Delayed dispersal in western bluebirds: teasing apart the importance of resources and parents

Delayed and localized dispersal are fundamental life-history traits associated with formation of family groups and kin neighborhoods. Although the field has focused mainly on resource benefits and ecological constraints as drivers of delayed dispersal, social benefits of nepotism can also be important. Resources and nepotism are theoretically correlated as the affordability of nepotism scales with resource abundance. Rarely have both been analyzed simultaneously within a single analysis. Western bluebird sons (Sialia mexicana) stay in family groups on mistletoe-based territories for winter, disperse locally to form kin neighborhoods in spring, and have a low level of facultative helping by sons, brothers, and grandsons. Although a son's tendency to remain on the natal territory increased with the number of parents present, mistletoe volume on the natal territory was a good predictor of sons staying home only for groups where the mother alone was present. Overwinter survival of sons was exceptionally high (95%) such that neither resources nor parental presence predicted survival advantages during the first year of life. When sons stayed on their natal territory for winter, their spring presence increased with the volume of mistletoe on their winter territory and was higher if they wintered with at least 1 parent. Stay-at-home sons acquired a portion of their parents' territory in spring, including mistletoe that scaled with their parents' mistletoe wealth. Our results indicate that although resources are undoubtedly important for the maintenance of family ties, the importance of maintaining connections with parents is underappreciated in studies of cooperative breeding.