Post-Breeding Season Behavior of Bald Eagles (Haliaeetus leucocephalus) at Avenue Point, San Juan Island

Abstract

After considerable increase in numbers over the past decades, Bald Eagles (Haliaeetus leucocephalus) may have reached carrying capacity in western parts of Washington. I studied the behavior of a Bald Eagle pair at Avenue Point, San Juan Island, Washington from 8-20 August 2011. Specifically, I recorded four behavior types (sitting, scanning, preening, and moving) at four-minute intervals during an observation period. I also noted eagle vocalizations and the relative distance between pair members along the shoreline. Behaviorally, eagles spent the majority of their time sitting (46%), followed by scanning (26%), preening (20%), and moving (7%). This might be justified energetically, due to the necessity during the post-breeding season of conserving energy for migration to wintering grounds. I saw only two predation attempts, both entailing fish prey and both unsuccessful, though other observers reported successful fish and gull predations during my study period. Vocalizations were separated into “chatters” and “peals”. Chatters were 8 times more likely to be used when two or more eagles were present, whereas “peals” were 5 times more likely to occur when either one or > two eagles were present. This result suggests that chatter calls are used for communicating within a pair, while peals are used for territorial purposes. The pair of eagles I studied spent >70% of their time between 5 m and 300 m apart from one other. This suggests that during the post-breeding stage they prefer to separate themselves spatially along their territory’s shoreline, as opposed to perching close together. However, evidence of the pair’s bond was still seen during this period, and I would occasionally see playful nipping of each other’s bills and mutual greetings.