Dominance hierarchy and status signalling in captive tree sparrow (Passer montanus) flocks

Abstract

Although the tree sparrow (Passer montanus) is a widespread species, its social behaviour in winter flocks is poorly known. In this study, we investigated whether (1) flock members form dominance hierarchies in their winter flocks, and (2) whether the size of the black throat patch (the 'badge') of birds is correlated with their dominance ranks, i.e. may have a status signal function. We found that tree sparrows formed linear dominance hierarchies in two of the three captive flocks we studied. The dominance rank of birds was positively correlated with their aggressiveness during feeding, i.e. high ranking birds initiated a higher proportion of their aggressive interactions than subordinates. Our results do not support unambiguously the status signalling function of the badge. On the one hand, badge size was the only significant predictor of dominance rank in one flock. In this flock, fighting frequency increased with increasing similarity of badge sizes and fights were especially common when both opponents had large badges. On the other hand, dominance rank was unrelated to badge sizes in the other two flocks. We suggest some factors that may contribute to the observed differences among flocks in the association between badge size and dominance rank.