Group-specific vocal signatures and neighbour–stranger discrimination in the cooperatively breeding green woodhoopoe

Abstract

Group displays involving vocal choruses are a prominent feature of many avian cooperative-breeding systems. The existence of group-specific vocal signatures within these choruses could assist territory owners when assessing the threat posed by different intruding groups. To investigate the possible presence of such signatures, I examined the choruses (‘rallies’) given by 22 green woodhoopoe, Phoeniculus purpureus, groups over two consecutive seasons. A rally involved the combined cackling of adult group members. Statistical analysis of midrally vocalizations, when all group members were calling, revealed that samples from the same group clustered in principal coordinate space, and the group clusters segregated from each other to a significant degree. This segregation might theoretically arise from differences in group size and sex ratio, especially since the vocalizations are sexually dimorphic. However, groups containing the same numbers of males and females had rallies that were significantly different acoustically. Groups that maintained the same composition of individuals produced acoustically similar rallies across seasons, while those that changed membership produced significantly different rallies. The group signature is therefore most likely to result from group members producing their own individual vocal programmes, but participating in a similar way for every rally. In a playback experiment, groups responded significantly more rapidly to strangers and to neighbours on the wrong boundary than to neighbours in their expected place. Woodhoopoes therefore appeared capable of distinguishing between groups on the basis of their vocal signatures, representing the first evidence of the ‘dear-enemy’ phenomenon in a group-living species.