Multiple functions to duet singing: hidden conflict and apparent cooperation.

joint territory defence, mate guarding and pair bond formation are the three principal hypotheses put forward to explain the occurrence of duet singing in birds and primates. We tested predictions from all three hypotheses in a subspecies of the plain, or canebrake, wren, Thryothorus modestus zeledoni. This species performs one of the most complex antiphonal duet songs described, and we examined the function of these duets through a combination of observations and playback experiments. There has been little consensus on how to distinguish between the different hypotheses, and we combined experiments and observations to help to distinguish between the hypotheses in the field. Duets in this species appeared to function primarily in cooperative joint territory defence, but our results suggest that both males and females also used their contribution to the duet to mate-guard. We found evidence of learning duet performance after pair bond formation but, contrary to predictions, not in the temporal coordination of a pair's duets. Instead, more established pairs were more consistent in their duet repertoires than newly formed pairs, which combined their song types more randomly. We also found plasticity in the speed of duet performance, with pairs apparently adjusting the temporal pattern of their duet in response to different stimuli. We discuss the implications of these results for the use of duets in this species. (c) 2006 The Association for the Study of Animal Behaviour Published by Elsevier Ltd. All rights reserved.