Alternate functions for duet and solo songs in magpie-larks, Grallina cyanoleuca

Several possible functions have been proposed for antiphonal duetting in birds, including pair-bond maintenance, cooperative territorial defence and acoustic mate guarding. Previous work has suggested that duetting in magpie-larks was consistent with both defence of the territory and guarding the mate against usurpation. We conducted playback experiments designed to distinguish between these two functions. Responses of males and females to simulated intrusion (broadcasts of unfamiliar solo songs) suggested functional differences for solo songs and duets. Both sexes initiated more songs in response to same-sex conspecific song, suggesting that solo songs are used to deter same-sex rivals in the context of territory defence. Contrary to predictions of the mate-guarding and pair-bond maintenance hypotheses, the sex of playback song had no effect on the likelihood of either sex answering their partner's song to form a duet. Thus, duets are most likely to be performed for the purpose of cooperative territorial defence