The influence of food and vegetation on bird distribution in tropical deciduous forest and dry oak woodland in western Mexico
Bird species densities, richness and diversity were estimated on 11 7 plots in a dry forest and oak woodland in western Mexico. The counts were performed during autumn 1990 and 1991 and spring 1991 and 1992. The plant composition and stratification were measured on each plot. Arthropod densities were estimated for most trees and shrubs during the two autumns. The relationship between bird species diversity and the plant associations was inspected by means of a canonical ordination. The plant variables explaining the species richness, diversity, total density and evenness were obtained by means of multiple regressions. The two methods were complementary and the results suggest that food abundance might be related with species richness and total number of individuals. An ordination of the sampling plots, based on the bird species counts, separated the main plant associations. Nevertheless, there were no discreet sets of birds corresponding to each associations. Bird species distribution was individualistic with loose groups of species sharing different associations. The plant variables with highest correlation coefficients in the ordinations corresponded to the vegetation type and in general they were not used directly by the birds. Birds were grouped into guilds according to foraging strategies and the plant species preferences were estimated. Even though food does not seem to control the bird species distribution for non-insectivorous species, birds favour those plants offering the most appropriate food type for each guild. The influences of food on the distribution and plant choice was estimated more closely for the insectivorous guilds. In addition to a significant correlation between gleaning insectivores and lepidopteran larvae densities in the first year, insectivores had a significant preference for those plants with highest lepidopteran larvae and homopteran densities.