The function of collective signalling in a cuckoo

Abstract

Collective signalling occurs in diverse animal groups. A particularly well-studied form of this sophisticated communication behaviour is vocal duetting in birds, in which members of a mated pair coordinate their songs on short temporal scales. Many hypotheses have been proposed to explain the function of bird duets, but the experimental evidence for them is still somewhat conflicting. Studies on songbirds often provide support for the joint resource defence or mate-guarding hypotheses. We investigated these hypotheses for a nesting cuckoo, the white-browed coucal, Centropus superciliosus, by presenting territorial pairs with four types of playback treatment: male–female duets, female solos, male solos, and a heterospecific control. The observed birds reacted aggressively to the conspecific playback by approaching the playback loudspeaker and singing. Pairs responded with duets more often to duet playbacks than to playbacks of female and male solo song. Also, neither sex treated simulated unmated intruders (solo playback) as more threatening than mated intruders (duet playback). Taken together, these findings argue against the mate-guarding hypothesis and instead suggest that duetting in white-browed coucals functions primarily in joint territory defence. In almost all cases, males were the first sex to sing in response to playbacks and, overall, they sang more than females. This suggests that males take a primary role in territorial defence. However, females also responded strongly, especially to the simulation of mated or unmated female intruders, hinting at a sex-specific division of labour in territorial defence.