Interspecific Aggression and Habitat Selection by Amazonian Birds

Abstract

We report that interspecific territoriality is a common spacing mechanism among closely related bird species distributed along a primary successional gradient in the meander belt of an Amazonian whitewater river. In the first phase of the research, we mapped the territories of more than 330 bird species in large census plots encompassing the complete successional gradient. We found species pairs in over 20 genera that showed contiguous but non-overlapping territories, such that early stages of the successional gradient were occupied by one member of the pair, and later stages by the other. Other species pairs showed additional types of spatial relationships, including partially overlapping and completely overlapping territories. Using reciprocal heterospecific playback experiments, we tested for the existence of aggressive interactions between the members of species pairs (usually congeners) showing all three types of spatial relationship. Among 12 species pairs showing contiguous, non-overlapping territories, 10 showed evidence of interspecific aggression (approach to the playback speaker). In eight of these 10 cases, the responses were markedly asymmetric. Target individuals of one species approached the speaker, whereas individuals of the other species remained in place or moved away. The heavier species was consistently the aggressor. In six species pairs showing partial territorial overlap along the successional gradient, only one species displayed statistically significant interspecific aggressiveness, although some individuals in all six pairs approached or avoided the speaker. In species pairs distributed with completely overlapping territories, we found interspecific aggression in only one of five genera tested. We suggest that spatial segregation of congeneric bird species on habitat gradients (presumably including elevational gradients), is commonly underpinned by interspecific territoriality mediated through directed, asymmetrical interspecific aggression. Aggressive asymmetry suggests a despotic model of habitat occupancy, in which the larger species occupies the more productive end of habitat gradients, and the smaller species occupies less productive habitats. The pronounced successional gradients characteristic of Amazonia may explain much of the increased species richness, especially within genera, of Amazonian bird communities.