Sex-specific mortality costs of dispersal during the post-settlement stage promote male philopatry in a resident passerine

The costs and benefits of dispersal that select for sex-biased dispersal are still poorly understood. Many studies examine fitness consequences of dispersal after first breeding, while dispersal costs are most likely paid before first breeding during the movement, settlement, and post-settlement stages. We studied survival correlates of dispersal between flock settlement and first breeding during the first winter of juvenile willow tits (Poecile montanus), a small passerine that has female-biased natal dispersal, but shows no dispersal-associated survival differences after first breeding. This resident food-hoarding species winters in small stable non-kin territorial flocks. We collected capture-recapture data by following flocks from autumn to the following spring. We compared monthly survival and return rates of juveniles that were born and recruited within the study area (philopatric juveniles) and juveniles that originated from outside the study area (immigrant juveniles). Among males, survival was highest for philopatric juveniles whereas survival of females was higher among immigrant juveniles, providing one explanation for the female-biased natal dispersal observed in the species. Philopatric males may benefit from prior residency either through increased site familiarity and knowledge of winter food resources and/or by gaining higher social ranks during flock establishment. However, rank data provided little support for the latter hypothesis. Other mechanisms such as increased ability to find high-quality flocks and mates may be important for female survival. Our results provide further evidence that dispersal costs are paid mainly before first breeding and that sex-specific costs of dispersal play a role in the evolution of sex-biased dispersal. This paper shows that female-biased dispersal can be a consequence of sex-specific costs and benefits of dispersal during the post-settlement stage of the dispersal process which is a very poorly understood stage in dispersal theory. By examining correlates of dispersal before rather than after first breeding as it is usually done, our study aids in understanding the selection pressures modifying dispersal strategies. Our results have wide applicability because there are many resident taxa similar to the willow tit (our study species) that have a settlement stage and a prolonged non-reproductive phase before their first reproduction.