Ecological Relationships of Two Interspecifically Territorial Vireos

The foraging behavior and habitats of territories of Red—eyed Vireos (Vireo olivaceus) and Philadelphia Vireos (Vireo philadelphicus) were studied in northeastern Ontario. Discriminant analysis of data on forest structure and composition showed no differences in habitats of the territories of the 2 vireos. Principal components analysis did indicate that Red—eyed Vireos occupied a wider range of habitats than did Philadelphia Vireos. However, both species had higher breeding success in areas they occupied jointly than Red—eyed Vireos did in areas where it occurred and Philadelphia Vireos were absent. Similar analyses of the foraging data indicate that the 2 species are similar in frequency of use of common foraging methods, but do differ in the frequency of use of uncommon foraging methods. These vireos are interspecifically territorial, and the results used are to evaluate the predictions of several authors about the evolution of interspecific territoriality. I conclude that interspecific territoriality can be maintained by natural selection, and that close ecological similarities among interspecifically territorial species do not violate the competitive exclusion principle. The particular relationship resulting from selection (mutual convergence, social mimicry, or divergence) in interspecifically territorial situations may be a function of the extent to which the species are ecological generalists or specialists.