Comparative Home Ranges and Food Habits of Bald Eagles Nesting in Four Aquatic Habitats in Western Washington

Abstract

Bald eagles (Haliaeetus leucocephalus) nest widely among Pacific Northwest habitats, including rivers, lakes, marine shorelines, and tidal bays. From 1986 to 1997, I investigated differences in home range characteristics, foraging behavior, and diets of bald eagles nesting in these habitats in western Washington. Home ranges (harmonic mean contours) for 53 eagle pairs averaged 4.9 km2 (SE = 0.5), and were smaller than ranges reported for other bald eagle populations. Core areas averaged 1.2 km2 (SE = 0.2). Eagles ate 78% fish, 19% birds, and 3% mammals based on 926 prey captured at 78 nest sites. Gulls (Larus spp.) and ducks were identified most often among 269 prey items. Home range characteristics varied significantly among habitats and were affected by foraging habits specific to each habitat. Eagles nesting in bays had the largest ranges, had the greatest available shoreline, and used the most perches due to expansive, tidally influenced foraging areas. A relatively high proportion of prey was scavenged (19%) and pirated (11%). Eagles along rocky, marine shores had ranges that were 50% smaller, but with irregular shorelines and islands that increased the length of shoreline available for foraging. Diets were similarly diverse. Eagles on freshwater lakes and rivers had the smallest core areas, least shoreline, and fewest perches within ranges, and spatial use was more restricted by the area of open water available for foraging. They captured >93% of prey alive, were highly efficient (88% successful) on rivers, and consumed the fewest fish (58%) in any habitat. Identification of key habitats for bald eagle nests in the Pacific Northwest can be made knowing that: 1) there is a progressive increase in the average home range size (2 to 7 km2), core area size (0.4 to 1.7 km2), shoreline length (2 to 4 km), and number of perches (14 to 32) used by nesting eagles on lakes, marine shorelines, rivers, and bays; and 2) the most important emphasis for habitat protection is <600 m from nests (average radius of core area) that includes most key perches and foraging sites.