We studied between-year dispersal of adult females within a population of cooperatively breeding Red-cockaded Woodpeckers (Picoides borealis) in the Sandhills of North Carolina, using data collected between 1980 and 1995. We tested four hypotheses about the cause of breeding dispersal: inbreeding avoidance, mate choice, site choice, and social constraints. In addition, we assessed relationships among age, reproductive failure, and breeding dispersal, and we estimated cost of breeding dispersal by plotting mortality against dispersal rate as a function of circumstance. Breeding dispersal in the population that we studied is associated with multiple factors. Inbreeding avoidance influences dispersal of females whose sons inherit their natal territories. Mate choice influences dispersal of females whose mates have died; these females acquire older, higher quality mates by dispersing. In this study, there was no clear relationship between site choice and breeding dispersal. Social constraints do not appear to affect breeding dispersal in this population: no evidence was found to suggest that dispersal is associated with female-female competition, within-group competition for resources, or reproductive competition between mothers and helper sons. The effect of reproductive failure on breeding dispersal changes with female age. Reproductive failure is associated with breeding dispersal in young females only (those <3 yr old). Estimated mortality rates for breeding females that attempt to disperse vs. those that do not attempt to disperse were 59% and 26%, respectively; the difference between those rates is the estimated cost of breeding dispersal in this population, an additional 33% probability of mortality. Thus, breeding females more than double their risk of mortality by dispersing.