The paradox of age-related dominance in brown-headed cow birds (Molothrus ater)

Abstract

Studies of brown-headed cowbirds (Molothrus ater) suggest that yearling males are as successful at acquiring mates as adult males. Our aim in this study was to determine if dominance status of males was also independent of age when they were competing for access to food. Five mixed-age (adult and yearling) groups of 9 or 10 male brown-headed cowbirds were maintained in aviaries. Using 6829 interactions recorded at a feeding dish we computed dominance hierarchies, all of which proved to be linear and stable. In two of the five groups all adult males were dominant to all yearlings, with less clear-cut age differences in the remaining three groups. Adults were dominant in 54 of the 66 dyads consisting of an adult and a yearling in which one individual won a significant proportion of their interactions. Although adults were significantly heavier and larger than yearlings, weight and size contributed little to the outcome of dominance interactions. These results suggest that dominance status in one social arena may be a poor predictor of social status in another for brown-headed cowbirds. Our results do not explain why young male cowbirds are successful in competing for mates.