Do juvenile males affect adult males’ reproductive success in brown-headed cowbirds (Molothrus ater)?

The present study was motivated by a recent anomalous finding between individual competitive performance and reproductive output in brown-headed cowbirds (Molothrus ater). White et al. (2002c) found that adult males who were housed with juveniles outcompeted adults who were housed with other adults when they were brought together to compete for mating opportunities. Although the males in groups that contained both juveniles and adults appeared to be better in securing reproductive opportunities, these groups were found to produce fewer eggs than groups with all adult males (White et al., 2002c). Because adult males were housed with juveniles, it was unclear whether the lower egg production was simply due to juveniles not reproducing or whether the adult males suffered decreased reproductive output. Is the presence of juvenile males advantageous to adult males or do they suffer reproductive consequences? In the current study, we studied four groups of captive cowbirds that differed only in the presence of juvenile males to determine what influence juvenile males have on adult male reproduction. At the end of the breeding season, we performed microsatellite parentage analysis on these four groups. Females in the two groups that contained juvenile and adult males produced fewer fertile eggs compared to females in the all-adult male aviaries. Furthermore, parentage analysis revealed that female reproductive patterns were similar in the two conditions, but females in the juvenile-adult aviaries produced fewer fertile eggs. Similarly, the males in the juvenile-adult aviaries showed a similar pattern of reproductive success as the males in the all-adult male aviaries, but had consistently lower reproductive output.