Function of pair duets in the eastern whipbird: cooperative defense or sexual conflict?

Abstract

Paired male and female eastern whipbirds, Psophodes olivaceus, sing precisely coordinated, male-led duets. Four broad explanations have been proposed for the function of duets: 1) cooperative resource defense, 2) prevention of partner usurpation, 3) defense of an individual's own position within the partnership, or 4) mate identification and localization. These 4 hypotheses make different predictions about how male and female residents should respond to simulated intrusion by other pairs or individuals. We compared the behavioral and vocal responses of 20 pairs of eastern whipbirds to simulated territorial intrusions by: 1) a solitary singing male, 2) a solitary singing female, and 3) a duetting pair. Males and females did not coordinate their approach to the playback speaker and showed sex-specific responses to playback. Males did not respond differently to duetting versus solo singing intruders. By contrast, females approached more closely during solo female song than during solo male song or duet playback. Females also produced specific vocalizations only in response to duet and solo female playback. Both sexes approached the speaker more closely and quickly during playback of same-sex solo songs than opposite-sex solo songs. Finally, females answered more of their mate's songs during simulated intrusion by a lone female than during simulated intrusion by a lone male. Our results suggest that duets in this species primarily function to allow females to defend their exclusive position in a partnership. Mate defense by females is unusual in birds but may be promoted in eastern whipbirds by a female-biased sex ratio and the need for exclusive access to male care. Thus, duets result from independent and conflicting strategies of mate and territory defense in males and females.