In many species of birds, pair mates sing duet songs. It has been hypothesized that coordinated singing is an adaptation to mediate conflict with rivals that would usurp territory or replace one of the pair mates. Duets should be more effective than solo songs at deterring rivals if duet songs signal that pair mates will defend cooperatively. While it has been argued that cooperative defence is incompatible with males pursuing their own fitness interests, counter arguments suggest several conditions in which cooperation may benefit both males and females. Data from observational studies of duetting birds provide some evidence of cooperative defence, but more quantitative studies are needed. Experimental removals of one pair mate have failed to demonstrate that being paired reduces the risk of territory loss. These experiments, however, have not been conducted over the relevant time scales and appear prone to Type II error. A meta-analysis of 19 song playback and decoy presentation experiments reveals that duetting species are significantly more cooperative (i.e. respond with a weaker same-sex bias) than non-duetting species. In summary, empirical evidence supports the hypothesis that duetting pair mates defend their territories and/or one another cooperatively, but fails to link cooperative defence to fitness benefits.
Keywords: cooperation, sex roles, duetting, antiphonal song, tropical birds