Territorial vocal rallying in the green woodhoopoe: factors affecting contest length and outcome

Most studies of asymmetric contests have focused on interactions between individuals. We examined territorial contests between groups of green woodhoopoes, Phoeniculus purpureus, which take the form of vocal rallying displays. The distribution of encounter durations was bimodal: interactions were generally either decided within 5 min (short contests) or took longer than 15 min to reach an outcome (extended contests). As short contests progressed, there was an escalation in the length of rallies, and these encounters were longer when the competing groups were more evenly matched in size. Residents won the majority of short contests, whereas the difference in the sizes of the competing groups was not a significant predictor of the outcome. The resident group appeared to match the rally length given by the intruding group, tending to lose the contest when it no longer achieved this. In extended contests, there was no further escalation in rally length after the first 5 min. There was also no resident advantage, but larger groups were more likely to win. These extended contests may be a test of stamina. To achieve a similar length of rallying, individuals in smaller groups must contribute more effort than those in larger groups. We discuss the possibility that physiological constraints prevent smaller groups from sustaining the same level of rallying as larger groups, and that this difference decides the outcome of extended contests. We consider the different possible functions of short and extended contests, and discuss our findings in relation to existing models of agonistic interactions.