Species names in all available languages
|English (United States)||Bald Eagle|
|French||Pygargue à tête blanche|
|French (French Guiana)||Pygargue à tête blanche|
|Lithuanian||Baltagalvis jūrinis erelis|
|Romanian||Codalb cu cap alb|
|Serbian||Beloglavi belorepan (beloglavi orao)|
|Spanish (Cuba)||Aguila calva|
|Spanish (Mexico)||Águila Cabeza Blanca|
|Spanish (Puerto Rico)||Águila Calva|
|Spanish (Spain)||Pigargo americano|
|Turkish||Ak Başlı Kartal|
David A. Buehler revised the text, with contributions by Peter Pyle on the "Plumages, Molts, and Structure" page, Guy M. Kirwan on the "Systematics" page, and Andrew J. Spencer on the "Sounds and Vocal Behaviors" page. Steven G. Mlodinow edited and copy edited the account. Claire Walter also copy edited the account. Rachel E. Post and Qwahn Kent managed the references. August Davidson-Onsgard and Arnau Bonan Barfull curated the media. Ricardo Cruz updated the distribution map.
Haliaeetus leucocephalus (Linnaeus, 1766)
- leucocephala / leucocephalos / leucocephalus
The Key to Scientific Names
Bald Eagle Haliaeetus leucocephalus Scientific name definitions
Version: 2.0 — Published October 7, 2022
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Demography and Populations
Bald Eagle fecundity and survival, and hence its overall numbers, have undergone large changes as populations were adversely affected by environmental contaminants, pesticides, and human persecution during much of the 1900s, and then rebounded as these limiting factors were addressed through environmental protections and public education. Population restoration efforts have been successful to the extent that the species now breeds throughout its former range in Canada and the United States. Recent genetic analysis has shown that surprising levels of genetic structure and diversity are still intact, in spite of severe population bottlenecks and potential homogenization from translocations, with 4–5 distinct population segments (Alaska, Arizona, Midwest, and eastern [1–2 segments]; 308). The most recent population estimate for the contiguous United States based on integrated population models that included aerial surveys of high-density strata, eBird data, and survival data was 316,700 individuals, with 71,467 occupied nests (109), a > 400% increase from an estimate in 2009 (309).
Measures of Breeding Activity
Age at First Breeding; Intervals Between Breeding
Capable of breeding in its fifth year (4+ years of age), after Definitive Plumage is attained. In dense populations with high competition for nest sites, eagles may not begin breeding until 6 or years of age (147). In populations well below carrying capacity, with limited potential for mates, breeding may be attempted at earlier ages (including when in immature plumage), though success is generally poor. There are numerous reports of younger birds breeding, including a successful attempt by a pair of immatures, in Georgia (310), a successful attempt by a 3-yr-old female and 4-yr-old male in Kansas (311), a successful attempt by 3-yr-old male with an adult female in Tennessee (312), and a successful attempt by a female in “eye-stripe” plumage (probable 3-yr-old) with an adult male in Alaska (198). Two immatures in the Greater Yellowstone Ecosystem attempted to breed (built nest and in one case laid eggs) but were unsuccessful (157). Breeds once per year, though replacement clutches are possible if a clutch is lost during laying or early in incubation (274).
Clutch Size and Number of Clutches per Season
The typical clutch size is 1 to 3 eggs. Data summarized across 16 studies showed 17% of nests with 1 egg, 79% with 2 eggs, and 4% with 3 eggs, with a mean clutch size of 1.87 eggs (18). Bent (8) reported 4-egg clutches, but suggested they were product of two females laying in same nest. In 1986 on Chesapeake Bay, a 4-egg clutch was successful with no sign of an extra female (DAB). One 4-egg clutch in Florida may have involved an extra female (S. Nesbitt, personal communication). Capable of laying 7 eggs in captivity (233).
Annual and Lifetime Reproductive Success
There has been wide variability in reproductive success across populations over time as individuals experienced reproductive impairment from DDT and attendant eggshell thinning after 1947 in many populations followed by recovery from those effects after the ban of DDT in 1972 (Appendix 2). Nest success is defined as the percentage of occupied nests in a population in which at least 1 young fledged, and productivity is defined as the number of young fledged per occupied nest. In Chesapeake Bay and Florida, pre-DDT nest success was very high at 79% and 70%, respectively, as was productivity at 1.6 and 1.2 young produced per occupied territory (313, 171). On Chesapeake Bay, nest success declined to only 14% by 1962, with an average of only 0.2 young produced per occupied nest (314). In Florida, nest success declined to 14% by 1952, with only 3 successful nests found in 1958 compared to about 100 successful nests one decade earlier (315).
Reproduction data across six regions from Alaska to Florida showed regional variability in the affect of DDT; nest success ranged from only 10% in areas around the Great Lakes to nearly 70% in Alaska and inland in Wisconsin; productivity ranged from 0.14 young per active nest along the Great Lakes to 1.00 young per active nest in Alaska and inland Wisconsin (316). Nest success and productivity in most populations increased during the late 1970s and the 1980s (see Appendix 2). In Florida during the 1980s, nest success and productivity approached pre-DDT levels (Appendix 2, 317).
Nest success and productivity during the 1990s and 2000s continued to improve in regions where DDT affects were previously significant (e.g., Maine, Great Lakes). All regional populations now have a nest success > 50% and productivity > 0.7, which are required to produce stable populations (316).
The causes of nest failure are not well documented and likely vary across the species' range, dependent on food availability, human disturbance, environmental contaminant loads, and weather. In Oregon from 1980 to 1987, pesticides, particularly DDE, accounted for 28 (31%) of 89 nest failures; additionally, 20 nests (22%) failed because eggs were not laid for unknown reasons, interference interactions with neighboring pairs probably accounted for 10 failures (11%), infertile eggs from unknown causes accounted for seven failures (8%); human disturbance accounted for two failures (2%), and mate replacement accounted for 1 failure (318). Twenty-one (24%) of 89 nests failed for unknown reasons.
Lifetime reproductive success has been documented in northern California where 19 banded individuals over a 20-year period produced from 0 to 36 young and occupied the same territory for 1 to 16 years (319). Stemming from a base population of 48 fledglings, two adults produced 33% and five adults produced > 50% of the 123 young produced during the 20-year monitoring period (319). One marked female in Saskatchewan began nesting at age 6, and then nested for 13 consecutive years (until the end of the study); she was successful in 11 years, including 10 straight, and produced a total of 23 fledged young, which equal 1.77 young per year (147). A banded male in Kansas nested for at least 29 seasons, was successful in 28 of those seasons, and produced 63 nestlings (150).
Number of Broods Normally Reared per Season
Proportion of Total Females Breeding in Population
The proportion of females that breed has not been well documented because an insufficient number of adult females have been marked in a given population to be monitored for breeding activity. Recent estimates of the number of breeding pairs in the conterminous United States and the total population estimate (109) cannot yield an estimate of the proportion breeding because the total number of adults is unknown. Data reviewed here pertain to the percentage of adults breeding, regardless of sex, and probably underestimates the percentage of females breeding. The proportion of adults that breed is highly variable between populations and depends on the relationship of population size to carrying capacity and the reproductive success in any given year. Populations that are well below carrying capacity are likely to have all adult females mated. In Alaska and coastal British Columbia, where populations are probably at or close to carrying capacity, the percentage of adults that do not breed is highly variable, probably related to nest site and food availability in a given year; nonbreeding adults composed 19% of the adult population on Amchitka Island, Alaska (198), 40% on Kodiak Island, Alaska (261), 16–86% during the 1970s in southeastern Alaska (250), and 56% in British Columbia (320). In the Greater Yellowstone Ecosystem (primarily Wyoming) from 1972 to 1974, 39% of adults were nonbreeders (157); in Saskatchewan, 27–40% of adults do not attempt to breed, presumably mostly 4-yr olds and 5-yr olds (147).
Life Span and Survivorship
Longevity records in the wild are 38 years in New York (321), 33.4 in Wisconsin (322), 33 years in Kansas (150), 32.8 years in Maine (322), 31.3 years in Michigan (322), 30.9 years in Michigan (322), and 29.75 years in Ontario and 29.6 in Michigan (322), 28.5 years in Montana (160), 28 years in southeastern Alaska (323). The longevity record in captivity is 36 years (324). Two individuals banded as adults survived to 22 years of age in northern California (319).
Bald Eagle may have a survival pattern similar to that of other raptors, with a relatively low first-year survival followed by increasing survival to adulthood. Early estimates of juvenile survival ranged from 21% (band return data, 41) to 37% (wing marker data,13) and are much lower than recent estimates. Radio-telemetry studies have shown excellent survival for most age classes across much of range. On Chesapeake Bay, 100% of 39 radio-tagged nestlings survived from 8 weeks of age to nest departure at 10 to 12 weeks of age, and 100% survived their first year (325). In Texas, 134 of 138 marked nestlings (97%) survived to nest departure (46). In Florida, 41 of 44 (93%) of eaglets radio-tagged at 8 weeks of age survived to nest departure at approximately 11 weeks; at least 63% survived their first year; mortality was documented during migration (173). In California, 10 of 13 (77%) radio-tagged juveniles survived their first year (306). In Arizona, 9 of 13 (69%) radio-tagged juveniles survived their first year, with a minimum of 6 (43%) surviving to adulthood (180). In Montana, 10 of 11 (91%) of juveniles survived their first year (123). In Yellowstone National Park, 13 of 15 (87%) radio-tagged juveniles survived their first year (146). In Maine, a minimum of 73% of juveniles survived their first year (170). In Prince William Sound, Alaska, after the March 1989 Exxon Valdez oil spill, Bald Eagles (n = 68) experienced a first-year survival of 71% (326). Hodges et al. (120) reported a much lower first-year survival rate (50%) in southwestern Alaska, but the sample size was small (n = 8). Mean first-year survival was 69% (95% CI: 62, 78) and mean after-first year survival was 91% (95% CI: 90, 92) from banding data from 1994‒2018 collected form the lower 48 United States (327).
Annual adult survival was generally high in Chesapeake Bay (80% minimum; 325), Florida (100%; 173), Prince William Sound, Alaska (88%; 326), and northern California (90%; 319). Cumulative survival to adulthood (4.5 years of age) was similar in Chesapeake Bay (55% minimum; 325), Florida (50% minimum; 173), Arizona (46% minimum; 180) and Prince William Sound, Alaska (61%; 326), but somewhat lower in Yellowstone National Park, Wyoming (< 30%; 146) and northern California (39%; 319). No difference in survival between sexes has been found (173, 326). Differences in survival rates between Florida and Chesapeake Bay juveniles suggest that the survival rates of migratory populations may be slightly lower (325, 173).
In saturated nesting populations (e.g., Chesapeake Bay and Florida), new nest sites often are located closer to human development/activity than traditional nest sites from previous decades. An in-depth assessment of rural v. suburban nests (n = 60 each) showed similar rates of nest success and fledgling pre-dispersal survival. However, juveniles from suburban nests had lesser survival rates in the first year (69-72%, n = 35) compared to 89% first-year survival in juveniles from rural nests (n = 35). Suburban juveniles were more apt to die from anthropogenic causes, such as collisions and electrocutions (279).
Disease and Body Parasites
There are few comprehensive assessments of disease in raptors because sick birds that die are rarely recovered and necropsied. The Bald Eagle is an exception because dead, sick, or injured eagles were often turned in to state or federal agencies and sent to the U.S. Geological Survey National Wildlife Health Center (NWHC) for necropsy (328). Only 2% of 1,428 Bald Eagles examined during the 20-year period (1962–1981) died directly from disease (104). A more extensive compilation of mortality from 1982–2013, reported disease as the cause of mortality in 155 of 2,980 individuals (5.2%) necropsied (328), with leading infectious diseases being aspergillosis (22.6%), avian pox, Staphylococcus infections, septicemia of unknown origin, avian cholera or pasteurellosis (5.2%), and West Nile virus (3.8%) (328). Necropsy of 1,490 carcasses recovered in Michigan from 1986–2017 documented disease as the cause of death in 6.2% of individuals, with infection by West Nile virus and botulism being the most common diseases (329). Avian vacoular myelinopathy (AVM) has been implicated in several major Bald Eagle die-offs in the southeastern United States after it was first reported from DeGray Lake, Arkansas, where deaths attributed to AVM included 29 Bald Eagles during winter 1994–1995 and 26 during winter 1996–1997 (330). AVM is caused by the bioaccumulation of toxins produced by cyanobacterium (Aetokthonos hydrillicola) growing on aquatic plants (Hydrilla verticillata) in reservoirs in the southeastern United States. Bald Eagles bioaccumulate the toxins when preying on American Coot (Fulica americana) that pick up the toxin while feeding on hydrilla (331). A widespread outbreak of highly pathogenic avian influenza (strain H5N1) in North America in 2022 resulted in Bald Eagle mortalities documented in 29 states across the entire continent, with > 200 documented mortalities during spring and summer (see USDA APHIS). The ultimate effect of this outbreak on Bald Eagle populations is unknown at this time. Numerous other diseases have been diagnosed in Bald Eagles. Reported diseases leading to death from 1975 to 1977 included peritonitis, pneumonia, enteritis, septicemia, avian cholera, aspergillosis, hepatic necrosis, and myocardial infarction (332). Additional documented diseases include infections due to a hematozoan, Plasmodium polare (333) and Streptococcus zooepidemicus (334). A herpesvirus isolated from a nestling had no apparent pathogenic effects (335).
Comprehensive data on parasites is generally lacking. Necropsies of 84 Bald Eagles collected when dead or moribund were examined for endoparasites and ectoparasites. One protozoan, two genera of trematodes, one genus of Acanthocephela, seven genera of nematodes, and three genera of Mallophaga were found. None of the parasites was implicated in the death of the individual, although several genera found have been reported to cause significant disease in other species (336). The examination of 40 dead Bald Eagles from Florida documented 20 Helminth spp. (9 trematodes, 9 nematodes, 2 acanthocephalans); infections caused no significant lesions (337). Parasites are common on nestlings. Haematosiphon inodorus (Hemiptera), was implicated in death of two eaglets in Arizona (338). Protocalliphora avium, a botfly (Diptera), occurred on all nestlings examined in Saskatchewan but did not appear to have pathological effects (339). Blood parasites (Leucocytozoon toddi) were detected in 13 or 21 nestlings in Michigan and Minnesota, although the degree of infection was light (340).
Causes of Mortality
Bald Eagles found dead were often necropsied at the U.S. Geological Survey, National Wildlife Health Center (NWHC). Extensive but nonsystematic data exist on the causes of mortality. Bias may exist because data represent only individuals turned in for necropsy, not total deaths. Humans, either directly (e.g., shooting, trapping, poisoning) or indirectly (e.g., powerlines and other structures), represent the single greatest cause of mortality. Of 1,428 individuals necropsied by NWHC from 1963–1984, causes of death were 23% trauma (primarily impact with wires and vehicles), 22% gunshot, 11% poisoning, 9% electrocution, 5% trapping, 8% emaciation, 2% disease, and 20% undetermined (104). During that period, at least 68% of deaths were human-caused, and some of the deaths attributed to disease and emaciation could have been indirectly related to human actions as well. Shooting during the period continued to be single leading cause of death, even though prohibited by law with stiff fines up to $20,000 levied under the Bald Eagle Protection Act and the Endangered Species Act. A similar review of Bald Eagles necropsied by NWHC from 1982–2013 (n = 2,980) assigned the following causes of death: 25.6% poison, 22.9% trauma, 12.5% electrocution, 10.2% gunshot, 5.9% emaciation, 5.2% disease, 2.0% trapping, and 10.2% undetermined (328). The majority of these deaths were human-caused, although mortality due to shooting and trapping declined significantly from the 1960s-early 1980s. Necropsy of 1,490 carcasses from Michigan from 1986–2017 documented humans as the leading cause of mortality, with vehicle trauma (n = 532) and lead poisoning (n = 176) being the most common mechanisms. The incidence of death due to gunshot and trapping decreased during the thirty year period, whereas vehicle trauma and lead poisoning increased (329).
Environmental contaminants are also a significant source of mortality (see Conservation and Management: Effects of Human Activity). Bald Eagle may ingest lead pellets from waterfowl carcasses, leading to lead poisoning (341). Lead poisoning was primary reason for admission of 138 (22%) of 634 individuals admitted to the Raptor Center at University of Minnesota from 1980–1995 (342). Lead poisoning was reported in 338 Bald Eagle and Golden Eagle (Aquila chrysaetos) turned in from 34 states to NWHC from 1963 to the early 1990s (343). Lead poisoning continued to be the most significant source of poisoning from 1982–2013 NWHC analysis, accounting for 63.5% of all poisoning cases (328). A recent (2010–2018) analysis of blood and tissues from live and dead Bald Eagles (n = 237 live, 343 dead) from 30 states in the U.S. documented the contemporary extent of lead exposure (344). From bone concentrations, 47% of the sample experienced chronic, long-term lead exposure, whereas from blood concentrations, 28% experienced acute, short-term exposure. Documented exposure rates were estimated to reduce population growth rates by 3.8%. The relative frequency of admissions for lead poisoning actually increased after the 1991 ruling by the U.S. Fish and Wildlife Service that required use of steel shot for waterfowl hunting, suggesting that the species is likely picking up lead from sources other than waterfowl (328). Blood lead levels were greater during the big game hunting season in Wyoming, and the voluntary use by hunters of non-lead rifle bullets (24% and 31% of hunters in 2009–2010) was linked to reduced blood lead levels in Bald Eagle (345). Lead poisoning is estimated to reduce Mercury exposure is also a health concern for the Bald Eagle because of the potential for bioaccumulation (346, 347). In Maine, the mercury levels in the blood and feathers of nestlings and adults often exceeded levels known to be harmful, though direct mortality from mercury was not noted (347). Mercury concentrations in Bald Eagle feathers collected from 23 nest sites in Idaho in 2004 and 2006 were four-fold greater than the concentrations known to be harmful (346). Other heavy metals (cadmium, chromium, selenium and lead) were also documented.
Can tolerate extreme cold, wind, and snow if food is available, and so there is little mortality directly attributable to exposure. However, extreme weather that leads to food shortages can lead to increased mortality.
See Behavior: Predation.
Population Spatial Metrics
On Kruzof Island, Alaska, there is one breeding pair per 0.8 km of shoreline, including 130 nests along a 16 km stretch, which equals one nest per 0.2 km (348). The distance between nests on Amchitka Island, Alaska, averaged 3.1 km from 1969 to 1972 (198). Historically, there has been about one nest per 1.6 km of shoreline along Chesapeake Bay (106).
Few data exist on spacing within communal roosts or at communal foraging sites. Multiple birds will perch in the same roost tree, with up to 30 individuals in a single roost tree in Utah (196), and birds will occasionally perch on the same limb but at least one wingspan apart. Spacing at communal foraging sites may depend on food abundance. During food scarcity, aggressive interactions increase, and the tolerance of close approaches by aggressors decreases (200).
Estimates of territory size (territory defined as the defended portion of home range) vary widely based on nesting density, food supply, and method of measurement. The most reliable estimates are from telemetry studies. Stalmaster (18) suggested that 1–2 km2 is the typical territory size. The average territory radius (n = 10) was 590 m in Minnesota, based on defensive reactions to the presentation of decoy eagle (239); assuming circular territories, the average territory size was 1.1 km2. The minimum territory size was 4 km2 for a radio-tagged pair in Saskatchewan (248). The greatest reported nesting density of is on Kruzof Island, Alaska; assuming that half the distance between nests was defended and that territories were circular, the average territory size was 0.5 km2, probably a minimum size for this species (348). The nesting-period home ranges in the Puget Sound of Washington, based on direct observation, averaged 4.9 km2 (n = 53 pairs), although core areas (which are similar to defended territories) averaged 1.2 km2 (218).
Home Range Size
Home range estimates vary widely depending on the breeding status of the individual, season, food availability, location, and based on the technology used to measure home ranges (VHF radio telemetry versus satellite tracking technology). The minimum home range of breeding adults in Saskatchewan was 7 km2. Range size did not differ between the male and female of a pair (147). Home ranges during breeding period on the Columbia River, Oregon averaged 21.6 km2 based on 95% contours of harmonic means and did not change seasonally (251), whereas home ranges in Klamath Lake, Oregon averaged only 6.6 km2 (349). Three satellite-tracked adults in Louisiana had an average breeding period (winter) home range of 45.9 km2. The same adults satellite-tracked during the nonbreeding period (summer) had an average home range of 128.9 km2 (47). Immature Bald Eagles, in contrast, occupy huge areas and may not consistently use an area as do breeding adults, but they may instead wander nomadically in response to changing food availability. A Kentucky adult male had a breeding season home range of 1.6 km2 (95% kernel) and similarly a nonbreeding season home range of 1.3 km2 (350). Immature Bald Eagles hatched on Chesapeake Bay used the entire Chesapeake annually and occasionally ranged north to Maine and the Maritime Provinces in summer and south of the Chesapeake in winter (165). Annual home ranges for these birds on Chesapeake Bay covered tens of thousands of square-kilometers (165). Home ranges (95% kernel) of immatures tagged and tracked via satellite from Florida (n = 44) which migrated northward in spring and summer, averaged 25,218 km2 the subsequent winter and 6,166 km2 during the following summer (351). An immature tracked via satellite wintered in Arizona, where it had a home range of > 40,000 km2, and summered in Northwest Territories across a > 55,000-km2 area (183). A Michigan immature tracked via satellite had a winter range > 21,000 km2, between Lakes Michigan and Huron, and a summer range of > 55,000 km2, north of Lake Superior (183). Winter home ranges were smaller in Colorado, averaging 310.7 km2; ranges for mated birds were less than that for unmated birds, 128.0 km2 and 545.7 km2, respectively (125). A minimum winter home range for an adult male in New Mexico apparently was only 16.0 km2, but its range was based on only 29 days of tracking (268). The mean minimum winter home range of 4 immature eagles in Arizona was 401.2 km2, within the range of Colorado values (352). Adults and immatures radio-tagged during their migration through Glacier National Park, Montana, had winter ranges from 102 to 3,925 km2 across a variety of wintering areas in the Intermountain West (123). In Missouri, the minimum winter home ranges of adults and immatures were small but changed year to year based on food availability; the mean home range = 48.2 and 18.5 km2 in 1976 and 1977, respectively (353).
The Bald Eagle has been extensively surveyed on both breeding and wintering grounds, although breeding populations in some regions (especially Canada) are remote and less well surveyed.
Recent integrated population models have estimated the population in the conterminous United States, excluding the southwestern states, to be 316,700 individuals and over 71,400 occupied nests (109). Adult populations along the northern Pacific Coast from southern British Columbia to the Alaska Peninsula have been monitored by aerial survey using a universal random plot design since 1967; in 2010, an estimated 58,000 adults occupied this region (354). Breeding populations are building along the Gulf Coast from Alabama to Texas, including over 400 nesting pairs in Louisiana (355) and > 150 pairs in Texas (356). In 2009, there was limited breeding (< 10 pairs) in New Mexico (357). Breeding populations persist in Mexico but are still perilously close to extirpation, with < 10 breeding pairs located in Baja California Sur on Magdalena Bay, in southern Sonora, and in Chihuahua; these tiny populations may benefit from the dispersal of birds from growing populations in the southwestern United States (e.g., Arizona, Texas).
Breeding. Breeding populations were counted by aerial and/or ground surveys annually in almost all of the United States and parts of Canada from 1970 to 2000; survey data are available for some populations for > 40 years. Throughout its range, populations have shown tremendous growth since ban of DDT in 1972. In 1963, there was an estimated 417 breeding pairs in the conterminous United States, a number that increased to > 5,000 pairs in 1997, a growth of almost 8%/yr on average, in spite of the lack of growth in many states until after 1980 (Figure 4; Figure 5, based on 358). Based on systematic aerial surveys in southeastern Alaska, the number of adults increased 66% from 7,230 in 1967 to 12,026 in 1997, about 2% growth/yr over the 30 year period, though there was little increase from 1987 to 1997 (359). Analysis of aerial survey data from 1967–2010 suggests that the northern Pacific Coast (Alaska and British Columbia) adult population has stabilized since 1990–2000 at apparent carrying capacity (354). The increase in Alaska is the result of reduced mortality, not elimination of DDT, as DDT was not a problem in Alaska (316). The breeding populations in the conterminous United States have continued to grow exponentially since 1997, approximately doubling in 10 years to 9,789 pairs in 2007 (360), and increasing by approximately 300% to 31,304 (± 2,511 SE) nesting territories in 2018–2019, based on dual-frame sampling design modified to account for detectability of nest structures (109).
Migration. At Hawk Mountain in eastern Pennsylvania, data have been recorded since 1934; the historic peak count there was 116 (0.18 individuals/observer hour) in 1950, the low count was 12 (0.03/observer hour) in 1972, which increased to 105 individuals (0.12/observer hour) in 1995, and 559 individuals in fall 2020 (0.57/observer hour) – far exceeding any count since 1934 (Hawk Mountain Sanctuary, unpublished data). Counts at Hawk Ridge, Minnesota similarly documented a dramatic increase, from a low of 23 individuals in 1972 (0.08/observer hour), to 4,368 in 1994 (5.9/observer hour), and an all-time peak of 6,177 in 2019 (7.2/observer hour; Hawk Ridge, unpublished data).
Wintering. Estimates during winter are problematic because detectability varies widely among areas, survey methodologies differ (aerial surveys versus a variety of ground methods), and observer effort is also variable. A comprehensive winter count in 1982, with > 4,000 observers, recorded almost 14,000 individuals in the contiguous United States (90). Some states discontinued counts by 2000 (e.g., Michigan and Virginia) whereas other states initiated counts only after 1995 (e.g., Mississippi, Ohio, West Virginia, and Wyoming; 361). Wintering population trend estimates from 1986 to 2010 vary by state and region: northeast and northwest regions showed significant population increases, the southeast region population was stable, and the southwestern region showed a significant population decrease (361). Based on Christmas Bird Count data, wintering populations in Canada increased by 5.4% per year from 1970–2016 (Status of Birds in Canada 2019). Winter counts are generally lacking in Alaska, Canada and some of the contiguous 48 states such that generating a rangewide winter population estimate is not possible.
Although the Bald Eagle can live for a long period of time (see Demography and Populations: Lifespan and Survivorship), it has relatively low reproductive rates. As a result, factors that affect survival are more influential in population regulation than factors that affect reproduction. A 10% decrease in mortality in some populations can turn a population decline into a population increase with stable reproduction, whereas a 10% increase in productivity is likely to have a much smaller effect on population growth (362, 363, 325, 364). Emigration and immigration also contribute to population regulation in a given area.
In areas where populations are close to carrying capacity (e.g., Alaska) and human interactions are limited, populations are probably regulated by the effects of food availability, weather, and intraspecific competition on survival and nesting (365, 18). Populations in coastal British Colombia appear to be regulated by both reproduction and overwinter survival: Density-dependence during the breeding period was expressed by lower fecundity overall (and not by an apparent increase in nonbreeding adults, or floaters), while survival in late winter decreased, apparently as salmon runs were depleted, eagles switched to alternative prey, which exposed them to additional sources of mortality (366).
Productivity is regulated by the percentage of adults that breed and the relative success of those attempts. The percentage of adults breeding is determined in part by competition for breeding sites. The success of breeding is limited by food availability, weather, and a myriad of other factors. Weather effects explained 63% of variation in reproductive output in Greater Yellowstone Ecosystem, with reduced output during cold, wet springs (157). Reproduction was similarly limited by cold springs in Saskatchewan (367). Spring rains are also related to the density of successful nests in southeastern Alaska but accounted for relatively little of the variation in productivity (368), and weather did not explain variability in nest success in the interior of Alaska (369). Cold, wet weather may decrease food availability and increase energetic demands on both parents and young; food availability during incubation may be more important in population regulation, because most significant egg losses may occur prior to hatching (368, 369). Productivity may not be related to winter food supply, as winter supplemental feeding within 20 km of the feeding site did not increase productivity in Maine (166). Food abundance in southeastern Alaska during the breeding period was strongly related to the percent of nests that were active and nest success, and there was intense competition for quality nesting sites (sites near reliable food sources; 365). Vegetative habitat characteristics were, at best, weakly related to nest success.
Human activity related to land use, use of environmental contaminants, collisions with vehicles, structures and electrocutions, and disease have been reported to have variable effects on reproduction and survival across the range (see Conservation and Management: Effects of Human Activity), and thus may be important in population regulation. Human development may limit nest-site availability and access to food resources in aquatic areas, thus limiting the size of the breeding population and the productivity of individual territories (105). Human use of environmental contaminants, such as DDT (103), has historically been shown to limit reproduction and survival, and thus regulate populations.