Geographic variation in social behaviour of white-browed sparrow-weavers, Plocepasser mahali
White-browed sparrow-weavers are social birds occurring in the semi-arid parts of Africa. Large variations in the density (abundance) of these birds in different parts of the continent probably result from differences in food supply, and are correlated with differences in vegetation and rainfall. Despite these local differences in environment, the social system of the species is surprisingly uniform: group size varies by a factor of less than two, and the presence of helpers within groups has been recorded at all the study sites. This contrasts with the published data for a number of other social vertebrates. The selective force shaping sparrow-weaver sociality may be constant throughout the geographic distribution of these birds. If so, this selective factor is not related to food supply, but probably to a high mortality rate.