Effects of sociality on short-distance, female-biased dispersal in tropical wrens

Abstract

1. Dispersal from the natal area, or between breeding sites, marks a critical transition for individual animals, and a fundamental demographic and genetic process. Limitations on dispersal in natural populations are important in predicting population dynamics and structure. Tropical habitats that can be naturally patchy are also being fragmented rapidly, and these habitats harbour many social species. 2. We studied dispersal in cooperatively breeding stripe-backed wrens (Campylorhynchus nuchalis) in the Venezuelan savanna, using 20 years of data from marked populations over 40 km2. We test the hypotheses that dispersal is limited by competition for social position and that such limitation is sex-specific. 3. Dispersal is strongly female-biased, and results mainly in attainment of breeding status in neighbouring groups. Reproductive success is determined by the structure of the breeding group, and is concentrated in a small minority of individuals. 4. Natal dispersal is delayed, and represents a dramatic shift from indirect contribution to kin production (helping) to the direct approach of breeding. Monogamy limits breeding opportunities, compared to other social species, and both natal and breeding dispersal are constrained to short distances, particularly when compared to noncooperative breeders. 5. Males are more philopatric compared to females, for which more frequent and distant dispersal leads to greater reproductive success. Competition for breeding status in an established social order impedes dispersal, in part because experience and previous familiarity are important, particularly in males that rarely disperse to productive large groups. 6. Diverse patterns of sex bias in dispersal among cooperatively breeding birds can be explained by the comparative diversity of reproductive alternatives available to males and females. A 'breeding diversity' hypothesis is illustrated by comparison with male-biased dispersal in brown jays. 7. Social resistance to movement establishes a brake on gene flow and demographic interaction among populations that has important implications for population viability.