Aerial Piracy by Bald Eagles: Success of Aggressors and Followers

We studied piracy among bald eagles (Haliaeetus leucocephalus) during the 1984 and 1985 autumn migration concentrations in Glacier National Park, Montana. The nearly exclusive prey at this site was kokanee salmon (Oncorhynchus nerka). We quantified, during aerial piracy attempts, the success of aggressors (eagles that aggressively tried to pirate salmon from other eagles) and followers (eagles that remained behind the interaction and made no attempt to directly pirate food) in obtaining food dropped by the host (the eagle in possession of the fish). During these interactions (N = 62), the host frequently dropped the salmon (55%), which was often recovered (62%) by the aggressor, a follower, or an eagle perched nearby. Fifty-two percent of the piracy attempts observed had at least one follower. A host was more likely to drop the salmon as the group size (aggressor and followers) increased. On a per-individual basis, aggressors and followers were equally successful at obtaining food lost by a host. Aggressors were more successful when one follower was present (17%) than when no followers were present (10%). One aggressor to one follower was the only ratio profitable to both.