Nest Placement Relative to Food and Its Influence on the Evolution of Avian Coloniality

Abstract

A model proposed by Horn, in which he showed that animals can reduce their travel distances by nesting in the center of their foraging arena whenever food is unpredictable, is often cited as a potential cause of avian coloniality. We investigate to what degree the model may account for the evolution of colonial nesting in cliff swallows (Hirundo pyrrhonota) and other birds. Two critical assumptions of the model, that foraging arenas are externally bounded and that potential nesting sites are distributed uniformly, do not hold for cliff swallows. Consequently, the model cannot apply to cliff swallows or explain why they live colonially. Rigorous evaluation of these assumptions is necessary before testing the model for a given species because, as we show for swallows, the model's predictions can be upheld even when both assumptions are violated. Reduction in travel distance by nesting at a central site is probably a general result for any central place forager that exploits a spatiotemporally variable food source. This result per se does not mean that the geometrical model explains coloniality. Because the model's critical assumptions probably seldom hold for natural populations, the model is unlikely to explain the evolution of avian coloniality in general.