Avian duetting is a poorly understood phenomenon despite many hypotheses as to its function. Contrary to the recent view that duetting functions for mate guarding and is a result of conflict between the sexes, Australian magpie-larks, Grallina cyanoleuca, do not use duetting as a paternity guard. I used a playback experiment to investigate the role of antiphonal duetting in territorial defence and pair bond maintenance, two traditional hypotheses about the function of duetting. The experiment showed that, like many nonduetting species, magpie-larks recognize neighbours on the basis of song. It also provided evidence of functional differences between duetting and solo singing which indicate that temporal coordination of song between partners is used to maintain the territory and pair bond. Duets were more threatening territorial signals than solo songs: males initiated more vocalizations in response to playback of duets than playback of solos. Simulated intrusion also caused males and females to approach the speaker together and coordinate more of their vocalizations to form duets. Females did not engage in sex-specific territorial defence, responding equally strongly to playback of male and female song, and maintaining both territory and pair bond by attempting to exclude intruders of either sex. Males initiated more vocalizations in response to playback of male than female song, and their likelihood of duetting appeared to be related more to threats to the pair bond, in particular desertion by their partner. Copyright 2000 The Association for the Study of Animal Behaviour.