OFFSPRING RETENTION IN THE SIBERIAN JAY (PERISOREUS-INFAUSTUS) - THE PROLONGED BROOD CARE HYPOTHESIS

We examined aggressive behavior in Siberian jay groups containing both retained offspring and immigrant juveniles during winter feeding and during breeding. Selective tolerance of retained offspring by parental bids in winter suggests that cooperation evolved through kin selectio n. Parents exhibited a self-restraint in aggression towards retained o ffspring at food in winter. Comparatively, nonkin immigrants were aggr essively prevented from sharing food by the local pair. Parental toler ance in winter could bring inclusive fitness gains through the direct kin component if retained offspring experience relaxed competition and enhanced survival. Parental tolerance would then favor the evolution of delayed dispersal. There is no evidence that delayed dispersal amon g Siberian jays should have evolved because of indirect fitness benefi ts to retained offspring from helping to raise younger siblings. Offsp ring retained by parents did not participate in incubation, feeding or nest defense.