AbstractSex-biased natal dispersal is predicted to be a consequence of differences in the costs and benefits of dispersal perceived by each sex. Although female-biased dispersal has been described for numerous birds in heterogeneous landscapes, studies documenting sex-related differences in the causes and consequences of natal dispersal in fragmented habitats are comparatively scarce. We used capture-mark-recapture data and genetic analyses in a population of Thorntailed Rayadito (Aphrastura spinicauda) from north-central Chile in order to investigate how the possible causes and consequences of dispersal are linked to sex-specific dispersal behavior in a naturally isolated and fragmented forest habitat. We analyzed 36 recaptured post-fledging birds to test whether female-biased dispersal reflected differential responses between sexes to potentially long-term acting selective pressures related to habitat fragmentation and high population densities. In this population, females disperse long distances (median distance: 780 m), whereas males are mostly philopatric (median distance: 85 m). Results suggest that female dispersal is possibly a response to the local density of breeding birds and mate availability, and comes without apparent reproductive costs. Inbreeding avoidance was not evident, but females may already be decreasing the probability of mating with relatives by moving away from their natal neighborhood where male kin usually remain. Our findings also indicate that male dispersal, while reduced in relation to females, can result from a tradeoff between the social benefits of settling near the natal territory and the associated costs of high breeding densities and kin competition. An increase of paternity loss was evident for males moving long distances. Follow-up studies should assess the differences in the costs and benefits of within- and between-fragment dispersal in this population.