Threat signaling in female song--evidence from playbacks in a sex-role reversed bird species

Prominent research areas such as animal communication and sexual selection use birdsong as a model system. Most studies on these subjects are conducted on species with typical sex roles with male-biased song production. Accordingly, the functions of birdsong, mate attraction, and territorial defense have hardly been studied in females. We investigated the territorial function of female song in the sex-role reversed African black coucal (Centropus grillii) to test whether females in such species demonstrate the same principles as male birds in species with typical sex roles. When territorially challenged, female black coucals changed their vocalizations in comparison to when they were singing spontaneously: They altered the composition of songs, lowered the pitch, and increased the duration of song elements. When challenged, larger females vocalized at lower pitch than smaller ones suggesting that pitch might be a reliable indicator of competitive abilities. To study whether females pay attention to such variation, we exposed them to playback experiments in which songs varied 1) in the composition and 2) in pitch and duration of song elements. Females did not respond differently to stimuli that varied in the composition. However, they reacted more cautiously to low-pitched and long stimuli compared with unchanged stimuli. This suggests that females were intimidated by the songs with low-pitched and long elements and that those songs signaled a higher level of threat. Thus, female black coucals paid attention to song parameters that reliably indicated competitive abilities. This confirms the general role of intrasexual selection in vocal communication of birds. Copyright 2010, Oxford University Press.