Birds of Many Feathers: The Formation and Structure of Mixed- Species Flocks of Forest Birds

Considering the differences in the social and ecological requirements of the different participants, the cohesive movement of animal groups is an amazing phenomenon. Associations composed of many species, each with its own intraspecific social organization and species-specific requirements, seem even more improbable. Yet mixed-species fish schools (Ehrlich and Ehrlich 1973; Alevizon 1976), ungiilate herds (Fitzgibbon 1990), primate troops (Gartland and Stnihsaker 1972; Terborgh 1983; Peres 1992a; Cords, chap. 4, this volimie), and bird flocks are commonplace. Nowhere else in the animal kingdom is the phenomenon of mixed-species associations more widespread than it is in birds. Mixed-species associations of birds are particularly well developed in forests, occurring from the most diverse tropical forest to the species-poor taiga. During the nonbreeding season in particular, a majority of bird species participate in mixed-species flocks. Mixedspecies forest flocks contrast with flocks found in open habitats (blackbirds, shorebirds, finches, etc.) in their smaller and more consistent sizes, more regular membership, the presence of a consistent flock home range, and participation by a small number of individu-