Sex ratio and habitat limitation promote delayed dispersal in superb fairy-wrens

Abstract

IN a small proportion of bird species (<3%), offspring delay their dispersal beyond the time when they are reproductively capable, remain on their natal territory, and usually assist the resident breeders, normally their parents, to raise other young^1–3. This pattern of social organization and care-giving behaviour, most frequently termed cooperative breeding, is controversial with respect to why offspring delay their dispersal^2–8. In this study we investigate the interaction between habitat limitation, habitat quality, and sex ratio in influencing dispersal decisions in a population of superb fairy-wrens (Malurus cyaneus). We show that every non-breeding male (31 of 32; 96.9%) given the opportunity, dispersed and bred. The sex ratio was male-biased and although males did not disperse into vacant but previously occupied territories in the absence of females, the reintroduction of females prompted dispersals. Our data suggest that young males delay their dispersal in response to a limited number of mates and secondarily to habitat limitation.