Function of duet coordination in a territorial socially monogamous bird

Duetting, a cooperative vocal behaviour performed by mated pairs, is a distinctive vocal behaviour among many species in specifically primates and birds. The exact features of duets that may make them a stronger territorial signal is still inconclusive. One hypothesis is that the precision of duet coordination can indicate the quality or dedication of a pair, and thus the degree of threat posed to a rival pair. To address the implications of duetting precision in a territorial context, we determined to what extend the duetting behaviour in the chirruping wedgebill (Psophodes cristatus), a territorial, socially monogamous passerine, is affected by the precision of duet coordination. We tested this with playback experiments where we broadcast coordinated and uncoordinated duets at mated pairs, predicting that pairs would exhibit stronger responses to coordinated duets than to uncoordinated ones and sing more coordinated after the simulated intrusion. We found that neither response intensity nor coordination of either sex differed between coordinated and uncoordinated duets. Since chirruping wedgebills did respond consistently to playback, we suggest that either (1) coordination of duetting does not hold a function in joint resource defence in this species, (2) playback stimuli were too threatening for them to adjust their coordination on a level we could detect or (3) they do not discriminate between our coordinated and uncoordinated playback treatments. We highlight the notion that there may be variety in functions of duetting at play within and across avian species, and that different aspects of duets such as coordination and intensity may hold different functions.