A DESCRIPTION OF MIXED-SPECIES INSECTIVOROUS BIRD FLOCKS IN WESTERN MEXICO

The species composition of flocks changed markedly and predictably among 5 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 species that participated in mixed-species flocks was significantly greater in the highlands (61.3%) than in the lowlands (24.6). About 1/2 of the flock participants in both undisturbed lowland and highland habitats were north temperate migrants, ranking W Mexican flocks among the most migrant-rich in the world. In highland flocks, the maximum, number of individuals per attendant species was generally 2-3, but there were often 6-12 individuals belonging to each of several nuclear species. The lowland deciduous forest flocks seemed to lack nuclear species. -from Author