Winter ecology of the gray vireo Vireo vicinior in Sonora, Mexico

Abstract

A marked population of wintering Gray Vireos (Vireo vicinior) was studied for two seasons (1985-1986 and 1986-1987) on two study sites near Puerto Lobos, Sonora, Mexico. Eleven of the 15 individuals banded in the first winter returned to the same areas in the following winter. Territories were defended throughout the winter and averaged 0.9 ha in size (N = 9). Territorial interactions occurred frequently along boundaries as neighbors trespassed to forage on each others' territories. All birds appeared to defend individual territories. First year birds appeared to occupy marginal territories on the periphery of the best habitats. Fruit from the elephant tree, Bursera microphylla, became a dominant part of the vireo's diet as winter progressed and the fruit ripened. The importance of B. microphylla to the vireos' winter diet and the high degree of overlap between the winter range of the vireos and the distribution of the plant suggested a mutualistic interaction between them. Gray Vireos acted as the primary dispersers for the plant.