The developmental ecology of acoustic reactions: Approaches to song playbacks by male cowbirds change across their first year of life

We examined whether captive, wild-caught juvenile male cowbirds (Molothrus ater) would react to playbacks of vocalizations of conspecifics and heterospecifics and if so, whether those reactions would vary across development. We played five different types of vocalizations to juveniles: adult male cowbird song, adult female cowbird chatter, juvenile male cowbird plastic song, songs from species that are common cowbird hosts, and songs of species that cowbirds do not parasitize. We played the vocalizations to different groups of juvenile males at four times during their first year of life (summer, autumn, winter, and spring). Juveniles approached the playback speaker in response to broadcasts of specific vocalizations, but these responses differed across the seasons. Early in summer, young males approached the speaker more often when adult conspecific vocalizations were played. In the autumn, subjects approached the speaker more often during broadcasts of juvenile cowbird plastic song. In the winter, juveniles approached conspecific and heterospecific vocalizations equally. In the spring, they once again approached adult conspecific vocalizations more than any others. Also, only in the spring, the time subjects spent near the speaker during the male cowbird song playbacks was positively related to the song's attractiveness to females (its ability to elicit copulation solicitation displays from females). These responses suggest that song reactivity may serve several functions across development, including species recognition, song learning, and group organization.