Evolution of Promiscuity in the Brown-Headed Cowbird

Abstract

A study of the social behavior of Brown-headed Cowbirds (Molothrus ater) in 1974 and 1975 suggests that in prairie communities the predominant mating behavior is promiscuous. Individually marked cowbirds of both sexes were observed copulating with more than one individual, suggesting that long-term pair bonds were non-existent. On the study area cowbirds parasitized three ground-nesting species and foraged among grazing cattle. Thus, cowbirds bred and fed in the same area. Since the cowbirds foraged with grazing cattle, the latter became a "moving resource." It is hypothesized that this situation selects against the evolution of territorial behavior, and for the evolution of promiscuity.