Winter Participation by Neotropical Migrant and Resident Birds in Mixed-Species Flocks in Northeastern Mexico

Abstract

Mixed-species foraging flocks in northeastern Mexico are large, diverse, and include Neotropical migrant and resident species. I characterized the species composition and social organization of mixed-species flocks in four habitats of the El Cielo Biosphere Reserve during the winters of 1993-1995. Thirty-seven percent of all species found in the Reserve participated in mixed-species flocks, including 16 migrant species. Flocks in the dry pine-oak forest had more migrant individuals than flocks in the humid oak-pine/cloud forest or tropical semi-deciduous forest, but flocks in the tropical semi-deciduous forest had more migrant species than resident species. Migrant species rarely have been reported as nuclear species that attract other species to the flock and maintain flock cohesion, yet both Ruby-crowned Kinglet (Regulus calendula) in the dry pine-oak forest, and Blue-gray Gnat-catcher (Polioptila caerulea) in the tropical semi-deciduous forest, are migrant species that behaved as nuclear flock species. Bridled Titmouse (Parus wollweberi) and Crescent-chested Warbler (Vermivora superciliosa), Mexican resident species, were nuclear species in the dry pine-oak forest and the humid oak-pine/cloud forest, respectively. I found that Ruby-crowned Kinglet and Crescent-chested Warbler were follower species (species that occurred regularly in flocks but did not behave as nuclear species) in habitats where they were not nuclear species, indicating that the social role of a species within a flock may depend on habitat, resource availability, species density, or the other species present in the mixed-species flock.