A spatial approach to duet function in Adelaide's warbler, Setophaga adelaidae

In many species of birds, mated pairs sing duets. In some duetting species, only females answer their mates' songs to form duets. Functional studies of these ‘female-controlled duets’ offer a unique opportunity to focus on the female's decision to form a duet. We used observational methods to test predictions of two hypothesized functions of duetting and gain insight into the evolutionary pressures that act on species with female-controlled duets. The joint resource defence hypothesis states that song answering signals to would-be intruders that both mates will cooperate to defend their shared resources. According to the mate localization hypothesis, answering helps mates locate one another. Our study species, Adelaide's warbler, is a tropical, nonmigratory New World warbler (family: Parulidae). We used radiotelemetry to simultaneously record focal pairs over the course of several mornings. Females were more likely to answer their mates' songs to form a duet when the female was close to the territory boundary or close to the mate, and around the time of aggressive encounters. Females tended to move towards their mate after he sang, but we did not find evidence that birds of either sex use duets to locate the mate. Our results suggest female answering functions to defend the territory and the partnership, but not to localize the mate. We propose that the resource defence and mate defence functions of duetting may be inextricably linked in species like Adelaide's warbler.