The Ecological Consequences of Interspecific Territorial Behavior in Birds

Abstract

When considering cases in which territorial aggression occurs between species, we find that the relationship varies with the species' geographical and ecological distributions. If similarly sized species are narrowly sympatric or are widely sympatric but normally occupy different habitats, they both are interspecifically territorial and maintain mutually exclusive territories. If similarly sized species are widely sympatric and are common in the same habitats, one species is interspecifically aggressive and the other species flies away when chased. In this paper I present a model that explains these observations. I propose that interspecific territoriality can account not only for these differences in behavior and habitat but also for differences in foraging behavior and bill shape between sympatric species. The last three differences have usually been explained to be consequences of interspecific competition for food. The assumptions of the model proposed here are that (i) interspecific territoriality is misdirected intraspecific territoriality, (ii) territorial behavior prevents some individuals from breeding in optimal habitat, (iii) the individuals of one species are usually dominant over the other species in interspecific encounters, and (iv) of the habitats available to two interspecifically territorial species in their area of sympatry, one habitat is optimal for them both. This model is unusual in that a corollary of assumption (i) is that interspecific territoriality is not an adaptive characteristic, contrary to the assumptions of earlier writers. Nevertheless, the model is interesting and worth consideration because of the range of observable phenomena that are predicted.