Vocal duetting occurs in diverse animal groups. Members of a mated pair may duet to communicate with each other or with other individuals. Many hypotheses have been proposed to explain the function of duets, and studies often provide support for the joint resource defence or mate-guarding hypotheses. We evaluated these hypotheses for the happy wren, Pheugopedius felix, using a two-speaker playback experiment. We observed the responses of happy wren pairs to playback of solo male, solo female and male/female duet songs, and compared these with heterospecific song control trials. Happy wrens responded aggressively to conspecific song playback by moving closer to their mate, approaching the playback speakers and increasing singing rates. Both sexes increased singing and especially duetting rates in response to conspecific playback. There were no differences in which sex initiated or terminated duets nor did birds vary the proportion of their partner's songs answered across conspecific treatments. Furthermore, neither sex treated unmated intruders (solo playback) as more threatening than mated intruders (duet playback). Together, these results argue against the mate-guarding hypothesis and instead indicate that duetting in happy wrens functions primarily in cooperative territory defence. Overall, males sang more than females, moved closer to the speakers and were more likely to answer their partner's songs, suggesting that males take a primary role in territorial defence. However, females also responded strongly, especially when female intruders were present (duet or female solo playback), which suggests a sex-specific division of labour in their territorial defence. (C) 2011 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.