An avian dominance hierarchy at a supplemental water source in the Patagonian steppe

Birds often compete and engage in interspecific agonistic interactions for access to resources such as food and breeding territories. Based on the observed outcomes from such interactions (i.e., patterns of displacements) dominance hierarchies can be established. Knowing which species can outcompete others for essential resources allows researchers to make predictions about the broader ecological impacts of interspecific interactions. We constructed an interspecific dominance hierarchy of twelve avian species which visited an artificial water source in an arid region of coastal Patagonia, Argentina. Displacements were categorized into four types, based on the behaviors involved in the interaction, and we tested if they could predict the difference in dominance between the interacting species (the difference between calculated dominance coefficients for the two focal species). Indirect displacements, involving only the arrival of the dominant species to the water source without direct aggression toward the subordinate bird, occurred more frequently between species with a large difference in dominance. The most dominant bird observed was the kelp gull (Larus dominicanus), which, due to an increasing population and expanding range, in part due to food supplementation from fisheries waste, is likely to outcompete terrestrial and marine avian species for other scarce resources.

Fighting over food unites the birds of North America in a continental dominance hierarchy

ABSTRACTThe study of aggressive interactions between species has, to date, usually been restricted to interactions among small numbers of ecologically close competitors. Nothing is known about interspecific dominance hierarchies that include numerous, ecologically varied species. Such hierarchies are of interest because they could be used to address a variety of research questions, e.g. do similarly ranked species tend to avoid each other in time or space, and what will happen when such species come into contact as climates change? Here, we propose a method for creating a continental-scale hierarchy, and we make initial analyses based on this hierarchy. We quantified the extent to which a dominance hierarchy of feeder birds was linear, as intransitivities can promote local species’ coexistence. Using the existing network of citizen scientists participating in Project FeederWatch, we collected the data with which to create a continent-spanning interspecific dominance hierarchy that included species that do not currently have overlapping geographic distributions. Overall, the hierarchy was nearly linear, and largely predicted by body mass, although there were clade-specific deviations from the average mass–dominance relationship. Most of the small number of intransitive relationships in the hierarchy were based on small samples of observations. Few observations were made of interactions between close relatives and ecological competitors likeMelanerpeswoodpeckers and chickadees, as such species often have only marginally overlapping geographic distributions. Yet, these species’ ranks—emergent properties of the interaction network—were usually in agreement with published literature on dominance relationships between them.Interspecific dominance hierarchy, aggression, displacement, citizen scienceLAY SUMMARYWhen it comes to fighting over food, bigger is better but woodpeckers are best. The outcome of aggressive encounters between birds frequently determines which individual gains access to contested resources like food, but until now, little was known about such encounters between individuals of different species. We partnered with citizen scientists to record interspecific behavioral interactions at bird feeders around North America, and assembled these interactions into a continental dominance hierarchy.

Can Lanchester's Laws Help Explain Interspecific Dominance in Birds?

We studied the applicability of Lanchester's laws of combat to explain interspecific dominance in birds. We focused on 10 species of Australian birds in the arid zone of New South Wales that foraged at an established locust trap. Consistent with the “linear law,” larger species usually dominated smaller species in one-on-one encounters. We found no support for the “N-square law,” which predicted that large numbers of smaller species could dominate larger species when more abundant. Further analysis of the most abundant species revealed that it was less likely to visit the locust trap when larger, more dominant heterospecifics were present. Body size, and not numerical superiority, seems to be an important determinant in interspecific foraging decisions in birds.¿Puede la Ley de Lanchester Ayudar a Explicar la Dominancia Interespecífica en Aves?Resumen. Hemos estudiado la aplicabilidad de las leyes del combate de Lanchester en explicar la dominancia interespecífica en aves. Hemos focalizado este estudio en 10 especies de aves australianas de la zona árida de New South Wales, las cuales se alimentaron en trampas de insectos establecidas para tal fin. Consistente con la “ley lineal,” las especies de mayor tamaño usualmente dominaron a las especies más pequeñas en los encuentros uno a uno. No encontramos evidencia que apoye la “ley cuadrática,” la cual predice que un gran número de especies de pequeño tamaño podrían dominar a especies de tamaño mayor cuando las primeras son más abundantes. Posteriores análisis sobre la especie más abundantes revelaron que la probabilidad de visita a las trampas de insectos es menor cuando individuos heteroespecíficos más grandes y más dominantes están presentes. El tamaño corporal, y no la superioridad numérica, parece ser un importante factor en las decisiones de forrajeo en las aves.