Parental aggression in black-capped chickadees

Black-capped chickadee (Parus atricapillus) adults are aggressive toward their young after the young have fledged. We examine two hypotheses to explain the function of this aggression. Eight families were observed throughout the period after fledging to see if aggressionfunctions in discouraging the youngs' dependence on their parents for feedings or in encouraging dispersal of young from the natal territory. Aggression consisted mostly of chases, supplantings, and attacks. Most aggressive interactions occurred when young were following a parent closely or begging. After an aggressive act, fledglings tended to move away from the parent and forage on their own. The frequency of aggressive acts peaked at 13 days after fledging, then decreased. This peak occurred when fledglings were making the transition from dependence on parental feedings to independent foraging. Most fledglings had not dispersed by 22 days after fledging. Our results support the hypothesis that parental aggression encourages fledglings to forage independently, rather than to disperse from the natal territory.