Golden Eagle Aquila chrysaetos
Version: 2.0 — Published September 17, 2020
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Diet and Foraging
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Diet and Foraging
Generally takes prey of intermediate size (0.5–4 kg), but occasionally takes both smaller (e.g., mice, voles; 253) and larger (cervids and canids) prey. Also scavenge at all times of the year, but this behavior is most common during the non-breeding season. Globally, diets of this species are catholic, consistent with their wide geographic distribution, and often focused on locally abundant species (e.g., 302, 303, 16, 253), suggesting a role for learning and individual preference in dietary choices. Occasionally takes livestock, although most observations of Golden Eagle feeding on livestock are of scavenging, rather than predation. The majority of data on diet and foraging comes from observations of territorial birds. There is sparse information on diet and foraging of non-territorial birds during the nesting season.
Main Foods Taken
In North America, the Golden Eagle tends to take small to medium-sized mammals, mainly leporids and sciurids, during the nesting season (304, Table 2). Diet is diverse, and individuals in some parts of the range focus on other taxa, including many species of birds and larger mammals (195, 304). Throughout its range, the species scavenges extensively on carcasses of ungulates (241) and smaller and medium-sized species, especially during the non-breeding season. See Diet and Foraging: Diet.
Microhabitat for Foraging
Takes most prey on or near the ground in each of the many habitat types occupied; rarely takes food over water. For details on broader foraging habitat use, see Habitat.
Food Capture and Consumption
Hunts from flight, either when soaring or in low contoured flight, or from a perch (305, 306, 243, 29). Hunting strategy is determined by weather conditions, topography, prey species, and the prey's escape response (243, 2). Hunts from a soaring flight more often on sunny and windy days, and from perches on overcast, calm, or rainy days. May hunt by flying close to the ground (“contour flight”) in broken topography, and they tend to hunt from a soar at higher altitudes in open habitats, although both types of flights occur in both types of habitats. They use contour flight to surprise prey that might escape to burrows. Contour hunting is said to be most common overall (2), but perch hunting is most common in southwestern Idaho where habitat is open and perches (power lines, canyon rims, and rock outcrops) are abundant (306). Aerial attacks are most often from upwind (29). See Behavior: Locomotion: Flight for additional details.
When hunting, often attacks prey using one of at least seven techniques (2). These include: (1) “high soar with glide attack” from a thermal with a long (≥ 1 km), low angle glide to attack solitary or widely dispersed prey (hare, grouse [Phasianidae]); (2) “high soar with a vertical stoop” from a high soar to attack slow-flying or flocking prey, such as geese (Branta spp.) and cranes (Grus spp.) (307), and Greater Sage-Grouse (Centrocercus urophasianus) (EHC); (3) “contour flight with a short glide attack” from low-level flight quartering over the ground to surprise colonial prey (ground squirrels of many genera); (4) “glide attack with tail chase” from a low angle stoop to flush, chase, and capture agile mammals and birds; (5) “low flight with slow descent attack” from a low-level quartering flight and slow “parachute” stoop to capture slow-moving prey (tortoises and snakes); (6) “low flight with sustained grip attack” to kill ungulates by landing on the victim's back or neck, and riding it until the animal dies (308, 309, 310); and (7) “walk and grab attack” to forage for insects (Acrididae) and to capture quarry protected by an obstruction (311, 312; M. Collopy, personal communication).
In a “walk and grab attack”, an eagle lands next to or close to its potential prey, usually a slow-moving or defenseless individual (2), and then walks up and grabs it (CLM). This behavior is frequently observed in Denali National Park and Preserve, Alaska, where eagles land near and walk up to colonies of arctic ground squirrel (Urocitellus parryii) and hoary marmot (Marmota caligata) in search of prey. Eagles also walk when searching for nestlings of ground-nesting birds such as Willow Ptarmigan (Lagopus lagopus) (CLM). Ellis (78) observed eagles running very quickly while pursuing red fox (Vulpes vulpes). They also may dive onto prey, and run after them when the prey species tries to take cover under shrubby vegetation (CLM; R. Swisher, personal communication).
Frequently feeds on carrion, especially during winter, even when live prey are available (313, 314, 2, 304). Some pairs also consume carrion during the nesting season (315, 316). Locates carrion from high-soaring flight, often cueing on the activity of crows (Corvus spp.) and other scavengers (2). When approaching carrion, they use a variation of a “walk and grab” attack, often landing some distance from a carcass (> 40 m) and walking to it (TAM, TEK, P. Bloom, M. Lanzone, unpublished data).
Mated pairs sometimes cooperatively hunt jackrabbits and other prey. It is not known if unpaired eagles may hunt in tandem. During tandem hunts, pairs pursue prey with the female usually following the male at a lower height (317). One pursuer diverts the prey's attention by stooping while the second makes the kill (318, 319, 317). In southwestern Idaho, males were more likely than females to hunt solo, and tandem hunting was less successful than solo hunting (317). Overall capture success for all hunts by eagles was 20% (n = 115 capture attempts), with capture success at 4.6% for tandem hunting (n = 42) and 29% for solo hunting (n = 73).
Occasionally hunts cooperatively with conspecifics. This type of hunting usually involves large prey targeted in winter (e.g., ungulates, red fox, Wild Turkey [Meleagris gallopavo]; 320, 321, 308, 322). Groups usually number 2 to 4, but can be as large as 8 (322). Groups of 3 to 6 adult Golden Eagles were observed flying together during the nesting season in North Dakota (323). Those birds flew in “wing-tip to wing-tip” formations at “tree-top” height on ≥ 12 occasions. It is uncertain if these groups were hunting because no capture attempts were ever observed.
May engage with canids in joint hunting behavior, described as either cooperative associations or exploitative competition (324, 325, 326; R. Bruesewitz, personal communication in 327). On one occasion in southwestern Wyoming, a Golden Eagle and a red fox were observed hunting a white-tailed jackrabbit (Lepus townsendii) at the same time (TEK). The eagle dove at the fox while the fox was chasing the hare. The hunt appeared unsuccessful, but a wounded jackrabbit was later seen ~500 m from the scene of the chase. It is unlikely that the two hunters coordinated their efforts, but the activity suggests that eagles may take advantage of disturbance by other predators to surprise prey (see below for similar responses of eagles to human disturbance of potential prey).
Less common feeding behaviors include kleptoparasitism, piracy, nest-robbing, and fishing. There are records of eagles taking prey from corvids (328, 329), foxes (330), Great Horned Owl (331), Northern Harrier (Circus hudsonius; MNK), Red-tailed Hawk (Buteo jamaicensis; 243), Prairie Falcon (Falco mexicanus; J. McKinley, personal communication), and other Golden Eagle (243) (see Behavior: Social and Interspecific Behavior: Kleptoparasitism for additional details). Also takes eggs and young from nests of many species including Canada Goose (Branta canadensis), Ferruginous Hawk (Buteo regalis), Prairie Falcon, Gyrfalcon (Falco rusticolus), Common Kestrel (F. tinnunculus), harriers (Circus spp.), Great Horned Owl (Bubo virginianus), Barn Owl (Tyto alba), Common Raven (Corvus corax), Yellow-billed Magpie (Pica nuttalli), Black-billed Magpie (P. hudsonia), and Rock Dove (Columba livia) (332, 333, 334, 208, 335; U.S. Geological Survey, unpublished data, TEK). Fishing is rare, but Brown (336) observed ≥ 5 individuals frequently capturing live trout from shallow streams and pools in Arizona during winter, and fresh salmon carcasses have been found in an occupied nest on the western Seward Peninsula (S. Lewis, personal communication).
Cannibalism occurs rarely. Collopy (337) reported apparent cannibalism of a nestling by its sibling in a nest in southwestern Idaho. Partially eaten remains of a Golden Eagle nestling in a Montana nest suggest cannibalism by a sibling or parent (338). Korňan and Macek (339) documented infanticide followed by cannibalism in a Golden Eagle nest in Slovakia. Cannibalism may occur if a territorial battle results in death, with the loser consumed by the winner, as this has been noted for many raptor species including other Aquila eagles (TEK).
May also hunt in association with humans or human activity. For example, near Pikes Peak, Colorado, eagles captured two bighorn sheep (Ovis canadensis) lambs that had been hazed by people using helicopters to capture lambs for research (340). Similar eagle behavior was noted during capture of Dall sheep (O. dalli) lambs in the central Alaska Range, where eagles pursued young lambs that the helicopter had separated from their mothers (S. Arthur, personal communication). In Idaho, an eagle captured a Barn Owl that flushed from a its daytime perch when a helicopter passed by the cliff (EHC; T. Craig, personal observation). Golden Eagles apparently followed and pounced upon squirrels flushed by a horse-drawn plough in San Diego County, California (311). A pair of Golden Eagles in Georgia (USA) were observed to follow a farmer’s tractor to get rabbits and mice flushed when the farmer was bush-hogging (M. Robertson and L. Wren, personal communication in 327). In Idaho, Golden Eagles followed a person on horseback who was flushing jackrabbits in sagebrush habitats (KS). On two separate occasions eagles killed Greater Sage-Grouse flushed by a horseback rider (D.W. Ellis, personal communication), and in another case they killed a Greater Sage-Grouse flushed by hunters (EHC). Finally, in two separate events, Golden Eagles captured one gunshot and one unwounded Chukar (Alectoris chukar) that were flushed by a hunter (M. McGee, personal communication).
Major Food Items
A recent meta-analysis summarized diet in 35 nesting-season studies conducted in 45 areas of western North America (304). That study concluded that during the nesting season in western North America, Golden Eagle feeds mainly on mammals (84% of prey items) and secondarily on birds (15%); reptiles and fish are less frequently (2% and 0.2%, respectively) taken; insects occasionally taken (312). In parts of Europe, Golden Eagle relies heavily on avian prey, but mammalian prey are still generally the most important by biomass (302, 303, 16, 253)
Mammalian prey observed at nests in western North America are primarily either leporids (jackrabbits, other hares, or cottontail rabbits) or sciurids (ground squirrels, marmots, or prairie dogs; Table 2). These two groups combined constitute, on average, 44–97% of prey items recorded, with leporids being primary prey in 78% of studies and sciurids primary prey in 18% of studies (304). Black-tailed jackrabbit (Lepus californicus) is the main prey in the southwestern United States, and white-tailed jackrabbit (L. townsendii) is more commonly taken in Wyoming and Montana (304). Hares and ground squirrels are primary prey in parts of Eurasia (341, 20) .
Avian prey observed at nests of Golden Eagles in western North America are predominantly gallinaceous birds (pheasants, grouse, and partridge) or waterfowl (342, 304). Ptarmigan (Lagopus spp.) are important prey in western and interior Alaska (343, 344), and waterfowl are taken frequently in arctic Canada and the Alaska tundra (134, 344). Ring-necked Pheasant (Phasianus colchicus) and Chukar are regularly taken in the Columbia Plateau of Washington, the Snake River Plain of southwestern Idaho, and the central Great Basin of Utah (345, 346, 347, 314). After wildfire caused large-scale habitat alteration and subsequently reduced jackrabbit numbers, waterfowl, mainly American Coot (Fulica americana) and Mallard (Anas platyrhynchos), were frequently taken by nesting Golden Eagle in southwestern Idaho (348). Golden Eagles that once nested in Maine took a wide variety of wading birds, ducks, seabirds, game birds, corvids, and other raptors (195). Chukar is taken in parts of Eurasia (20).
Scavenges extensively on, and occasionally kills, large prey, including seals (Phocoidea), ungulates, such as mountain goat (Oreamnos americanus), bighorn sheep (O. canadensis), Dall sheep (O. dalli), Asian mountain goat species (Pseudois, etc.) , chamois (Rupicapra rupicapra) , musk ox (Ovibos moschatus), caribou (Rangifer spp.), deer (Odocoileus spp., Cervus spp.), pronghorn (Antilocapra americana), Mongolian gazelle (Procapra guttarosa); carnivores including coyote (Canis latrans), badger (Taxidea taxus), arctic fox (Vulpes lagopus), bobcat (Lynx rufus), and Pallas’s cat (Felis manul); and large birds of many types, including wild turkey, geese , Trumpeter Swan (Olor buccinator) and Tundra Swan (O. columbianus), Sandhill Crane (Grus canadensis) , Whooping Crane (G. americana), and Demoiselle Crane (G. virgo), Osprey (Pandion haliaetus), Upland Buzzard (Buteo hemilasius), Eurasian Eagle-Owl (Bubo bubo), Common Raven, and Great Blue Heron (Ardea herodias) (73, 349, 320, 350, 342, 307, 9, 351, 352, 353, 354, 309, 310, 355; R. Ritchie, unpublished data; J. Rose, personal communication). When ungulates are taken, primarily young individuals are killed, though there are reports of Golden Eagle killing adults (318, 248, 308, 327; J. Toynbee, personal communication). There are numerous on-line videos of Golden Eagle attacking ungulates; some of these videos are staged and some of the birds filmed are clearly flown by falconers.
Also preys on or scavenges domestic animals, including sheep, goats, calves (Bos taurus), pigs, poultry (Gallus gallus), dogs (Canis familiaris), and cats (Felis catus) (73, 356, 342). May kill livestock, even when principal prey are available (357), but such predation events are rare. Livestock remains, including both carrion and eagle kills, accounted for only 1.4% of 7,094 prey items identified in studies throughout the western United States (342). In studies in which domestic sheep and goat remains were found at nests, these species constituted 0.2 to 13.9% of remains (358, 356, 359).
In Japan, the species apparently specializes on Japanese hare (Lepus brachyurus; 231). Other important prey were a variety of snakes and Copper Pheasant (Syrmaticus soemmerringii). In Arabia, hunts hares, gazelle, and spiny-tailed (Uromastyx) lizards (230).
Approaches to assessing diet and biases in those approaches
The methods used for sampling dietary information affect how the results are interpreted and applied. Golden Eagle dietary data are available primarily for the nesting season and are most frequently based on analyses of pellets and prey remains collected at nests (304). However, dietary data also may be obtained through direct observation of prey deliveries at nests (337) or, more recently, from motion sensitive cameras at nests (360, 361). Some of the earliest dietary estimates were based on analyses of stomach samples (362, 313), but the quantity of data collected from stomach samples from individual raptors is minimal compared to that collected via other methods (363). Stomach content analysis, collection of pellets and prey remains beneath roosts, and motion-sensitive trail cameras set over carcasses, are some of the only ways that dietary data have been collected during the non-breeding season.
Prior work suggests that analysis of prey remains and regurgitated pellets collected at nests tends to underestimate total prey biomass compared to direct observation, but the two methods do not differ significantly with regard to percent biomass or percent frequency (337, 364, 304). In contrast, cameras can detect more prey species than does identification of prey remains (360), and the probability of detecting small prey items may be higher for camera images than from analyses of prey remains and pellets (253, 361).
The frequency of data collection can influence results of dietary studies (253). Video or observational data are essentially continuous when a camera is operational or when a person can watch the nest. However, cameras often are installed after nestlings have reached a certain age (361), and observers cannot be present all day every day at nests. Similarly, the frequency of visits to nests for collection of pellets and prey remains can vary from as regularly as every 3–5 days during the nesting season for 10 years (365) to one collection per nest for only a single season (see Appendix 1 of 304).
Major Prey in North America by Geographic Region—nesting season
The relative importance of prey taxa varies by ecoregion (diets reviewed in Bedrosian et al. , Table 2). Arctic ground squirrel is generally the primary prey in the Boreal Cordillera, Alaska Tundra, and Southern Arctic ecoregions (> 60° N; Table 2), but only after the squirrels awake from hibernation. Snowshoe hare, arctic hare (L. arcticus), and ptarmigan are secondary prey (Table 2; see Bedrosian et al. (304) for ecoregion descriptions and maps), and are the primary prey species available while squirrels hibernate. Leporids (jackrabbits [mainly black-tailed jackrabbit] and cottontails) dominate Golden Eagle diets in western North America below 50° N latitude (Table 2). Sciurids (rock squirrels and California ground squirrels) are primary prey in the Western Cordillera and Mediterranean California ecoregions (Table 2). Sciurids (yellow-bellied marmot, prairie dogs, rock squirrel, and numerous ground squirrel species) are common secondary and tertiary prey (Table 2, 304). Many species of birds, including but not limited to Black-billed Magpie, ducks, Chukar, Greater Sage-Grouse, Ring-necked Pheasant, forest grouse, and Rock Pigeon, are secondary and tertiary prey in the West Central Semi-Arid Prairie and Cold Desert ecoregions (304, Table 2). Reptiles comprise a higher proportion of Golden Eagle diet in the Mojave Desert than in any other area in western North America. Gopher snake (Pituophis catenifer) and chuckwalla (Sauromalus ater) were tertiary prey in two studies in warm deserts but do not comprise more than 10% of prey items (304). Other prey of Golden Eagles include gulls (Larus spp.) and tree squirrels (Sciurus spp.) in the Channel Islands and southern California, and tortoises in southern California and New Mexico (360, 304; B. A. Kimsey, personal communication).
Recent studies suggest that diets of some Golden Eagle populations have changed over time in response to alterations in habitat (348). White-tailed jackrabbit and cottontails were the most commonly taken prey in south-central Montana during the 1960s (366, 358). However, jackrabbits accounted for < 4% of prey items in this same study area 50 years later, and Richardson's ground squirrel and cottontails are now the most frequently taken prey (R. Crandall, unpublished data). Similarly, black-tailed jackrabbits and cottontails accounted for 54% of prey items collected from Golden Eagle nests in southwestern Idaho between 1971–1981 (348). Thirty years later, jackrabbits and cottontails accounted for only 13% of prey items, and American Coots and Mallards increased from < 2% of prey items to 27% (348). Most studies of diet have been conducted prior to 2008 (304) and these two long-term studies suggest that historical knowledge about eagle diets may not accurately predict future diets.
Diet data from the nesting season are scarce for eastern North America. The few published historical records on the diet of breeding Golden Eagles in eastern Canada suggest that they feed on birds (particularly waterfowl and wading birds) with greater frequency than do eagles in western North America (195, 5). A high proportion of prey remains in Golden Eagle nests in Maine were from American Bittern (Botaurus lentiginosus), Canada Geese, and Great Blue Heron (367, 194). Observations on the Canadian breeding grounds suggest that Golden Eagles prey on ducks in northern Labrador (368), and indirect evidence suggests that Golden Eagles take fish-eating seabirds (Procelliformes) and scavenge marine mammal carcasses (TAM). However, snowshoe hare, cottontails, geese, and marmots also are thought to be common prey in eastern North American (216, 142).
Major Prey in North America by Geographic Region—Non-breeding Season
Information on diet of Golden Eagles during the non-breeding season (the period when eagles are not feeding young at nests, usually fall and winter) is much more limited than for the nesting season (304). That said, one meta-analysis of these limited data suggested that winter diet of Golden Eagles in western North America does not appear to differ appreciably from nesting season diets except that hibernating sciurids are not available and thus not taken and carrion is more frequently taken (304). Carrion is a highly important part of the winter diet of some eagles, and the amount consumed might be influenced by winter severity and local availability of other food (304). Prey taken during the non-breeding season by territorial pairs in northeastern Wyoming and southwestern Idaho varied greatly among territories (236, 176). Winter diet in central Utah is reported to be composed almost entirely of black-tailed jackrabbit (97%; 305), but it is unclear how changes in jackrabbit abundance have influenced winter diet of those birds. Eagles prey on young of the year and occasionally adult pronghorn in the Wyoming Basin during winter (313, 369, 308, 310, 304). Hares and rabbits comprised 51% of 65 prey individuals identified in stomachs of 50 Golden Eagles killed in Colorado in March 1948, and leporids also comprised 59% of items identified in 63 eagle stomachs collected between November and March from 15 states throughout the western United States (362, 313). Sheep and goats constituted 11% of items in that study. Golden Eagles also prey on waterfowl during winter (313, 370).
Wintering eastern Golden Eagles regularly feed on carrion of white-tailed deer (Odocoileus virginianus), moose (Alces alces), and caribou (Rangifer tarandus; 371, 216, 196). Golden Eagles have been observed attacking wild turkey during winter (352), and evidence suggests attempted predation on a porcupine (Erethizon dorsatum; 372, EHC). Waterfowl are thought to be important in winter diets on Chesapeake Bay and eastern coastal areas (D. Buehler, personal communication).
Food Selection and Storage
Golden Eagles in North America focus on prey that range in size from 500 to 4,000 g (2, 304; see Diet and Foraging: Diet). The exception is the high percentage of smaller ground squirrels (200–300 g) taken in northern California and southwestern Idaho (365, 348; B. Woodbridge, unpublished data in 304). In southwestern Idaho, size of prey ranges from 10 to 5,800 g (geometric mean 690 g, n = 2,203 items; 365). Generally eats large prey at kill sites, however, fresh limbs of young ungulates in nests suggest that eagles may disarticulate large prey before bringing parts to the nest (313, MNK; T. Craig, personal communication).
Parents may bring more food to nests than young can eat. Excess food is only sometimes carried away from the nest (see Breeding: Parental Care). Over-provisioning of prey with extensive fat reserves can have negative consequences from excess oiling of nestlings (373).
Rarely caches prey. However, there is a report that a pair in Scotland deposited prey on a cliff near the nest before feeding it to the young (374).
Foraging Response to Changes in Prey Availability
Some Golden Eagle populations exhibit dietary shifts in response to changing abundance of primary prey species. An increase in diet breadth and a decrease in the frequency of leporids in the eagle diet was correlated with a decrease in relative leporid abundance in Montana (358). Dietary breadth was negatively correlated to cottontail abundance in the Bighorn Basin of northwestern Wyoming, suggesting that as cottontails increase in abundance, eagles eat fewer alternative prey (375). Similarly, a large increase in diet breadth and a shift in major prey consumed has been related to habitat alteration in the Columbia Basin (364, 348).
The most comprehensive information about diet composition and dietary change of the Golden Eagle in North America comes from Morley Nelson Snake River Birds of Prey National Conservation Area (NCA) in southwestern Idaho where > 2,200 individual prey items were identified from 1971 to 1981 (365) and > 1,160 items from 2014–2015 (348). In these studies, the proportion of main prey in the diet varied annually, and the proportion of jackrabbits in the diet correlated positively with jackrabbit density in the environment. Black-tailed jackrabbit was preferred over Piute ground squirrel, and ground squirrels were preferred over all other, non-jackrabbit prey. Diet breadth of these eagles was smaller than co-occurring Red-tailed Hawks but larger than that of Prairie Falcon. After wildfires caused large-scale habitat alteration and reduced available jackrabbit habitat, eagles in the NCA shifted from taking mostly leporids (mainly black-tailed jackrabbit) to waterfowl (American Coot and Mallard; 348). Diet breadth increased after wildfires, and the altered diet included more birds and fewer black-tailed jackrabbits and cottontails compared to pre-burn years. Jackrabbits, however, still contributed the most biomass to the post-burn eagle diet (348).
Diet also varies within the nesting season, often reflecting opportunistic hunting and suggest a functional response to changes in prey availability. In southwestern Idaho, Ring-necked Pheasant is the most common prey in nests in April, coinciding with the peak of pheasant breeding activity; however, once pheasants began to incubate eggs, they were commonly detected as prey in eagle nests (199). Diet in the early nesting season in interior Alaska, when sciurids are hibernating, consists mainly of hares and ptarmigan. Ground squirrels (mainly juveniles) form a large proportion of prey delivered to nests later in the season (CLM). In Norway, captured fewer mountain hare (Lepus timidus) and Willow Ptarmigan (Lagopus lagopus) and more thrushes as the nesting season progressed, (Turdus spp.; 253).
Nutrition and Energetics
Golden Eagle pairs delivered ~0.885 kg of prey biomass per day to nests in western Texas (376) and ~1.417 kg per day in southwestern Idaho (249). Pairs in Montana brought an estimated 1.47 kg of prey per day to a nest (377). Eaglets in multiple-young broods receive more food from adults than do eagles in one-young broods (249). See Breeding: Parental Care, for additional information on prey delivery rates.
Estimates from feeding trials suggest that between 24 and 33 kg of food is needed to raise a nestling from hatching to fledging (10 weeks; 378). Prey biomass consumed by nestlings increases during brood-rearing and peaks at 7–9 weeks of age (249). The amount of food consumed daily by 2 male and 2 female captive nestlings increased steadily from 11 to 15 days of age, peaked at 28–44 days, and declined slightly until experiments ended at 53–57 days (379). Food consumption did not differ between male and female nestlings. During late brood-rearing (47–57 days old), captive eaglets consumed 12–15% of their body mass per day. This is much greater than published consumption rates of free-flying adults and juveniles (5.7–6.6% of body mass per day; 380). Greater food consumption by nestlings likely reflects the intense energetic costs of producing body tissue and feathers. Assimilation efficiency (ratio of energy metabolized to energy ingested) of 4 captive nestlings averaged 74.4% (range 73.9–74.8%) and did not differ between males and females (379). In the case of adults, and probably for nestlings, greater food consumption results in birds being more able to withstand thermal stresses (381, 382).
Metabolism and Temperature Regulation
Mean gross and net energy efficiency (proportion of total ingested and metabolized energy, respectively, converted to feathers, fat, and other body parts) of the 4 captive nestlings described in Nutrition and Energetics was 31% and 42%, respectively, and did not differ between sexes (378). Metabolized energy is energy ingested minus energy egested in the form of feces and pellets. Growth efficiency (ratio of biomass produced to biomass consumed) of nestlings decreased linearly with age, from 27%, at 2 weeks of age, to < 5%, at fledging, and did not differ between males and females (379). As eaglets aged, more of their energy budget is allocated to maintenance. Trends in metabolized energy paralleled food consumption and peaked at ~2,500 kJ/d, with no difference between sexes. Metabolized energy of wild male nestlings peaks at 7–8 weeks of age at ~2,000 kJ/d, and females peaks at ~3,100 kJ/d at 8 weeks of age (379). Energy metabolism ranged from 4.3 to 4.0 W/kg for 2 other captive Golden Eagles (383). Body temperature of a telemetered nestling ranged from 38 to 39°C during 18 days when the bird was 35 to 53 days old (384). During a single nocturnal recording, the nestling’s body temperature dropped to approximately 38°C within 1 hour after sunset and remained near that temperature until about sunrise.
Drinking, Pellet-Casting, and Defecation
Drinks occasionally, but most or all liquid requirements, particularly for nestlings, are met by ingesting prey (32). In Nevada and Arizona, reported to drink from small creeks and in mountain bogs and springs and to ingest snow near or above timberline (385, 386; D. Driscoll, personal communication). Trail cameras commonly capture Golden Eagle drinking at stock tanks and “guzzlers” in desert environments (120, TEK). Drinking can be a frequent daily activity of captive individuals (387, 388).
Usually casts pellets once per day, often early in the day (M. Collopy, personal communication). When casting pellets, an eagle arches its neck with face down and forward and gapes widely while rapidly shaking its head laterally. This behavior is repeated several times, with brief pauses between head shakes. Soft squeaks or whistles often accompany casting. Individuals often bob their heads prior to casting, and conspicuous swallowing often follows a casting attempt. Young cast 1–3 pellets per day from age 20 days to fledging, but some young do not cast every day (78). Two captive male and two captive female young produced an average of 8 g and 7 g per day of pellets (all measurements dry mass), respectively (378). The same captive eaglets defecated an average of 57 and 60 g per day, respectively (378). Number of defecations per day increases linearly to about day 20 in wild nestlings (n = 4), and then levels off to 10–16 per day until fledging (78).