Functions of perch relocations in a communal night roost of wintering baldeagles

We investigated the functions of perch relocations within a communal night roost of wintering bald eagles (Haliaeetus leucocephalus) along the Nooksac k River, Washington, during two winters. We tested seven predictions of two nonexclusive hypotheses: (1) bald eagles relocate within roosts to assess foraging success of conspecifics and (2) bald eagles relocate to obtain the rmoregulatory benefits from an improved microclimate. Additionally, we gath ered descriptive information to allow refinement of further alternative hyp otheses. We rejected the hypothesis that relocations are a means of assessi ng foraging success. Contrary to our expectations, immature eagles did not relocate to be closer to adults, and relocations were less frequent when fo od was less abundant. Our data support the hypothesis that eagles relocate within night roosts to obtain a favorable microclimate during winters when they are subjected to cold stress and food stress. In both winters, relocat ions were more frequent in the evening than in the morning. In both winters , most evening relocations were to the center of the roost rather than to i ts edge, and the frequency of relocation to the center was greater when tem peratures were low. The microclimate hypothesis, however, explains only a l imited number of relocations. Based on our findings, it is likely that relo cation has multiple functions, including establishing and (or) maintaining foraging associations, establishing and (or) maintaining social-dominance h ierarchies when food is less abundant, and nonsocial activities.