Thraupis larvata ACKNOWLEDGEMENTS Though our results in dominance factors were similar, other aspects of our study differed from that of Daily and Ehrlich (1994), - who found the most dominant species during the dry season at liipuleole and Devan Tatemichi for helping us set up our feed- naturally fruiting trees were the buff-throated saltator (Saltator ing stations, which were made with the help of the Las Cruces maximus), clay-colored thrush, Cherrie’s tanager (Ramphocelus passerine) and palm tanager (Thraupis palmarum), while the most Michelson for statistical advice, Wendy Townsend for her support subordinate species were the bayheaded Ttnager (Thraupis gyro- and observations and Michael Breed for his help and suggestions. la), golden-hooded tanager (Tangara larvata- nia and Tennessee warbler (Vermivora peregrine). The differences the National Science Foundation (NSF) for funding. B. Thornton we observed in the dominance hierarchy of Daily and Ehrlich - - orado Boulder for supplemental funding and especially Dr. Sue ing during the wet season, in addition to our observations being made at novel, man-made food resources. Repeating our study in the dry season, and including both natural and man-made food REFERENCES sources, would shed light on whether these possible explanations are supported. Bernstein, I., & C. Ehardt. (1985). Intragroup agnostic behavior in While there were occasional bouts of aggression between Macaca mulatta). International Journal of Primatology, 6(3), 209-226. Case, T. J. & M. Gilpin, M. (1974). Interference Competition and crowned motmots, simultaneously displaced multiple birds at the Niche Theory. Proceedings of the National Academy of Sci- ences, 71(8), 3073-3077. were not exclusively feeding. Blue-gray tanagers also showed Chase, I. D. (1982). Dynamics of hierarchy formation: the se- surprisingly high levels of dominance when compared to other quential development of dominance relationships. Behaviour, birds of a similar weight, and often displaced tanagers of similar 80(3-4), 218-240. Status on Individual Foraging and Community Structure in a The positions of these species in the dominance hierarchy may be Bird Guild. Oecologia, 100(1-2), 153-165. - and it would be interesting to include these measurements in fu- mal behaviour. Behaviour, 125(3), 283-313. ture studies. ArcGIS (Version 9.3.1.) [Computer software]. Redlands, CA: ESRI. CONCLUSIONS French, A. R. & T. B. Smith, T. B. (2005). Importance of Body Dominance patterns are particularly interesting due to their ap- Tropical Frugivores. Biotropica, 37(1), 96-101. plication to a diverse range of topics in ecology, including food Hawley, P. H. (1999). The Ontogenesis of Social Dominance: A Strategy-Based Evolutionary Perspective. Developmental Re- (Daily & Ehrlich, 1994; Holway, 1999; French & Smith, 2005). view, 19(1), 97-132. For instance, as some species are removed from or introduced to a community, positions in a dominance hierarchy could potentially Displacement of Native Ants by the Invasive Argentine Ant. shift to accommodate the change, impacting patterns of seed dis- Ecology Durham, 80(1), 238-251. persal and forest structure (Holway, 1999; French & Smith, 2005). Lebbin, D. J. (2008). Aggressive interactions and preliminary evidence for reversed sexual dominance in Ramphocelus Tana- gers. Ornitologia Neotropical, 19(3), 329-334. to adapt to a changing environment, which could be useful in de- Morse, D. H. (1974). Niche Breadth as a Function of Social Domi- signing conservation programs. For future studies, comparisons nance. The American Naturalist, 818-830. Pratt, T. K. (1984). Examples of Tropical Frugivores Defending for gaining a more complete understanding of this complex be- Fruit-Bearing Plants. The Condor, 86(2), 123-9. havioral system, such as we did here with our comparison to Daily Journal of Zoology, 149(3), 344-364. into the unusually high aggression observed in blue-gray tanagers Rowell, T. E. (1974). The Concept of Social Dominance. Behav- - ioral Biology, 11(2), 131-54. ferent cause. Snow, D. W. (1981). Tropical Frugivorous Birds and their Food

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