The Adaptive Significance of Colonial Nesting in the Brewer's Blackbird (Euphagus Cyanocephalus)

The adaptive significance of colonial nesting for the Brewer's blackbird was studied in a sagebrush desert in eastern Washington. The colonies were small enough (5—30 pairs) to allow analysis of the behavior of individual birds and their reproductive success. The structural simplicity of the study site allowed easy access to the nests and accurate measurement of their spatial distribution. The effects of predation on breeding success were analyzed with respect to colony shape and the spatial distribution of the nests. On the study area the predation pattern favored more widely spaced nests in liner colonies, but more closely spaced nests in round colonies. I examined the role of colonial nesting in aiding the exploitation of a spatially and temporally variable food supply by taking food samples from nestling birds, measuring patterns of food availability, and observing the foraging patterns of the adult birds. An idealized model relates the spatial distribution of nests to the efficiency of individual exploitation of different distributions of food. In addition to conforming to the conditions of the model, Brewer's blackbirds take advantage of each other's foraging success. In conclude that colonial nesting in the Brewer's blackbird is primarily adaptive to the variable nature of their food supply.