SPECIES

Bald Eagle Haliaeetus leucocephalus Scientific name definitions

David A. Buehler
Version: 2.0 — Published October 7, 2022

Diet and Foraging

Introduction

The Bald Eagle is an opportunistic forager. Its diet varies greatly across its range and can be site-specific, based on local food availability (186, 18, 122). In most regions, it favors aquatic habitats for foraging and fish are taken most frequently (187, 188, 119, 18, 189), though aquatic birds are often taken, and aquatic mammals (e.g., muskrat, Ondatra zibethicus), reptiles, amphibians, and crustaceans are sometimes taken. Prey are taken live, fresh, or as carrion.

Feeding

Microhabitat for Foraging

Forages predominantly where live fish are present near the surface, such as areas with relatively shallow water (186, 190, 122, 191) or at locations likely to have carrion. Live fish are usually hunted within 500 meters of a shoreline with adequate perches (122). Notable "habitats" for seeking carrion (or near-carrion) include hydroelectric facilities (the turbines of which kill or stun fish), shallow gravel bars on rivers of Pacific Northwest and Alaska, where salmonid carcasses accumulate after spawning (192, 120), iced-over bodies of water (which can cause sporadic fish-kills as well as concentrate foraging opportunities; 122, 189), and shallow waters during summer temperature peaks, which can produce fish-kills due to oxygen depletion. Bald Eagle also frequents places where waterfowl are hunted, where it may search for unretrieved kills and wounded birds (185).

Food Capture and Consumption

Forages throughout day, with foraging concentrated in early morning hours (193, 194, 122). Food is obtained by direct capture, the scavenging of dead prey, or by stealing food from other Bald Eagle, other avian raptors, or mammals. It hunts in flight, from perches, by wading in water, by walking on the ground (seldom), and through cooperative hunting (18). Live prey is hunted mostly from perches or on the wing, and adults are more apt than immatures to capture live prey because of superior foraging ability and experience (186, 125). In flight, searches for prey by soaring, and when prey is spotted, it suddenly stoops and attempt to capture the prey item with one or both feet.

Most commonly soars overhead when searching for live prey, and when prey is spotted, the eagle suddenly stoops, attempting to capture it with one or both feet. Repeated stoops are often required when hunting waterfowl that are sitting on the water, and success is often poor. Bald Eagle hunting fish during winter in Nebraska had a 24% success rate (kills made/foraging search; n = 1,997 attempts) and were successful in 73% of strikes (n = 667); gizzard shad (Dorosoma cepedianum) composed 91% of all captures (n = 489), reflecting the high susceptibility for this species, especially below hydroelectric facilities (195). Bald Eagle in Utah hunted, flushed, and killed black-tailed jackrabbit (Lepus californicus) cooperatively (196). One nesting pair in Florida hunted cooperatively for Cattle Egret (Bubulcus ibis), which appeared to increase capture success (197). Most prey items are taken to a nearby perch for consumption, although small items may be consumed on the wing. In areas with a high concentration of Bald Eagle, successful foragers are often chased; at such places, successful foragers select perches away from the foraging birds for less conspicuous prey consumption.

Scavenging opportunities are usually discovered through soaring flights over likely habitat. Carrion that is too large to carry off is eaten on site. Some large items, such as mammalian carcasses, may be fed on for many days. Sometimes displaces, or is displaced by, other species at scavenging sites, such as corvids, coyote (Canis latrans), bobcat (Lynx rufus), and feral dog (Canis lupus familiaris) (166). Will steal food from another Bald Eagle as well as other avian species, especially fish-eaters such as Osprey (Pandion haliaetus) and herons (Ardeidae) that are better at the capture of live fish but have limited defense ability. Rarely food is stolen from mammals, such as sea otter (Enhydra lutris) (198, 199). Food piracy is more common in fall, winter, and spring than during the breeding period (186, 192, 142, 122). Hunting and stealing food in Alaska were approximately equal based on cost-benefit analysis (200). On the northern Chesapeake Bay, dead fish composed 25–67% of all fish captured, even though dead fish abundance was low (122, 189).

Diet

Major Food Items

Differences in food items taken based on whether prey is captured live or eaten as carrion. Fish that are most often taken live include gizzard shad (Dorosoma cepedianum), threadfin shad (Dorosoma petenense), and white bass (Morone chrysops), all species that often occur near the water surface. Benthic-dwelling species, such as catfish (Ictalurus spp.), are often taken when dead and floating on the surface (122), contrary to reports of the live capture of benthic-dwelling species (201). The list of aquatic birds serving as live prey includes not only waterfowl and waterbirds like American Coot (Fulica americana), but also gulls (Laridae) and Great Blue Heron (Ardea herodias) (18) . Large mammals (i.e., cervids) are most often eaten as carrion, especially in winter; small to medium-sized mammals (i.e., hares [Lepus spp.], rabbits [Sylvilagus spp.], muskrat [Ondatra zibethicus], ground squirrels [Marmotini spp.]) may be taken live (157) . Known to eat garbage at dumps in Alaska (198) and British Columbia (202). There are very few documented cases of predation upon living livestock, and most of such cases attributed to Bald Eagle were actually attributable to the Golden Eagle (Aquila chrysaetos) (203, 204, 205).

Quantitative Analysis

Diet has been extensively studied and reviewed (18, 122). Early studies were based on stomach-contents analysis (206, 207, 208, 40). Other approaches include direct observation of prey delivery to nests (209, 210), examination of prey remains from nests and feeding perches, studies of egested pellets from nests and roosts, and direct observation of foraging. The methods of data collection used to study diet have several biases. Relative to direct observations, collections at nest sites overestimated birds, bony fishes, and medium-sized mammals while underestimating smaller mammals and smaller fish (211, 212). Importantly, the diet of an opportunist like the Bald Eagle is highly varied and highly variable from place to place and time to time. For instance, prey remains at nests in southern Florida changed from 1972–1973 to 2009–2010, including a reduction in fish and increase in waterbirds linked to changes in the aquatic communities of Florida Bay (213). Overall, the species selects fish throughout its range when available; combined data from 20 studies across the species' range found that the diet of nesting Bald Eagle was 56% fish, 28% birds, 14% mammals, and 2% "other" (18).

In Maine, 64 species of vertebrates and 2 species of invertebrates were identified from food remains collected primarily at nest sites (186); fish made up > 75% of identified remains. Fish use was greatest at inland nests, with bird and mammal use greatest in coastal areas. On Chesapeake Bay, pellets collected from communal roosts contained 34 bird species, 14 mammal species, several fish species, turtles, and blue crab (Callinectes sapidus); Canada Goose (Branta canadensis) and Mallard (Anas platyrhynchos) were the most common avian prey, and white-tailed deer (Odocoileus virginianus) and raccoon (Procyon lotor) the most common mammalian prey (122). Based on year-round foraging observations, 8 fish species were taken on Chesapeake Bay, with gizzard shad and catfish (Siluriformes) being most common (122). In Florida, nesting Bald Eagle consumed 10 fish species (brown bullhead [Ameiurus nebulosus] and catfish [Siluriformes], comprised 59% of total prey remains), 12 bird species (American Coot, comprised 19% of total), 9 mammal species, and 3 reptile species (214). At Lake Superior, prey remains (n = 156) collected at nests and perches included 50% fishes (especially suckers, Catostomus spp.), and 48% birds, primarily Herring Gull (Larus argentatus) (215). The average total length of fish consumed at Lake Superior was 354 mm (215). Wintering Bald Eagle in Nebraska along the Platte River and at associated reservoirs primarily consumed fish (97% of 489 observations); gizzard shad was the most common prey item, though waterfowl and carp (Cyprinus carpio) contributed the most energy to diet (195).

In the Pacific Northwest and Alaska, post-spawning salmon carcasses are the primary prey when available (e.g., 192, 120), and so the use of salmon during the nesting period is restricted mostly to Alaska as spawning in the Pacific Northwest occurs mostly during fall and winter (210). Herring (Clupea spp.) are also important in Alaska and British Columbia during nesting (216, 217, 210). Direct observations of eagles nesting Puget Sound in Washington indicated that prey was 78% fish, 19% birds, and 3% mammals (218). Of 1,198 items collected from nesting territories in the San Juan Islands, Olympic Peninsula, and Puget Sound, 53% were birds, 34% fish, 9% mammals, and 4% invertebrates (211), and a collection of 380 prey items under 67 nest trees in the Puget Sound and San Juan Islands consisted of 67% birds, 19% fish, 6.8% mollusks and crustaceans, and 6% mammal (218). The most common fish in these western Washington studies included flounder (family Pleuronectidae), plainfin midshipman (Porichthys notatus), dogfish shark (Squalus acanthias), sculpin (family Cottidae), rockfish (Sebastes spp.), ling-cod (Ophiodon elongatus), walleye pollock (Theragra chalcogramma), Pacific hake (Merluccius productus), Pacific cod (Gadus macrocephalus), cabezon (Scorpaenichthys marmoratus), red Irish lord (Hemilepidotus hemilepidotus), salmon (unidentified salmonids) and channel catfish (Ictalurus punctatus) (211, 218). Common avian prey included Mallard, scoters (Melanitta spp.), mergansers (Mergus spp.), Western Grebe (Aechmophorus occidentalis), Common Murre (Uria aalge), Glaucous-winged Gull (Larus glaucescens), Pelagic Cormorant (Urile pelagicus), and Great Blue Heron (211, 218). Nesting individuals on San Juan Island, Washington, preyed extensively on introduced European rabbit (Oryctolagus cuniculus), with 53% of prey (n = 89 total observations) delivered to nests being rabbits that were likely killed by cars or farm machinery; in addition, 5 fish species accounted for 43% of prey (219).

A historic nest site excavated on San Miguel Island, California, used by Bald Eagle from the 1800s through 1939, contained a diverse collection of prey remains: 61.2% were birds (45–48 species), 18.6% fish (12 families), 13.6% invertebrates (largely marine gastropods and bivalves, 29 species), 5.2% mammals (7 species), and 1.4% reptiles (1 species); fish were likely under-represented due to bone deterioration over time (220). At nests in Sonora, Mexico, 77% of 118 prey remains were fish (6 species, catfish most common), 20% birds (11 species), and < 3% mammals (3 species) (221). Two studies in Arizona examined predation by Bald Eagle, observing 481 prey captures (201) and 1,471 prey deliveries to nests (222); fish (primarily catfish) composed 73–76% of prey, mammals accounted for 5–18%, and birds only 1–4%. Wintering Bald Eagle in Arizona along the Colorado River foraged primarily on fish (99% of 1,327 attempts), capturing almost exclusively rainbow trout (Oncorhynchus mykiss) (223). In the Yellowstone ecosystem, carrion is abundant during winter, and Bald Eagle was found to feed primarily on ungulate carrion (93% of 148 foraging observations), noting that predation upon living birds and fish may have been less apparent and thus undercounted (157). Breeding Bald Eagle in the Greater Yellowstone ecosystem, however, preyed more upon birds than elsewhere in the species' range; of the 368 remains/pellets, birds comprised 43% (23 species), fish 43% (6 species), and mammals 14% (11 species) (157).

Food Selection and Storage

The Bald Eagle prefers fish when given a choice, but it is an opportunistic forager that selects prey based on availability (187, 189). When available, selects large fish (340–380 mm) over small fish (230–275 mm) during the breeding period, but there is little selection for size during the nonbreeding period (224). Not known to cache food, but will use carcasses of large items (fish, waterfowl, and large mammals) repeatedly until all is consumed. Food accumulates at the nest and may be used day after day by nestlings, unless fresh food is provided (225). Can consume large amounts of food (gorging), with birds storing food in their crop and then digesting it over several days. They are also capable of fasting for many days: a captive eagle was deprived of food for 16 days and lost 28% of its body mass, but upon resumption of feeding it consumed a 924-gram meal and recovered with no apparent ill effects (226). Another captive individual survived food deprivation for 32 days (102). Reported to have a poorly developed sense of smell; apparently has a sense of taste and avoids foul-tasting items (18).

Nutrition and Energetics

Stalmaster and Gessaman (227) documented food consumption and energy requirements of 4 captive individuals (2 adults, 2 subadults) across a range of ambient temperatures based on diets comprised solely of chum salmon (Oncorhynchus keta), black-tailed jackrabbit (Lepus californicus), or Mallard. The greatest daily consumption occurred with salmon (92.0 g/kg), was intermediate with jackrabbit (74.8 g/kg), and least with Mallard (65.1 g/kg). Relative consumption was inversely related to energy content of food type. Daily gross energy intake, existence metabolism, and excretory energy for all thee food types combined were 425.5, 341.9, and 83.6 kJ/kg at 5°C, respectively. Energy assimilation efficiencies were greatest for Mallard (85.2%) and similar for salmon and jackrabbits (75.0 and 75.4%, respectively). The minimum gross energy requirements for a 4.5-kg Bald Eagle through a 90-day winter at 5°C ambient temperature were estimated to be 13 salmon, 20 jackrabbits, or 32 Mallard. Energy and food requirement in the wild were estimated to be 10% greater than in captivity (142).

Free-ranging individuals in Washington were found to need 489 g/d of salmon to maintain body weight. Adults consumed 522 g/d of salmon, whereas immatures and subadults consumed only 410 and 459 g/d, respectively, and thus were energy deficient during winter (142). Based on field-measured metabolic rates, nestlings in Wisconsin required 1,458 grams of fish per day to support the eaglet metabolic rate plus growth and development needs, which is the equivalent of 5.4 28-cm bullheads (Ameiurus spp.) per day (228).

Metabolism and Temperature Regulation

Stalmaster and Gessaman (142) documented the metabolic rates for 4 captive individuals (2 adults, 2 subadults) under a variety of environmental conditions. The basal metabolic rate, as measured by oxygen consumption, was 11.595 kJ/g/h; the metabolic rate below thermal neutrality (10.6°C) was linearly and inversely related to the ambient temperature. The body temperature of free-ranging individuals averaged 38.8°C while resting at night, 40.7°C when resting during the day, and 41.2°C during flight. The metabolic rate increased by 9% and 21% in response to artificially induced rainfall of 6.1 cm/h and 22.2 cm/h, respectively (142). Based on field-measured metabolic rates using a doubly-labelled water method over a 5-d period, nestlings (n = 16) weighing 3.1–4.9 kg in northern Wisconsin (39 to 60 days of age) expended 2,247 kJ/d (228); this metabolic rate is almost twice the rate of nonbreeding adults in captivity (1,203 kJ/d, 229). The field-measured metabolic rate for eaglets was weakly positively correlated with age but not with mass.

Drinking, Pellet-Casting, and Defecation

Water is largely derived from prey and metabolic processes. Nestlings are not provided with water for drinking. Will drink water on occasion when bathing .

Casts pellets with indigestible food components (hair, feathers, some bone, large fish scales). The meal-to-pellet interval for captive raptors, including Bald Eagle, is reported to be < 1 meal/pellet (230). Multiple pellets can be produced in a 24-h period, including 16 pellets cast from a single meal for a captive individual, and 3 pellets from one meal for an individual observed in the wild (231). Pellets are generally cast at a roost, possibly at dawn in response to daylight (18). There is little information on defecation, but eagles often void watery, white-colored excreta after feeding (209).

Recommended Citation

Buehler, D. A. (2022). Bald Eagle (Haliaeetus leucocephalus), version 2.0. In Birds of the World (P. G. Rodewald and S. G. Mlodinow, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.baleag.02