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 selection. Parents exhibited a self-restraint in aggression towards retained offspring at food in winter. Comparatively, nonkin immigrants were aggressively prevented from sharing food by the local pair. Parental tolerance 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 among Siberian jays should have evolved because of indirect fitness benefits to retained offspring from helping to raise younger siblings. Offspring retained by parents did not participate in incubation, feeding or nest defense.