Facultative development of courtship and communication in juvenile male cowbirds (Molothrus ater)

We investigated effects of naturally occurring variation in experience with adult males on development of song and courtship competence in captive juvenile cowbirds. We studied birds in groups housed in large outdoor aviaries that allowed birds to regulate access to social stimulation. In two aviaries, we housed juvenile males and females either with or without adult males. Birds remained in these conditions from September 1999 through their breeding season. We documented social and vocal development of juvenile males in the two aviaries by measuring social assortment and patterns and frequencies of their song interactions. We then brought the juveniles from the two aviaries together to compete against each other for access to females. In addition, we recorded juveniles' songs four times over the study and played back their breeding season songs to females in sound-attenuating chambers to measure the effectiveness of songs in eliciting copulatory responses from the females. Compared to juvenile males housed with adult males, juvenile males housed without adult males developed atypical behavior patterns. They (1) displayed little intrasexual aggression or near-neighbor associations and (2) exhibited different patterns of courtship and copulation, but (3) were as successful at competing for copulations. Furthermore, they developed stereotyped songs sooner and developed more potent breeding season songs. These different outcomes could not be traced to one variable but to a cascade of effects involving diverging patterns of song acquisition and social interaction. The patterns of social skills that emerged indicate considerable plasticity in the mechanisms underlying acquisition of courtship competence.