Food Consumption and Energy Requirements of Captive Bald Eagles

Abstract

Thirty-six food consumption trials were conducted using 4 captive winter-acclimatized bald eagles (Haliaeetus leucocephalus) on 3 diets at 3 temperatures. Daily consumption for combined temperatures was highest on the chum salmon (Oncorhynchus keta) diet (92.0 g/kg), intermediate on the blacktailed jack rabbit (Lepus californicus) diet (74.8 g/kg), and lowest for mallard ducks (Anas platyrhynchos) (65.1 g/kg) which was inversely related to the wet energy contents (0.90, 1.22, and 1.96 kcal/g, respectively) of the diets. Dry matter, fat, protein, and ash contents accounted for the disparity in wet energy content. Daily gross energy intake, existence metabolism, and excretory energy for all diets combined were 116.9, 94.3, and 22.5 kcal/kg at -10 C, 101.8, 81.8, and 20.0 kcal/kg at 5 C, and 89.7, 69.1, and 20.6 kcal/kg at 20 C, respectively. Energy assimilation efficiencies were higher on the duck diet (85.2%) compared to salmon (75.0%) and rabbit (75.4%) and efficiency increased with decreasing temperature on the latter 2 diets. Minimal gross energy requirements of a 4.5-kg eagle for a 90-day winter period at 5 C is predicted to be 13 salmon, 20 rabbits, or 32 ducks. J. WILDL. MANAGE. 46(3):646-654 Wintering bald eagles are opportunistic scavengers and predators and range widely to locate food. Fish is the main food source, but waterfowl, seabirds, small mammals, and carrion are eaten as alternate prey (Spencer 1976). Winter diets of this species have been assessed in many regions (Imler and Kalmbach 1955, Spencer 1976, Steenhof 1978), but food requirements have received little attention. Food consumption by captive bald eagles was measured by Chura and Stewart (1967) and Duke et al. (1976) incidental to other experiments. Stewart (1970) documented food needs of a single, captive bald eaglet. Energy consumption has not been studied. Some information is available on food requirements of white-tailed eagles (Haliaeetus albicilla) (Love 1979) and golden eagles (Aquila chysaetos) (Fevold and Craighead 1958, Brown and Watson 1964, McGahan 1967, Collopy 1980). Hayes and Gessaman (1980) reported the effects of temperature, wind, and radiation on the e ergy metabolism of the golden eagle. Winter raptor populations are often food-limited (Newton 1979:80). Starvation may be a population-regulating mechanism for some bald eagle populations (Sherrod et al. 1977). Management practices are currently being developed that include preservation and enhancement of the food base with the goal of increased winter survival (Stalmaster 1980). Additional information on the food and energy requirements of wintering bald eagles is necessary for predicting carrying capacities of wintering areas, to understand impacts on prey populations, and in estimating food supply needs for artificial feeding programs. This study measured food and energy consumption and energy assimilation of 4 captive bald eagles on 3 common diets at 3 temperatures. Partial funding was provided by the Department of Biology and Ecology Center at Utah State University and the Washington Department of Game. W. English and J. W. Foster provided the I Present address: 10,000 47th Ave. S.W., Seattle, WA 98146. 646 J. Wildl. Manage. 46(3):1982 This content downloaded from 157.55.39.35 on Wed, 31 Aug 2016 04:32:15 UTC All use subject to http://about.jstor.org/terms BALD EAGLE FOOD AND ENERGY REQUIREMENTS. Stalmaster and Gessaman 647 experimental animals. R. L. Knight assisted in many aspects of this study. L. C. Stoddart and R. M. Stalmaster supplied food items. K. L. Hamilton and J. S. Kirkley assisted in handling eagles. We appreciate review comments of M. R. Fuller, J. W. Grier, K. L. Hamilton, J. R. Murphy, and J. R. Newman on an earlier draft of this paper.