Territorial vocal rallying in the green woodhoopoe: influence of rival group size and composition

Group territorial displays are a prominent feature of many avian cooperative-breeding systems. I used natural observations and playback experiments to investigate the territorial vocal rallying display of the green woodhoopoe, Phoeniculus purpureus. Rallies, which consisted of all adult group members cackling loudly while bowing up and down, were given both when unprovoked and in response to other groups. Unprovoked rally length correlated positively with group size, making it a potential indicator of group resource-holding potential. However, group members vocalized for longer in response to actual and simulated intrusions by larger groups than by smaller ones. The duration of the initial response rally was therefore strongly influenced by the rally length given by the intruding group, and consequently did not accurately reflect group size. I discuss the implications of this potential deception. Individuals differed in their contributions to response rallies depending on their sex and dominance status, and on the composition of the intruding group. Although males and females contributed equally overall, each sex expended more effort responding to intruders of its own sex, which might have been viewed as a greater threat. Individuals also approached playbacks of the opposite sex more closely than those of their own sex. Subordinates tended to cackle for longer than dominants, perhaps because they had more to lose from the increased foraging competition following the inclusion of additional group members. (C) 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.