A Description of Mixed-Species Insectivorous Bird Flocks in Western Mexico

Insectivorous bird flocks were observed in all types of forested habitats during the nonbreeding season in western Mexico. The species composition of flocks changed markedly and predictably among five categories of habitat type. The average number of species per flock in lowland habitats was 4.7, while a mean of 18.6 species participated in highland flocks, ranking the latter among the most species-rich flocks in the world. The mean proportion of the local insectivorous pecies that participated in mixed-species flocks was significantly greater in the highlands (6 1.3%) than in the lowlands (24.6%). About half of the flock participants in both undisturbed lowland and highland habitats were north temperate migrants, ranking west Mexican flocks among the most migrant-rich in the world as well. In highland flocks, the maximum number of individuals per attendant species was generally two to three, but there were often six to twelve individuals belonging to each of several nuclear species. The lowland deciduous forest flocks seemed to lack nuclear species.