Coordination between the sexes for territorial defence in a duetting fairy-wren

In territorial songbirds both sexes depend on possession of a territory, yet its defence is traditionally regarded as the domain of male birds. Studies on duetting species have examined both male and female roles in territorial defence, often focusing on potential reproductive conflict between the pair members. Surprisingly, interactions between defending pair members are rarely investigated. We combined observations on undisturbed pairs of purple-crowned fairy-wrens, Malurus coronatus, with experimental simulation of a strong territorial threat to quantify coordination between partners during territorial defence. Pairs formed strongly cohesive partnerships: males and females naturally spent a high proportion of time close together (75% within 5 m) in dense riparian habitat, and coordinated half their songs to form duets. Partners that spent more time close together also coordinated more of their songs to form duets. When we presented playback simulating paired intruders, females responded as aggressively as males. Intensity of approach to the playback speaker was highly correlated within pairs. Spatial coordination between partners increased dramatically when they responded to simulated intrusion so that they spent virtually all their time within 2 m of one another, and most of the songs they sang were coordinated to form duets. We conclude that the highly coordinated vocal and approach responses of male and female purple-crowned fairy-wrens to simulated intrusion in the nonbreeding season are not coincidental, or a consequence of sexual conflict over mating, but instead are likely to represent cooperative territorial defence of a valuable shared resource.