FORAGING BY COMMON RAVENS IN THE PRESENCE AND ABSENCE OF TERRITORY HOLDERS - AN EXPERIMENTAL-ANALYSIS OF SOCIAL FORAGING

Immature common ravens, Corvus corax, are vagrant and aggregate into groups at large animal carcasses. In contrast, breeding adults are territorial and defend carcasses. Immature ravens may benefit by foraging socially because groups can acquire defended food. This hypothesis was tested by quantifying accessibility of defended foods and feeding rates of dominant and subordinate immature ravens foraging both free in nature and in a large outdoor enclosure. Groups of 9-29 immature birds were more likely to overcome adult territory defence and fed at higher rates on carcasses than did immature birds in smaller groups or solitary individuals. These benefits resulted from reductions in the neophobia of the foragers and reduced aggression of the adult defenders as group size increased. Dominant immature birds fed at higher rates than subordinates, but social foraging was also beneficial to subordinates when carcasses were large and adults were defensive (adults focused their attacks on dominant immature birds). Groups of immature revens that arrived at carcasses from distant communal roosts were optimal in size. However, group size quickly exceeded the optimum because individuals that joined groups of 29 or more still fed at higher rates than those joining much smaller groups or those striking out on their own. © 1991 The Association for the Study of Animal Behaviour.