Duet duels: sex differences in song matching in duetting eastern whipbirds

The dynamics of vocal duels between rival male songbirds have been the subject of intense investigation over the last few decades. However, countersinging interactions between females and duetting birds have received considerably less attention. We compared patterns of song type matching in male and female eastern whipbirds, Psophodes olivaceus, using playback experiments to simulate intrusion by solo singing and duetting intruders. Eastern whipbirds sing a repertoire of sex-specific solo song types, which they combine nonrandomly to form specific duet types, initiated exclusively by the male. During playbacks, individuals could sing either a song type that matched the song type of a same-sex rival or one that fitted the song type of their partner. We found markedly different patterns of song type matching between the sexes. Both male and female eastern whipbirds matched the songs of same-sex intruders during solo singing song duels. However, when forming a duet, females selected a song type that fitted the song of their partner in preference to matching the song type of a female rival, whereas males preferentially matched a rival. We suggest that these sex differences in song type matching behaviour are the result of different strategies of mate and territory defence. Rather than being a purely cooperative signal, duets may be the result of sexual conflicts, with females forced to defend their position within the partnership, either by showing commitment to their partner or by deterring female rivals through display of their pair bond status.