Plasticity in Adult Development: Experiencewith Youngmales Enhancesmating Competence in Adult Male Cowbirds, Molothrus Ater

The social environment can play an important role in organizing organisms’ behavioural development. We studied the effect on adult male cowbirds’ communication and matingrelated behaviour of being housed in social groups with juvenile males. In two large outdoor aviaries, we housed adult males, juvenile females and adult females either with or without juvenile males. Conditions remained intact from September 1999 through the Ž rst half of the breeding season in May 2000. We observed them throughout this time, documenting singing interactions, patterns of afŽ liation, and song production. We then brought the two groups of adult males together by rotating individuals from the groups into a mating competency tournament, allowing the males to compete with each other for mating opportunities with a new group of females. Throughout the study prior to the mating competency test, there were few differences among adult males in the two conditions as measured by amount and use of song, the quality of their songs, or number of copulations they received. In the mating competency tournament however, signiŽ cant differences among males in the two conditions emerged. Compared with adult males that had been housed without juvenile males, adult males that had formerly been housed with juvenile males were more successful in the mating competition as measured by: success in getting copulations, number of copulations received, and latency to get copulations. They also engaged in more male-male singing interactions. These results provide evidence to suggest that development of mating competency is malleable throughout life in response to the social environment that individuals experience.