Factors influencing dispersal initiation and timing in a facultative cooperative breeder

Natal dispersal is a high-risk endeavor where decisions on whether and when to disperse have long-term consequences. Among facultative cooperative breeders, juveniles often forego dispersal and remain philopatric for one or more breeding seasons. This decision is key to the formation of cooperative breeding groups and could have significant effects on reproductive success. We investigated the probability and initiation of dispersal in the cooperatively breeding Brown-headed Nuthatch (Sitta pusilla) to determine the influence that social environments had on dispersal. This study was concurrent with another study where manipulation of population sex ratios increased the prevalence and size of cooperative groups. The concurrent manipulations enabled us to evaluate social effects on dispersal as a plausible mechanism driving the relationship between adult sex ratios and cooperation. We evaluated which factors best predicted whether males dispersed, and the timing of dispersal for both sexes. We considered variables related to the immediate nesting environment as well as characteristics of the local population. Social environments were related to dispersal for both males and females. Juvenile males dispersed earlier when a helper was present in the natal group. Females dispersed earlier in settings with more adult neighbors and when a lower proportion of those neighbors were males. Females with shorter tarsi relative to their siblings dispersed earlier, suggesting that size-based competitive interactions may also affect dispersal decisions. Our results suggest juveniles disperse more readily when they fledge in constrained social environments, and that competition with conspecifics is a major driver of dispersal in the Brown-headed Nuthatch.