Multistate estimates of survival and movement in relation to colony size in the sociable weaver

Abstract

We estimated survival and movement probabilities in relation to breeding-colony size in the sociable weaver (Philetairus socius) by using multistate statistical methods, in which survival and movement to time t + 1 is conditional on an individual's colony size at time t. The sociable weaver is a colonial, cooperatively breeding species that builds a massive communal nest, with colony size ranging from fewer than 20 to more than 500 individuals in some areas. We conducted an 8-year capture/mark/recapture study of sociable weavers near Kimberley, South Africa. By comparing the fit of different multistate models to our data, we found evidence that annual survival probability of adults was lower in small colonies (less than 30 individuals) and medium-sized colonies (30-60 individuals) than in large colonies (more than 60 individuals). First-year survival of birds banded as juveniles also increased with natal colony size. Statistically, however, these effects were weak, and models without an effect of colony size were equally well supported by our data. Movement probabilities illustrated that individuals seldom moved between colonies of different size classes, and showed a preference to use colonies of size classes similar to what they had occupied the preceding year, even when changing colony sites between years. The potential survival differences among birds in different colonies, if real, translated into differences of 39-45% in average lifespan. First-year survival is probably enhanced through antipredator advantages of being in larger groups during the fledgling period. Adult survival in larger groups may be increased by thermal advantages of a large nest during cold winter weather. The fidelity of weavers to a particular colony-size class between years may reflect phenotypic specialization for certain group sizes.