Communal singing in the cooperatively breeding subdesert mesite Monias benschi: Evidence of numerical assessment?

It has long been suggested that competing groups of social animals assess one another on the basis of communal vocalisations, but so far this has only been demonstrated in mammals. We investigated this idea in the subdesert mesite, a group-living bird that produces communal songs during aggressive territorial interactions with other groups. Where groups compete as units, differences in group size might outweigh inter-individual differences in determining the outcome of contests. Given this species' variable social system, we predicted that the composition of groups would also influence their resource holding potential. Playback experiments showed that the number of simulated intruders significantly influenced the strength of response by defending groups hearing the stimulus: groups were less likely to approach but gave more protracted and more communal vocal responses as the number of vocalising intruders increased. We found that the likelihood of a group approaching playback increased as the ratio between the number of males in the defending group and the number of males in the intruding group increased. Furthermore, the ratio of adult males to females in the defending group significantly affected the probability of approach, with the probability increasing as groups became more male-biased. This finding supports the idea that the social structure of groups may be important in determining the outcome of contests between groups. Overall this study provides some indication in birds of relative numerical assessment based on vocal cues.