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. Promiscuity is generally considered to be mating behavior in which there is no lasting pair bond, and males and females are both likely to copulate with more than one individual of the opposite sex (Verner and Willson 1966, Lack 1968). Promiscuous mating behavior in passerine birds should be rare because promiscuity implies almost complete freedom of males from parental care (Selander 1972) and, possibly, territorial responsibilities, if a female's choice of a male is unaffected by his territory. However, promiscuity may be more prevalent than has generally been thought, as is suggested by the results of Bray, Kenelly, and Guarino's (1975) study of vasectomized male Redwinged Blackbirds (Agelaius phoeniceus). In their study, females of vasectomized males regularly laid fertile eggs if there were fertile territorial males nearby. Promiscuous behavior of females suggests that differences among males may not be critical; this would be more probable if the parental care contributions of the males were