Favouritism or intrabrood competition? Access to food and the benefits of philopatry for red-cockaded woodpeckers

Abstract

In species showing delayed natal dispersal, broodmates vary in natal dispersal timing and strategy, where some choose to disperse early while others delay. In the cooperatively breeding red-cockaded woodpecker, Picoides borealis, typically only one juvenile male per brood delays dispersal despite the lifetime fitness benefits associated with delayed dispersal. We sought to determine whether adult favouritism or intrabrood competition over the distribution of natal food resources provides a mechanism for the persistence of individual variation in natal dispersal strategies. We show that fledgling red-cockaded woodpeckers form male-biased, linear dominance hierarchies as a result of frequent aggressive conflicts. For males, high nestling condition relative to male broodmates was a strong predictor of future dominance, and this condition–rank relationship persisted after individuals reached nutritional independence. Adults were never observed interfering with broodmate conflicts, and were only rarely aggressive towards fledglings. Adults showed no overt favouritism towards offspring when targeting individual fledglings during provisioning early in the postfledging period. However, conflict rates increased with decreasing targeted-feeding rates, suggesting that access to resources is an important function of dominance hierarchies. After fledglings were developmentally able to compete for positions near foraging adults, first- and second-ranking males were fed more often than subordinate females. Of the juvenile males surviving to spring, subordinates were more likely to disperse during their first year than were their dominant broodmates. Together, our findings suggest that for young red-cockaded woodpeckers, intrabrood social rank provides a mechanism for prioritized access to natal resources and variation in natal dispersal strategy.