Vigilance patterns of Bald Eagles feeding in groups
Patterns of vigilant behavior of wintering Bald Eagles (Haliaeetus leucocephalus) feeding on spawned salmon were examined in 1983-1984 on the Nooksack River in north-western Washington. Vigilance in feeding birds has, in general, been attributed to predator detection; however, we proposed an additional function of vigilance in socially feeding birds that are vulnerable to food robbery and possible injury by conspecifics. We tested predictions of two nonexclusive hypotheses: (1) eagles look up while feeding to detect danger from humans, and (2) eagles look up while feeding to detect pirating attempts or avoid injury by conspecifics.
Results suggest that the function of vigilance varies, depending on the size of the feeding group. Vigilance patterns of eagles feeding in small groups (1-4 eagles) and medium groups (5-7) eagles are consistent with hypothesis 1, whereas those of eagles feeding in large groups (8-14 eagles) are consistent with hypothesis 2. Eagles in small groups were more vigilant (measured as scanning time and rate of head raising) when feeding near potential danger (riverbank cover) than when far from danger. Adult eagles feeding in areas of intense human activity were more vigilant than immatures feeding at the same site and were more vigilant than both adults and immatures feeding at secluded sites. Vigilance declined as group size increased from 1 to 4 eagles, and increased as group size ranged from 8 to 14 eagles. Feedings eagles that were looking up at the time of a pirating attempt were more successful in keeping their food than eagles with their heads down. In feeding areas where human activity was minimal, eagles formed larger groups than at a more disturbed site.