Golden Eagle Aquila chrysaetos
Version: 2.0 — Published September 17, 2020
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Demography and Populations
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Demography and Populations
Golden Eagle is a long-lived, slowly reproducing, k-selected species, with low population density. Populations are stable or slightly declining in North America, although trends in the western United States may be declining slightly and those in eastern North America may be increasing slightly. Populations in much of Europe appear to be increasing; they are of unknown status in Asia and are likely declining in Africa, where the species is of high conservation concern.
Measures of Breeding Activity
Age at First Breeding
Golden Eagles usually attempt to nest only after attaining adult plumage, which generally occurs in the fifth summer (see Appearance: Plumages). However, they are capable of nesting earlier, and eagles in pre-adult plumages are sometimes observed on territories
Nine eagles marked as nestlings in southwestern Idaho were 4–15 years old when first detected on breeding territories (254; USGS, unpublished data). Three of four known male eagles were 4 years old when first detected, and the other male was 15 years old. Two marked males were 4 to 8 years old when observed nesting (254). The only female known to return was 7 years old. A female telemetered as a nestling in southwestern Montana first laid eggs at 3 years of age (R. Crandall, unpublished data). Over 12 years (1970–1981) in southwestern Idaho, the percentage of nesting pairs composed of at least 1 subadult ranged from 0–13% (538). In that study, the frequency of subadult nesting was highest when densities of wintering adults were lowest. In Denali National Park and Preserve, Alaska, the percentage of nesting pairs with ≥ 1 subadult averaged 2% per year (range 0–6, n = 14 yr; CLM). In that study, all 7 breeding subadults were females. In central California, between 1996 and 2000, the percentage of pairs observed with ≥ 1 subadults ranged from 0 to 6% (G. Hunt, personal communication). One of these pairs was composed of 2 subadults. Finally, in a low-density population in Norway, 51% of pairs had ≥ 1 subadult member (539).
Territorial adults may prevent subadults from nesting. As a consequence, subadults are more likely to nest in areas with fewer adults and usually in territories with high disturbance and high turnover rates (538) or where persecution has been high (539). Therefore, the proportion of pre-adults occupying territories may indicate the stability of the population, as for other eagle species (540, 541).
Intervals between Breeding
Highly variable and poorly understood. In general, the probability of breeding appears to respond to spatial and temporal variation in prey availability, such that reproduction is more likely in places or years with high prey density. Variation among sites can be at the territory level (some territories consistently produce offspring) or regional (breeding attempts are infrequent in some regions, frequent in others). Variation among years may be driven by fluctuations in density of cyclical prey species (e.g., ground squirrels, lagomorphs, colonial corvids). See Demography and Populations: Determinants of Variation in Nesting Success for additional details.
Clutch Size and Number of Clutches per Season
Renesting sometimes occurs after a nest fails early in the nesting cycle, but there are no records of pairs producing > 1 brood in a year. For information on numbers of eggs produced and egg laying behavior, see Breeding: Eggs: Clutch Size, Egg Laying.
Brood Size and Number of Broods per Season
Golden Eagles produce a single brood in a season, generally of 1–3 young. Populations at the northern end of the range generally have smaller broods and produce fewer fledglings than those in temperate areas (343; Table 4). For additional details, see Demography and Populations: Population Regulation.
Proportion of Broods with One, Two or Three Nestlings
In a survey of 21 study areas in North America ranging from Zacatecas, Mexico to the north slope of the Brooks Range, Alaska (Table 4), broods with one > 7 week-old nestling were more common than broods with 2 or 3. Broods with 2 nestlings were documented in all 21 study areas, and broods with 3 nestlings were documented in 13 study areas (Table 4). The proportion of broods with 1, 2, and 3 nestlings averaged 0.63 ± 0.14 SD (range 0.19–0.85), 0.35 ± 0.14 SD (range 0.15–0.80), and 0.02 ± 0.02 SD (range 0.00–0.07), respectively (n = 5,266 broods; Table 4).
Broods with 1 nestling were more common than broods with 2 or 3 nestlings in every study area except the Butte Valley, California, and southwestern Idaho (Table 4). The highest proportions of single-nestling broods were reported in study areas in Yellowstone National Park, Wyoming (0.85) and Quebec, Canada (0.82) (Table 4). The highest proportion of broods with 2 nestlings (0.80) was reported for the Butte Valley, California, study area (Table 4), where there is anecdotal evidence that a combination of abundant prey and favorable weather conditions over 16 years was correlated with high nesting success and fledgling production (B. Woodbridge, personal communication).
In the 13 study areas reporting broods with 2 nestlings, the overall proportion of broods with 2 nestlings ranged from 0.005 to 0.07, and the proportion of years when 3-nestling broods were detected ranged from 0.06 to 0.75 (mean = 0.34, SD = 0.21). The study area with the highest proportion of years when broods of 3 nestlings were detected (0.75; 6 of 8 years) was in Oregon. The study area with the highest overall proportion of broods with 3 nestlings (0.07) was in the Kisaralik River watershed, Alaska (Table 4). Curiously, the Butte Valley, California study area had the highest overall proportion of nests with 2 nestlings, but the lowest proportion of broods with 3 nestlings (0.005) (Table 4).
The frequency of broods with 3 nestlings varies by year in two study areas where the primary prey exhibit population cycles (Table 4). In the Kisaralik River study area, broods of 3 nestlings were detected in 4 of 5 consecutive survey years from 1991 to 1996, but in only 1 of 11 survey years between 1997 and 2014 (B. McCaffery, unpublished data). From 1976 to 1994 in the Morley Nelson Snake River Birds of Prey National Conservation Area, brood sizes of 3 were recorded only in the 6 years with the highest estimated jackrabbit densities in the preceding winter (KS). No broods of 3 young were found in the 13 years with the lowest estimated rabbit densities in the preceding winter (KS). As jackrabbit numbers have declined across southwestern Idaho, the frequency of large broods has declined. The number of broods with 3 nestlings was 36% lower between 1990 and 2009 than it was between 1970 and 1989 (MNK).
Determinants of Variation in Nesting Success
Several factors influence nesting success of Golden Eagles. It is thought that age of parents may affect nesting success, with older and younger birds being less productive than those of middle ages, as is the case for other k-selected raptors (e.g., Eurasian Sparrowhawk [Accipiter nisius; 542]; White-tailed Eagle [Haliaeetus albicilla; 543]).
In many regions, the combination of weather and prey densities interact to determine nesting success. In southwestern Idaho, jackrabbit abundance limited reproduction during 15 of 23 years, and weather influenced how severely reproduction declined in those years (474). Similarly, drought in the Diablo Range, California has no influence on territory occupancy, but it strongly influences the annual probability of successful nesting (438). In Denali National Park and Preserve, Alaska, reproductive success is related to numbers of hare and ptarmigan prey (544). In Wyoming, reproductive success increases with increasing cottontail rabbit populations (375). Finally, in eastern Utah, reproductive rates fluctuate with prey densities and weather conditions (204).
Laying rates for birds are usually related to conditions prior to the nesting season (545). Females are thought only to lay eggs if they are able to gain body mass and mobilize reserves for egg production. Likewise, insufficient food supplies or increased energy needs due to cold weather may prevent egg-laying. Many pairs do not lay eggs during periods of low prey abundance (420, 474, 343, 475, 544).
In southwestern Idaho, the percentage of pairs laying eggs is related positively to black-tailed jackrabbit abundance and inversely to winter severity (474). Similarly, in Denali National Park and Preserve, Alaska, the probability of laying eggs and the number of fledglings produced correlated positively with the abundance of prey (475). In both these studies, the percentage of pairs that lay eggs each year was the most variable reproductive component (474, 343, 475), varying from 38 to 100% in Idaho (mean 79% ± 15.5 SD [n = 22 yr]) and from 4 to 88% in Alaska (mean 56% ± 22 SD [n = 30 yr] 475, 544). Despite this inter-site variability, the percentage of laying pairs that were successful was more constant, with a mean of 60% ± 14% [SD] in southwestern Idaho (range 32–80%, n = 23 years; 474) and 61% in Denali National Park and Preserve (range 14–88%, n = 23 years; 475). Consequently, annual reproductive output often is influenced most strongly by the proportion of pairs that lay eggs, as opposed to brood size or nest failure rates (474).
There is substantial variation in the frequency with which eggs hatch. The percentage of eggs that hatch ranges from 57% in central Utah (n = 87 eggs, 44 clutches; 420), to 65% in southwestern Idaho (n = 282 eggs, 145 clutches; USGS, unpublished data), and to 86% in south-central Montana (n = 28 eggs, 14 clutches; 358). Likewise, once eggs hatch, there can be substantial variation in the frequency with which nestlings survive to fledging. The percentage of nestlings that survived to leave nests is 77% in southwestern Idaho (n = 302 young, 168 broods; USGS, unpublished data), 80% in central Utah (n = 50 young, 35 broods; 420), and 46% in south-central Montana (n = 24 young; 358).
As noted above, migratory populations that nest in northern parts of the species’ range may produce smaller broods and raise fewer fledglings than resident eagles in temperate regions (343). In Denali National Park and Preserve, Alaska (475), apparent nest success and mean brood size are unrelated to prey densities or other covariates considered (human disturbance, elevation). However, farther south, spring weather may affect survival of nestlings (478). In southwestern Idaho, nesting success and brood size at fledging is related positively to rabbit abundance and inversely to the frequency of hot spring days (474). It is thought that the inadequate food availability (i.e., low rabbit populations) may interact with high temperatures to cause nestling mortality (474, 479).
Human activity also may influence nesting success (546, 419; see also Conservation and Management: Effects of Human Activity: Disturbance at Nest and Roost Sites). Finally, frequent interactions between non-territorial birds and territory holders in areas of the Swiss Alps with high densities of eagles apparently reduces reproductive success of territorial pairs (228); this problem is not well studied in North America.
Annual and Lifetime Reproductive Success
Long-term productivity (number of fledglings per territorial pair) is ≤ 1 per year. Average annual reproductive success has been reported as 0.78 in Montana and Wyoming (500), 0.73 in the Bighorn Basin of Wyoming (but ranged from 0.38 to 1.32 in any given year; 375), 0.79 in southwestern Idaho (474), 0.82 in Utah (204), 1.08 in Oregon (547), 0.63 in interior Alaska (n = 23 years; 475), from 0.46 to 0.90 in California (6), 0.49 in Quebec (476), 0.80 in Scotland (548), and 1.24 in Iran (20). Despite the abundance of reports on this metric, they should be interpreted with caution, because the metric was not calculated in the same manner in each reports.
There are few reports of true lifetime reproductive success for the Golden Eagle. The data that do exist are anecdotal. In one case, a marked male began occupying a nesting territory in Snake River Canyon, Idaho at 4 years of age. That bird continued to occupy the same territory for 14 consecutive years. It bred successfully in 10 years and over that period produced 15 fledglings (USGS, unpublished data).
Life Span and Survivorship
The life span and survivorship of the Golden Eagle varies with age, geographic area, and even the approach used to measure survivorship. Recovery data are generally biased with regard to causes of death, but they are likely less biased with regard to life span. Telemetry data are less biased with regard to cause of death, but telemetry devices, especially earlier heavier systems, may influence life span of tracked birds.
The mortality rate during the post-fledging period (n = 48) in Denali National Park and Preserve, Alaska was estimated with telemetry at 2%, and first-year survival after independence was estimated at 18–46% (213). Estimated survival rates, based on conventional telemetry, of > 250 individuals near a wind turbine facility in west-central California, was 84% for juvenile eagles, 80% for 1- to 3-yr-olds and non-territorial adults, and 91% for territory holders (421). Analysis of banding data suggests that 50% of eagles in the Rocky Mountains live 3 years, 25% live 6 years, 5% live 13 years, and 1% live 20 years (549).
A recent analysis estimated annual survivorship using a dead-recovery model with Seber parameterization with data from 10,627 banded Golden Eagles in North America, of which 565 were recovered dead (422). Their best supported model estimated annual survival of 70% for juvenile birds, 77% for second-year birds, 84% for third-year birds, and 87% for birds > 3 years of age.
Banding data from the U.S. Geological Survey Bird Banding Laboratory indicate that at least 3 Golden Eagles have lived > 30 years in the wild. Two of these died at 30 years 9 months (1 in Wyoming, the other in Colorado) and a third died at 31 years 7 months. In Europe, there are reports of a life span of 46 years in captivity (390) and 32 years in the wild (550). Average life expectancy of adults in wild was estimated at 40 years in western Scotland and 12 years in Germany (2). Caution should be used when interpreting these metrics, as they are likely calculated in different manners, and an average life expectancy of 40 years seems extreme.
Disease and Body Parasites
Many ectoparasites are known to infect adults and nestlings.
Adult eagles can be infected by feather lice (Phthiraptera). These parasites appear primarily on the head and neck (551). Another parasite, Trombidiform mite larvae (Harpyrhynchus spp.), also can cause progressive feather loss on the head and neck (552).
Nestlings and eagle nests host at least two species of ticks (Ornithodoros concanensis and Haemaphysalis leporispalustris; 345, 553), three species of cimicids (Mexican chicken bug [Haematosiphon inodorus]; 554, 555, 535); human bed bug [Cimex lectularius]; cliff swallow bug [Oeciacus vicarius]; 345), a biting midge (Leptoconops herteszi; 345), and bird blowflies Protocalliphora spp. (Diptera: Calliphoridae; 556, 557).
External appearances suggest that small numbers of ticks do not seem to cause significant problems for eagles (MNK). As many as 48 ticks have been reported on a single eaglet, primarily around eyes and ears (345). The ears and nostrils of nestlings sometimes are infested by larvae of the bird blowfly. These insects live in nest material and periodically suck blood of nestlings (556, 557). When infestations are present, eagles have black crusts in and around ear openings. These infestations usually subside before eagles fledge and rarely result in eagle fatality (199).
Several species of non-parasitic arthropods occur in nests (345). Most have no effect on eagles and many, such as dermestid beetle larvae (Dermestes spp.) are widespread. However, Ellis (78) reported retarded growth and weight loss of nestlings from a nest in Montana, where dermestid beetle larvae consumed prey items in the nest.
Two emerging diseases have influenced local populations in North America. First, knemidocoptic mange, caused by a previously undescribed mite closely related to Knemidocoptes derooi, has injured and killed several eagles in California (558). Second, infestations of Mexican Chicken bug have caused failure of nesting attempts in Idaho, Arizona, California, and other western states (535; D. Driscoll, personal communication). At times, infestations are so severe that aggravated nestlings jump to their death to escape the parasites. Increased infestation of Mexican chicken bug reduces nestling mass and hematocrit and increases the probability that nestlings either fledge early or die in the nest (535). Heavily parasitized nestlings show signs of greater stress than non-parasitized nestlings, as reflected in higher corticosterone levels. In this study, use of aromatic nest material was positively associated with nestling hematocrit, suggesting that addition of this material reduced the effects of ectoparasitism on nestlings (535).
Experiences bacterial infections including avian cholera (Pasteurella multocida; 559), tuberculosis (Mycobacterium avium; 560, 561), and erysipelas (Erysipelothrix insidiosa; 562). Cholera affects eagles that ingest waterfowl that have died from the same infection (559). Bacterial infections can be serious for individuals, but their prevalence and demographic significance is unknown.
At least one viral disease, avian pox (Avipoxvirus spp.; 563), and one fungal disease, aspergillosis (Aspergillus spp.) also affect Golden Eagle. Pox is apparently rare, but exposure to Aspergillus spores more common. At the University of Minnesota, 13% of 30 Golden Eagles presented for treatment had symptoms of aspergillosis (564). Most of those eagles with aspergillosis had some other debilitating injury or illness (P. Redig, personal communication). Avian influenza affects many other bird species and has been reported for other birds of prey (565).
Infectious protozoans recorded include hematazoa (Leucocytozoon; 566), intestinal coccidia (Isospora buteonis; 567), and flagellates (Trichomonas gallinae). Leucocytozoon infections vary regionally (568). Cysts from a benign protozoan (Sarcocystis spp.) occur frequently on necropsy specimens (P. Redig, personal communication). Trichomonads cause the most well-known and widespread protozoan infections. Avian trichomonosis, or “frounce,” caused by the protozoan Trichomonas gallinae, is usually fatal to nestlings (569). Individuals typically become infected with Trichomonas after feeding on infected pigeons and doves. Symptoms of the disease include yellow, caseous lesions in the oral cavity, that can block the esophagus and cause starvation (570).
Between 1971 and 1981, the annual proportion of nestlings in southwestern Idaho with visible trichomonosis symptoms ranged from 0 to 42%, with the lowest incidence occurring in years when jackrabbits were abundant (USGS, unpublished data). In 2015, T. gallinae infection occurred in 13% (12/96) of eagle nestlings across 10 western states excluding Idaho, and in 41% (13/32) of nestlings in the southwestern Idaho population (569). The probability of T. gallinae infection is positively correlated with the proportion of Rock Pigeon in the diet of the nestlings. Nestlings with diets that consisted of at least 10% Rock Pigeon had a very high probability of T. gallinae infection (569). This work suggests that eagles may be particularly vulnerable to T. gallinae infection in degraded habitats, where changes in resources may cause raptors to switch from foraging on native prey to non-native avian species such as Rock Pigeon (569). Similar observations were made on Bonelli’s Eagle in northeastern Spain and southern Portugal—when this species increased consumption of Rock Pigeon as preferred prey populations declined, T. gallinae infection became a major cause of nestling mortality (571, 572).
Capillariasis, a disease caused by nematode worms, has been documented in Scotland (2) but not in North America. However, nematodes and strigeid trematodes have been found in a dead eagle from Washington (567).
Diseases caused by contaminants are important to the demography of the Golden Eagle. DDT, lead, anticoagulant rodenticides (both first and second generation), as well as a suite of other toxins all affect components of eagle demography. For details, see Conservation and Management: Effects of Human Activity.
Causes of Mortality
Causes of death of eagles have been studied two ways through analysis of necropsy data and through telemetry studies. Both approaches provide information, but interpretation of frequency of causes of death determined via necropsy is biased towards anthropogenic causes, because eagle carcasses are more likely to be found if they die in places that humans frequent (e.g., along roadways). Interpretation of frequency of causes of death determined via telemetry study is time consuming and expensive but thought to have fewer biases because telemetered birds can be recovered almost wherever they die. Finally, although not covered in detail here, there are a number of factors that do not directly cause death of eagles but that may influence survivorship. These factors include climate change, and habitat alteration and destructions, as well as the many processes that drive these (e.g., energy and agricultural development, urban sprawl).
Of 1,427 Golden Eagles found dead and turned in to the National Wildlife Health Center (NWHC) in Madison, Wisconsin from 1975–2013, the primary causes of death (> 75%) were human related (573). The two most common sources of anthropogenic mortality were accidental trauma (collisions with vehicles, power lines, etc.; 27% of all deaths) and electrocution (27%). Other important causes of death were shooting (14%), poisoning (8%), and trapping (3%). Non-anthropogenic causes of death in this study included starvation (6%), disease (3%), and drowning (0.2%). Other processes and unknown causes of death made up the remaining 13% of fatalities.
The U.S. Fish and Wildlife Service (USFWS) evaluated causes of death of 386 Golden Eagles that were tracked via telemetry from 1997–2013 (422). Cause of death was determined for 97 of the 137 recovered eagle carcasses. The Service then modeled these different causes of mortality in a Bayesian framework to estimate age-specific rates of mortality for each of the different causes of death. Because many tagged birds would not have been found if they had not been remotely tracked, the relative frequency of mortality causes differs from that resulting from necropsy-based studies.
An estimated 44% of tracked eagles in the USFWS (422) study died from natural causes. The importance of anthropogenic causes of death increased with age of instrumented eagles: from 34% of deaths for first year birds, to 57% for birds aged 2–3, and to 63% for birds > 3 years old. The most frequent cause of death was non-anthropogenic starvation and disease (together 22% of all fatalities). Starvation was more common for young birds, and an estimated 58% of first-year eagles tracked died from starvation. As birds age, starvation becomes a less common cause of death (8% of deaths of birds > 3 years old), but fighting and injury become relatively more important (26% of deaths of birds > 3 years old). Other non-anthropogenic causes of death included interspecific conflict (11%), other injury (7%), drowning (2%), and predation (1%). These patterns are consistent with the expectation that older birds are skilled at foraging, but that they expose themselves to risk to obtain and maintain a nesting territory (255, 209).
Poisoning, lead, and rodenticides made up another 20% of fatalities. Other anthropogenic causes of death included shooting (15%), collision (9%), electrocution (8%), and trapping (4%). A follow-up analysis with additional data that focused exclusively on the western United States was similar, except it suggested that poisoning was relatively less important and shooting relatively more important than was apparent in the original study (B. Millsap, personal communication).
In a similar study in Scotland, fates of 131 telemetered eagles were studied between 2004 and 2016 (574). Of these, 10 died of natural causes, and 5 were known to be killed by humans. The tags of another 41 eagles stopped transmitting but diagnostic information suggested they had not malfunctioned. The authors interpret this “stopped but not malfunctioned” circumstance as evidence of likely human-caused mortality. This argument is further bolstered because transmitters that stopped without malfunctioning were generally clustered into one of eight areas (i.e., they were not distributed randomly across the landscape). Furthermore, several of these clusters were associated with moor management for red grouse, a popular game species. Fatalities were not associated with wind energy facilities. Human caused mortality in Scotland is predominantly poisoning, with other birds being shot or caught in illegal traps.
Starvation and disease may be especially relevant to migratory populations in which young birds must leave their natal territory much earlier than in non-migratory populations (see Movements and Migration: Dispersal and Site Fidelity: Natal Philopatry and Dispersal). For example, starvation and dehydration were the cause of death for 9 of 10 first year eagles from Denali National Park and Preserve, Alaska tracked for 11 months (213); these data were also included in the USFWS (422) study above.
Some deaths result from injuries from porcupine quills. Presumably these are incurred when eagles attempt to kill porcupines (575, 372). Exposure to porcupine quills may be most common for young and inexperienced birds (372). However, several free-ranging eagles captured in east-central Idaho had a porcupine quill or quills in their feet but appeared otherwise healthy (EHC; T. Craig, personal communication).
Finally, some putatively non-anthropogenic causes of death may be influenced by human activity. For example, raptors may be more likely to drown in a stock tank than in a natural water body (576, 577), and eagles may be similarly at risk. Likewise, the cause of an apparently natural injury can be difficult to diagnose and may stem from natural actions (fighting, accidents while flying or foraging) or be related to human activity (collision with a power line or vehicle; 213).
For additional details on causes of death of eagles, see Conservation and Management: Effects of Human Activity.
Nestling eagles are faced with a different set of stressors than are flighted eagles. Range-wide, nestling Golden Eagles are susceptible to thermal stress during first six weeks after hatching (478; see Breeding: Nest Site). As a consequence, heat stress appears to be an important source of nestling mortality. As a consequence, heat stress can drive estimates of nesting success and mean brood size at fledging (473, 474, 479). Other causes of death of nestlings include the presence of ectoparasites, (See Demography and Populations: Disease and Body Parasites), disturbance and abandonment by parents, falling from the nest, extreme precipitation events (rain or hail) and, rarely, predation (CLM). See Conservation and Management: Effects of Human Activity for anthropogenic causes of nest failure.
Population Spatial Metrics
Golden Eagle pairs may nest in close proximity to other nesting Golden Eagle pairs, or they may nest long distances from them. In 12 different regions of Scotland, average nearest-neighbor distance among nests ranged from 4.2–10.8 km, although some nests were as close as 1 km apart (2). In Quebec, distances between the closest nests of neighboring territories averaged 13.3 km, but ranged from 5.9–32.8 km (476). In a study of 25 nests in the Brooks Range, Alaska, nearest-neighbor distances were 6.4 ± 4.0 km (EHC; J. Herriges, T. Craig unpublished data). In a small population in the forest steppe of northern Kazakhstan, 4 nests were > 10 km apart from each other, but in south Kazakhstan foothills with high prey densities, nests were often 2–3 km apart (TEK).
The area within the home range that an animal defends is considered a territory, which is typically similar to or smaller than the home range in size. Generally it is assumed that, during the nesting season, territorial Golden Eagles defend an area close in size to that of their home range. During the non-breeding season eagles may range farther and are unlikely to defend the large area they cover.
Home Range Size
Home ranges are areas where animals confine their movements seasonally (578), and is typically defined as the area where an animal spends 95% of its time during a specific time period. Within a home range, animals concentrate their movements in a core area, which is typically defined by the area where an animal spends 50% of its time.
Nesting season home ranges of territorial adults are large, but tied to a specific nest site. Home ranges of non-territorial birds, whether migratory or non-migratory, are much larger. Recent telemetry studies suggest that non-territorial eagles may show some fidelity to areas they use in summer (CLM).
Home range size has been linked to habitat quality, which is influenced by availability and accessibility of prey, nest sites, roosting sites, and updrafts that support flight (176, 407, 579, 165, 580, 144). Additionally, home range size varies depending on time of year, migratory status, breeding status, and age (Table 5). Sex related differences in home range size have not been documented for adults but may be a factor influencing range size of pre-adults (306, 103, 144).
Resident pairs may maintain a home range year-round, with shifts in intensity of use from nesting season to winter (306, 176). Some pairs in Idaho and Wyoming appear to occupy smaller home ranges during the non-breeding season than during the nesting season. For example, a pair in southeast Wyoming used a 14 km2 area in winter, and a 24 km2 in the nesting season (581), and 3 pairs in southwestern Idaho used a winter range of 9 km2 ± 7 SD (range 3–17), and 32 km2 during the nesting season (306). However, these birds were tracked with older technologies and newer tracking systems reveal more information on winter movements. For example, range sizes of 8 putatively resident pairs in southwestern Idaho were about 10 times larger during the non-breeding season than during the nesting season (176). These larger ranges resulted from periodic excursions that occurred when eagles may have been searching for other breeding and foraging opportunities. One eagle from this area and tracked with satellite telemetry made repeated non-breeding season trips to Montana and Wyoming (see Distribution: Nonbreeding Range). Similarly, eagles that nested in the Mojave Desert had larger home ranges in the non-breeding season because they engaged in substantial non-breeding season excursions to higher altitude areas (165).
Migratory adult eagles from Alaska and eastern Canada appear to occupy larger nesting season home ranges than do resident eagles in the conterminous western United States (Table 5). Regardless of migratory status and time of year, pre-adults tend to occupy larger ranges than do adults and range size appears to increase with age until a smaller nesting territory is established (Table 5; 103, 144). Likewise, seasonal variation of home range of migratory eagles may be different from those of non-migratory birds. Adult migratory eagles from eastern Canada were also found to occupy smaller home ranges during the non-breeding season (144).
Resident eagles in the Mojave Desert in California occupy the smallest monthly ranges during successful nesting years (165). Irrespective of nesting success, home ranges of eagles in the Mojave Desert and the nearby Tehachapi Mountains were smallest from November to January (165, 103).
The size of home ranges of younger, non-territorial, and migratory eagles is dramatically larger than that of territorial birds. Eagles tagged as nestlings in Denali National Park and Preserve wander extensively during their second summers, and often spend time north of the Brooks Range, Alaska, or in the Yukon Territory, Canada (220, CLM). Similarly, in eastern Canada, home ranges of subadult birds were larger than those of either juveniles or adult eagles (144). Some authors debate whether these movements are best described by the term home range; for simplicity we include them here, but we recognize that some may prefer to see this behavior described as wandering.
Ranges of neighboring pairs in southwestern Idaho overlap only slightly in the nesting season (mean 4% ± 2 SD; n = 10), but more during non-breeding season (mean 22% ± 9 SE; 176). Distance traveled from the nest does not differ among years or between sexes, but mean distance traveled during the nesting season (1,047 m ± 367 SE) is significantly less than during the non-breeding season (3,036 m ± 241 SE; 176). Estimates of movements provided from this study should be interpreted with the knowledge that these studies were conducted with conventional radio-telemetry, which can miss some types of movements. Nesting season range size tends to increase with total number of young fledged (176).
Among migratory individuals, some adults may spend both the breeding and non-breeding season together. A pair of Golden Eagles from Sweden and one from Manitoba followed separate migratory routes from their breeding ranges to shared non-breeding ranges in Finland and Kentucky, respectively (445; TAM, M. Lanzone, unpublished data).
In the Mojave Desert, breeding adults shift from lower elevation breeding home ranges, to cooler higher elevation non-breeding ranges during the hottest summer months (165). Some eagles breeding at higher elevations in Wyoming shift to lower elevation wintering areas (TAM, TEK). Breeding eagles also may take trips from the breeding area, occasionally traveling long distances and staying away for a few days and sometimes weeks (see also Distribution: Nonbreeding Range).
BirdLife International (197) estimates the global population of Golden Eagle at 100,000–200,000 individuals. The conservation status of the species is Least Concern, because of its large range, stable population trend, and large population size. Road transects, such as the Breeding Bird Survey in North America, tend to poorly estimate population size for the Golden Eagle (582).
Braun et al. (583) estimated up to 100,000 individuals in North America during the 1970s. Olendorff et al. (584) estimated 63,242 wintering individuals in 16 western United States. Early estimates of the number of breeding pairs by state have ranged from 3,381 in Wyoming (206), 1,200 in Nevada (585), to 500 in California (586). In 1977–1986, there were 804 known nesting territories in Wyoming, 506 in Oregon, 500 in Colorado, 430 in Nevada, 190 in Washington, 156 in in Idaho, and 50 in Montana (193). Current numbers of known nests are much larger and form the basis of predictive maps of nesting habitat developed by U.S. Fish and Wildlife Service.
Estimates of population size are now available for several parts of the western United States (587, 588, 589, 590, 422) and for Alaska, and eastern North America (591, 422). In 2016, the population for the United States, including Alaska and the eastern states, was estimated at 40,000 (422). An estimate for the four Bird Conservation Regions of the western United States (most of Golden Eagle range in the West, excluding California and Alaska) was ~27,400 individuals (587). This estimate was revised down as more data were collected and trends became apparent (589).
Estimates of population size in eastern North America are approximately 5,000 (591). These estimates include eagles that summer in Quebec, migrate through Pennsylvania, and winter in the eastern U.S., but they do not include eagles that summer in other parts of eastern Canada (especially Ontario). The population in Alaska was coarsely estimated at 2,400 (422). Migration count data suggests that the population size in Alaska may be substantially larger (281, T. Booms, personal communication).
Early estimates of population size suggested between 2,000 and 10,000 breeding pairs in Canada (592). More modern estimates of the size of the Canadian population of Golden Eagles are between 5,000 and 50,000 adults (593). Questions similar to those asked about the population size in Alaska (281) also could be raised about nesting populations in western Canada, especially given the USFWS (422) estimates for other parts of North America.
There are few data on population size in Mexico.
Population estimates here are from Orta et al. (594). In the 1980s, the total European population was estimated at ~4,500 – 5,000 breeding pairs, with the largest populations in Spain (~1200 pairs) and Scotland (~425 pairs). There are also >100 pairs each in Norway, Sweden, Finland, France, Italy, Switzerland, Austria, Greece, former Yugoslavia, Bulgaria, Turkey and European Russia. Smaller populations exist in Israel and Portugal.
The Japanese population was estimated at < 500 birds in the 1980s.
The Moroccan population was estimated at > 100 pairs in the 1980s, but has apparently declined since then (594). Similarly, the geographically distinct population in Bale Mountains, Ethiopia, is thought to be critically small and possibly headed for extirpation (595).
Nesting Golden Eagles in Denali National Park and Preserve, Alaska, occur at a density of approximately 28 km2/pair (343) and those in California at 18.5 km2/pair (437). In other regions, total area/pair ranges from 34 to 89 km2/pair (mean = 60) in Wyoming (206), 100–119 km2/pair in Utah (472, 305), 66 km2/pair in southwestern Idaho (199), 65–192 km2/pair in Montana (358), and 252 km2/pair in Nevada (596). Area per pair in Hudson Bay is reported to be much lower than in the western United States, at 961 km2/pair (211). However, more recent work suggests higher densities in the vast areas of Quebec that remain unsurveyed (143).
In places where territorial and non-territorial eagles coexist, relative abundance is more difficult to assess. During year-round surveys from 1970–1972, observers encountered 10 Golden Eagles per 1,000 km driven in Wyoming, 5 per 1,000 km in Utah, 3 per 1,000 km in Colorado, 2 per 1,000 km in New Mexico, 1 per 1,000 km in Arizona, 0.3 per 1,000 km in Texas, and < 0.1 per 1,000 km in Oklahoma (597). In two years of winter surveys in the Mojave Desert, observers counted 9 eagles along 1,230 km of road transect (239). During the same time period and in the same area, only ~3 were observed in a large number of survey transects conducted from fixed-wing aircraft (R. Neilson, personal communication). Mean winter densities along aerial transects conducted from 1973 to 1979 in 6 western states averaged 5 birds/100 km2 (598). Wyoming and northwestern Colorado had the greatest densities, with up to 18 eagles /100 km2, followed by Utah, Montana, Idaho, and New Mexico (598). Aerial transects conducted in the western United States after the nesting season but before arrival of northern migrants reported encounter rates from 0.76–0.97 eagles per 100 km2 (589). These translate into densities of 0.009–0.028 eagles/km2.
Winter densities in Texas and New Mexico were estimated from aerial transects conducted during 1963–1968. In general, density was greater in parts of New Mexico (0.2–3.5 individuals/100 km2) than in the Trans-Pecos region of Texas (0.16–1.4/100 km2) (599). The New Mexico counts were more variable over the entire course of winter, likely reflecting the arrival and departure of migrants.
North America (Continent-wide)
Long-term surveys show declines in nesting populations in the western United States (588), but not in Alaska or western Canada (582, 422). The more recent of these analyses suggest that eagle populations are held below carrying capacity by anthropogenically related mortality rates.
Breeding Bird Surveys (BBS) and Christmas Bird Counts (CBC) have limited value for detecting trends because of the low number of routes in Golden Eagle habitat and the small number of individuals counted. BBS data show no trend for nesting Golden Eagles, either on a regional or continental scale (600). CBC data suggest Golden Eagles increased significantly at 2.8% per year throughout United States and Canada from 1955 to 1999 (J. Sauer and W. Link, unpublished data).
Western North America
In general, recent data point to stable or declining trends in western North America. Recent Bayesian modeling using combinations of data from aerial transects and band recovery data suggested that eagle populations are either stable or slightly declining (422). Data from migration count sites, as summarized and analyzed by the Raptor Population Index (RPI) project, show recent downward trends in numbers counted at many sites (601; but see 602). The RPI authors point out that negative trends may reflect changes in migration behavior, rather than changes in population size.
Trends at sites in the conterminous United States where long-term nesting studies have been conducted show similar patterns. The number of occupied territories in the Morley Nelson Snake River Birds of Prey National Conservation Area (NCA) declined 34% (from 35 to 23 territories) between 1973 and 2016 (603). The number of occupied territories in a “comparison area” adjacent to and upstream from the NCA declined by 44% over a similar period (603). These declines have been associated with loss of shrubs and jackrabbit habitat due to widespread fires (604). Similarly, nesting populations in San Diego County, California, decreased from an estimated 85 pairs in 1900 to 40 occupied territories in 1999 due to extensive residential development (D. Bittner and J. Oakley, unpublished data). Recent data from the same area suggest that more territories have been lost since 1999 (P. Bloom, unpublished data). The number of nesting pairs in a Colorado study area declined from 10 pairs in 1972 to 7 pairs in 1990 (605).
Farther north, data on trends of nesting populations are sparser, but they suggest more stability. In Denali National Park and Preserve, Alaska, reproductive output declined but the number of occupied nesting territories did not change over 29 years from 1988 to 2016 (475, 544). Nesting populations and productivity in Canada may be stable (592). CBC data suggest there has been little recent change in wintering populations in southern Canada (593).
Eastern North America
Golden Eagle numbers in eastern North America have declined substantially over the past 100 years, although the trend in the past 20 years appears to be increasing again (5). Migration counts in the eastern United States and eastern Canada suggest a decline in Golden Eagle passage rates from the 1930s to the early 1970s, with stable or increasing trends from the early 1970s to about 2000 (606, 607, 608, 609). Anecdotal evidence suggests that recent increasing trends in the East may be due to reduction of DDT and improved winter survivorship associated with increased food supplies, especially deer and possibly Wild Turkey populations (5). Golden Eagle formerly nested in the eastern United States (5). The number of nesting pairs in the northeastern United States declined from about 8 pairs in 1951 to 2 pairs in the mid-1990s, to zero in the early 2000s (194, 195, 5; C. Todd, personal communication).
Golden Eagle is said to require three primary features: food, nesting sites, and updrafts (G. Hunt, personal communication), and the availability of these factors likely regulates populations. In stable environments such as northern Scotland, territorial behavior apparently limits number of nesting pairs (439, 610). However, recent evidence suggests that populations in both northern Scotland and North America may be suppressed below carrying capacity by anthropogenic fatalities (611, 422, 574). Within the western United States, these analyses suggest that if environmental conditions remain constant and food availability is not the limiting factor, that reduction in human-caused mortality could offset the Golden Eagle population declines.
Most populations include non-territorial adults often called “floaters,” individuals that cannot nest because all suitable territories are occupied. Floaters fill vacancies as they occur and thereby contribute to population stability (470). For most eagles and large raptors, floaters are a buffer against fluctuation in the nesting population. Reduction in the floater pool can indicate either a growing or declining population (612).
Populations can exhibit evidence of long-term stability consistent with the buffering effect of floater populations. Thus, despite substantial changes in reproductive output, the number of territorial pairs in southwestern Idaho and interior Alaska does not fluctuate annually with short-term annual changes in prey abundance and weather (474, 475, 544). However, both these areas are undergoing long-term habitat change that may affect the number of pairs that an area can support. The number of occupied territories in the Snake River canyon in southwestern Idaho declined significantly over the past 40 years (603; see Demography and Populations: Population Status: Trends: Western North America). Some pairs abandoned territories after wildfires destroyed jackrabbit habitat adjacent to Snake River Canyon. When this occurred, remaining pairs appear to have expanded their ranges and subsumed neighboring vacant territories (604). These pairs also have shifted their diet in response to changing food availability (see Diet and Foraging: Food Selection and Storage).
Population regulation in the non-breeding season is poorly studied but likely driven by the availability of food. Winter densities in southern Idaho correlate with abundance of black-tailed jackrabbit (613, 614). Food availability also probably determines the decision to migrate (see Movement and Migration: Control and Physiology of Migration).