Competition and hybridization drive interspecific territoriality in birds.

Abstract

Historically, aggressive territorial interactions between members of different species have been dismissed as relatively rare occurrences and unimportant selective forces. We conducted the largest-ever comparative study of interspecific territorial behavior, amassing a dataset of all published observations of territorial aggression between species of North American perching birds. We found that interspecific territoriality is common, with individuals from nearly a third of all species defending territories against one or more other species. Contrary to the prevailing view, we also found abundant support for the hypothesis that interspecific territoriality is an adaptive response to resource competition and reproductive interference, not just a rare occurrence restricted to recently diverged lineages, and that interspecific territoriality constrains the evolutionary divergence of territorial signals.