Aggression in female Red-Winged Blackbirds: A strategy to ensure male parental investment

Summary Female Red-winged Blackbirds (Agelaius phoeniceus) are often aggressive towards conspecific females during the breeding season. We hypothesize that the function of female-female aggression in this species is to guard the nonshareable portion of the male's parental investment.The investment-guarding hypothesis predicts that a female should be more aggressive toward another female evincing interest in mating with the territory-owning male than toward a female simply perching within the male's territory. Results of mount presentations to females with active nests confirmed this prediction. Nesting females attacked a stuffed conspecific female mounted in a precopulatory, soliciting posture significantly more often than a mount in a normal, perched posture.The male's nonshereable parental care consists of provisioning his young, and most of this care is invested in the brood of his primary (first-to-nest) female. It is therefore predicted that primary females should be more aggressive than secondary (later-nesting) females. Female mount presentations also confirmed this prediction. Primary females attacked the soliciting mount significantly more often than secondary females.