Summary
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.
Methodology
We conducted the study at the Queen's University Biological Station in eastern Ontario between 7 May and 28 June 1985 and between 5 May and 7 July 1986. In total, five groups of either 9 or 10 males were used. The first group (five adults, five yearlings) was captured 7 May 1985 in mist nets, using cracked corn to attract the birds. All subsequent birds were caught in decoy traps. Thus, within groups, all birds were caught by the same method, thereby avoiding potential biases associated with the capture method (Weatherhead and Greenwood 1981). The groups caught on 24 May 1985 and both 1986 groups also consisted of five adults and five yearlings. In one of these groups (5 May 1986), one yearling died in captivity and his interactions were not included in the analysis. The 3 June 1985 group consisted of four adult and five yearling males.