SPECIES

Bald Eagle Haliaeetus leucocephalus Scientific name definitions

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

Breeding

Phenology

Breeding phenology varies with latitude. In southern latitudes (e.g., Florida and other Gulf Coast states) the nesting period is prolonged, ranging from late fall to early spring, while in northern United States and Canada, nesting is fairly synchronous, beginning in early spring.

Pair Formation

Timing of pair formation is not well documented. It is thought to occur on the breeding grounds, but may also occur on wintering grounds (125). Once formed, the pair bond may last year-round for pairs that don't migrate (251). A radio-tagged adult male often associated with an unmarked adult female in late winter and attempted copulation in the wintering area (268). In Ohio, a female lost her mate and attracted another mate in only four days. In contrast, a female lost her mate on two occasions in an Ohio territory during late fall and did not return with a new mate for 11 weeks one time and 14 weeks the other (151). In Arizona, a male disappeared on 13 February during the incubation phase of nesting. A replacement male appeared in the territory only one day later and delivered prey and sticks to the nest by 19 February (269).

Nest Building

Nest building generally begins one to three months prior to egg-laying. In Florida, nest-building and maintenance begins from late September to early October (171). In Ohio, it usually begins in February, although adults repair existing nests all year long (151). On Amchitka Island, Alaska, nest-building begins as early as 20 January, though adults have been observed carrying nesting material year-round and often perch by the nest in December (198). In southeastern Alaska, nest-building begins in April (270). In Saskatchewan, adults build or repair nests in September, prior to migration, and resume building or repairing nests in April, upon their return from the wintering grounds (266).

First/Only Brood Per Season

See Figure 1. One brood per breeding period, though replacement clutches may occur if eggs are taken or destroyed during incubation, especially at southern latitudes. The timing of laying varies with latitude. In Florida, the breeding period is prolonged, with incubation beginning as early as October and as late as April, though instances of April breeding may be second attempts; most incubation is initiated in December and January (171). On Chesapeake Bay, incubation is initiated from the last week in January to end of February (DAB). In Saskatchewan, egg-laying is fairly synchronous, with 90% of pairs laying within a 10-day period in mid-April (266). In the Greater Yellowstone Ecosystem (Wyoming/Montana/Idaho), clutches are laid from early March to mid-April, with later dates at greater elevations (157). In Arizona, eggs are typically laid from late January to mid-February (271). Nests observed in Mexico had incubating adults on them in January, and therefore laying may have occurred from late December to early January (272). In Alaska and the Yukon Territory, egg-laying extends from late April to the end of May, and peaks in the second week of May (262, 273). Note that Bent (8) reported a range of egg dates (dates that eggs were collected from nests), but because incubation is long (35 days), and eggs persist in abandoned nests, these data do not accurately document laying and incubation phenology.

In a Florida study, eggs were removed from 58 nests, and 78% of pairs laid replacement clutches, with an average interval of 29 days between removal and relaying; there was no apparent relationship between the stage of incubation and the likelihood of relaying, although clutches removed prior to 1 January were more apt to be replaced than clutches removed after 1 January (274). In captivity, eggs were removed from nests within eight days of clutch completion, and in 9 of those 11 nests, the female laid a second set, with the first egg of the second clutch appearing 18 to 23 days after first clutch was removed (275).

Nest Site

Selection

It is unclear which sex is responsible for nest-site and nest-tree selection. One female lost her mate twice and found a new mate each time, and on both occasions they returned to her territory, suggesting that females may select nest site (151). Observations of females defending territories and attempting to attract mates has also reported from Saskatchewan (266). The minimum distance from a nest to human development in some populations is less than 100 meters, but the average distance in most populations is greater than 500 meters (276, 277, 278, 112, 113, 191). Relationships between nest-site selection/nest success and human development/activity are changing as more eagle-human contact occurs without adverse effects. In saturated nesting populations (e.g., Chesapeake Bay, Florida), new nest sites often are located closer to human development/activity than traditional nest sites from previous decades (279). Areas with considerable shoreline development or human activity (e.g., Florida, Chesapeake Bay, Minnesota, parts of the Greater Yellowstone Ecosystem, coastal Oregon) have nests that are located farther from the shoreline than nest sites in less developed areas, such as Alaska (280, 276, 278, 157, 112, 113).

Site Characteristics

In suitable locations, the nest tree is generally one of the largest trees present that has accessible limbs capable of holding a nest (209, 276, 157, 112, 113, 191). A large, super-canopy nest tree provides good flight access to the nest and good visibility of the surrounding area. Nest-tree branches must be capable of holding the first sticks brought to the nest as the breeding pair begin nest construction. Ground nests are built in treeless regions (e.g., Alaska, northern Canada, coastal islands, California, Arizona) and are rare elsewhere (e.g., Minnesota [281]; southern Florida [282]). Ground nest sites are usually on prominent ridges, cliff sides, or sea stacks with good flight access but limited ground access for potential predators (198). In Arizona, 9 of 17 nests were on cliffs (283).

The tree species used for nesting varies throughout the eagle's range because of regional differences in dominant trees; see Stalmaster (18) for review. Coniferous trees, including pines (Pinus spp.), spruces (Picea spp.), firs (Abies spp.), and Douglas-fir (Pseudotsuga spp.) are used where conifers are dominant in the canopy. Deciduous trees, including oaks (Quercus spp.), hickories (Carya spp.), and cottonwoods and aspens (Populus spp.) are used where large conifers are absent (primarily eastern United States). At the southern limits of the species' range, mangroves are used in southern Florida (282) and the Gulf of California, Mexico (284), and hecho cactus (Pachycereus pectinaboriginum) has been used in Sonora, Mexico (285).

The size of the nest tree is highly variable and depends on the tree species available and how it compares with the surrounding canopy. Stalmaster (18) reviewed nest-tree data from 14 areas across the species' range, and nest trees ranged from 20–60 m in height and 50–190 cm in diameter. On Chesapeake Bay, nest trees (both conifer and deciduous) averaged 82 cm in diameter and 28 m in height (276). Nest trees in Florida were smaller on average: 28 cm diameter and 23 m tall (113). The nest tree is usually live, although adjacent snags (dead trees) are used for perching (209, 110, 157, 112, 113, 191). The nest is usually placed in uppermost quarter of the tree, just below the crown. For instance, 52 nests in the Greater Yellowstone Ecosystem were, on average, 5 m below the tree crown, which averaged 27 m in height (157). Nests are usually at the highest point where large branches join the bole of tree, with the nest typically placed against the trunk. Sometimes nests are in a fork of large branches close to the trunk and near the canopy.

The distance between the nest and water varies within and between populations. In some cases, the distance to water is not as critical as the quality of the foraging area nearby. The quality of foraging area is defined by the diversity, abundance, and vulnerability of prey (191), structure of aquatic habitat, such as the presence of shallow water (190), and the absence of human development and disturbance (286).

Nest

Construction Process

Both sexes contribute to nest building, although the female may place the sticks within nest. Sticks used as building material are collected from the ground near the nest site or are broken off of nearby trees. One pair in Ohio carried sticks from as far as 1.6 km to the nest site (151). Once initiated, nest-building may take up to 3 months to complete, although nests may be completed in as little as 4 days (225). Additional materials are regularly added to the nest throughout the year, including daily additions during the breeding period (266), so that nests used for multiple years may achieve enormous dimensions.

Structure and Composition

The nest is constructed from an array of sticks placed in an interwoven pattern. Other materials are added as fillers and may include grasses, mosses, even corn stalks (151). Nest are lined with finer woody materials, especially Spanish moss in Florida, and ultimately lined with downy feathers from the adults (171). Nests often contain sprigs of greenery (225). Ground nests in Alaska, northern Canada, and coastal California are constructed from a variety of materials available at the site, including kelp and driftwood (198).

Peculiar items have been found in active Bald Eagle nests, including clothes pins, plastic bottles, golf balls, gunny sacks, light bulbs, stones, duck decoys, a rubber rat, and items of human clothing (287). The use of these items to adorn nests is presumably to indicate that it is occupied (18).

Dimensions

The Bald Eagle nest is among the largest nests of all birds. A famous nest site in Vermillion, Ohio, used for 34 years, was wineglass shaped and measured 2.7 m in diameter and 3.6 m high; it was estimated to weigh almost 2 metric tons (151). The largest nest on record, however, was in St. Petersburg, Florida; it was 2.9 m in diameter and 6.1 m tall (171). Typical nests are 1.5–1.8 m in diameter and 0.7–1.2 m tall, and the nest shape conforms to shape of tree, ranging from cylindrical or cone-shaped to flat (18). Cliff nests are generally larger with larger sticks than those of the Golden Eagle (Aquila chrysaetos) (283).

Microclimate

The nest is often shaded at southern latitudes, where heat stress may be a concern. One Chesapeake Bay pair abandoned their nest site after caterpillars of spongy moth (Lymantria dispar) defoliated and killed the tree; the pair rebuilt in a loblolly pine (Pinus taeda) that provided shade (DAB). Nests in Saskatchewan tend to be on east and southeast shorelines of lakes (facing west and northwest), possibly because these shorelines are protected from prevailing east winds during incubation (110). There is few other data on nest orientation with respect to prevailing winds and exposure to the sun.

Maintenance or Reuse of Nests

Nests often reused year after year. A nest in Ohio was used for 34 years before the tree blew down (209). Pairs often have alternate nests in their territory and may switch nest sites in successive years, especially after nesting failures. Across its range, averages 1.5 nests per pair, with up to 5 nests reported in several territories (18). In Saskatchewan, only 10% of pairs had alternate nests (266), and pairs on Amchitka Island, Alaska, did not have any extra ground nests (198). Nests are maintained year-round at southern latitudes. In Saskatchewan, nest maintenance takes place in September, prior to migration, and again in April, just after return from the wintering grounds (266). Immatures (2–4 years old) have been observed building “false” nests that are never used (266). Half of the nests (n = 48) at Besnard Lake, Saskatchewan were extant after 6 years (252).

Nonbreeding Nests

Not reported.

Eggs

Shape

The eggs are rounded-oval to oval (8).

Size

As with body size, the egg size increases with increasing latitude and can be predicted by regression equations (length = 67.38 + 0.14 × latitude; breadth = 53.09 + 0.07 × longitude) (18). Across its entire range, eggs varied in size from 5.8–8.4 cm in length and 4.7–6.3 cm in breadth. On average, eggs were 7.0–7.6 cm in length and 5.3–5.6 cm in breadth (n ~ 300;18).

Mass

Eight eggs in Alaska averaged 130 g (262); 17 fertile eggs in Saskatchewan averaged 114.4 g ± 2.57 SE (22).

Eggshell Thickness

There are few published data on eggshell thickness prior to introduction of DDT, although eggs from that era still exist in various collections. A large sample of eggs from Florida, collected prior to 1947, averaged 0.584 mm in thickness (288). Eggs collected from 15 states from 1969 to 1979 (DDT era) ranged from a low of 0.450 mm (–26%; i.e., 26% below pre-DDT norm) and 0.473 mm (–3%) for an egg from New York and another from Delaware to 0.62 mm for an egg from Ohio (+1%) and 0.584 mm (–5%) for 7 eggs from Alaska, where DDT contamination was limited. Almost all states showed some eggshell thinning when 1969–1979 values are compared with pre-1947 values. Eggshell thickness reported by Wiemeyer et al. (289) for eggs collected from 15 states from 1980 to 1984 showed little improvement; five states had eggshell thickness > 15% thinner than pre-1947 norm, including Wyoming (–24%), New Jersey (–23%), Minnesota (–21%), Delaware (–19%), and Florida (–17%). During the 1990s, eggshell thinning was still noted at failed nests in New Jersey (290), British Columbia (291), and Lake Superior (292) and eggshell thinning (–11% below 1947 levels) was present in 1994–1995 along the Columbia River in Oregon (293). Eggshell thickness returned to pre-DDT levels by 2000 (~0.61 mm) at Great Lakes nests in Ohio and Michigan (294).

Eggshell thinning of > 15% is correlated with reproductive failure (288), and eggshell thinning has been correlated with both DDE and PCB levels in Great Lakes (294). Reproductive failure, however, is more closely linked to DDE concentrations, with eggshell thinning probably a secondary effect (295). See Demography and Populations: Measures of Breeding Activity and Conservation and Management: Effects of Human Activity for more on DDE and impairment of reproduction.

Color and Surface Texture

Dull white in color with no markings except, rarely, brown spots (18). Rough-shelled.

Clutch Size

Generally 1 to 3 eggs, with 2 most common (see Demography and Populations: Measures of Breeding Activity). Reports of 4 eggs in a clutch are very rare, with unconfirmed reports of 2 females laying in the same nest (151, 8, S. Nesbitt, unpublished data, DAB).

Egg Laying

One egg laid per day, not always on successive days, with clutch generally completed in 3 to 6 days (151, 18).

Incubation

Onset of Broodiness and Incubation in Relation to Laying

Incubation begins after the first egg is laid; thus, young hatch over a period of several days (151, 262).

Incubation Patches

Both sexes have brood patches, although the female's patch is better developed than that of the male.

Incubation Period

Generally reported to be 35 days, although some authors report limited variance (e.g., ± 1 d; 151).

Parental Behavior

The female does the majority of incubating, but the male also participates. Captive birds nesting in open-air cage in Maryland incubated eggs 98% of a 24-hour day, with the female responsible for 72% of incubation (237). Video cameras at nests in Alaska indicated that eggs are incubated during 95% of daylight hours, with females incubating 53% of the time (4.5 bouts/d at 144 min/bout), and males 42% of the day (3.1 bouts/d at 164 min/bout) (240). Based on direct observation of successful nests over a 2-year period in South Carolina, the nest was incubated >95% of daylight observations, with the female accounting for 54‒75% of the incubation effort (296).

Hardiness of Eggs Against Temperature Stress: Effect of Egg Neglect

Eggs can tolerate some cooling, but at what temperature the embryo dies is unknown.

Hatching

Hatching starts with the young pipping the egg, and they emerge without assistance from their parents. Pipping may take up to 1 day (262). Hatching, like laying, is usually asynchronous, with 1 to 4 days between hatching; 13 of 16 two-egg clutches in Saskatchewan hatched 2 days apart (297). Adults of both sexes brood the young and incubate remaining eggs until all eggs hatch (18). The differences in hatch date gives the first hatchling a significant advantage in competing for food (266).

Young Birds

Condition at Hatching

On the day of hatching, the mass of six nestlings averaged 79% that of the egg (22). At hatching, the nestlings are covered with light-gray down, which is initially wet but dries quickly; the eyes are brown, the gape and legs are pink, and the skin is pink (22). New hatchlings are capable of only limited locomotion.

Growth and Development

There are few data except from Saskatchewan (22, 38), which is summarized here. The second down begins to emerge at 9 to 11 days of age. Flight feathers emerge at two to three weeks of age, body contour feathers first emerge with the humeral tract at three to four weeks of age. There is great variability in emergence of contour feathers, such that same-aged individuals may look different. Feathers on the head and back emerge at 4 to 5 weeks of age, on the lateral ventral surface at 4 to 6 weeks of age, and feathering on tarsi is the last to emerge, at 6 to 8 weeks of age. Daily weight gain is substantial, with a maximum average of 102 g/d and 130 g/d for males and females, respectively. Maximum growth is attained at about 3 to 4 weeks of age. Male develops flight feathers and asymptotic size more rapidly than female, such that measurement of wing chord, central rectrices, and eighth primary (p8) not sexually dimorphic during nestling period, although these characters show considerable sexual dimorphism in fully developed birds (22).

In Saskatchewan, the second chick hatched in a clutch showed delayed development compared to first (n = 38; 22), and chicks that hatched two days after a sibling showed a greater delay than those that hatched one day after (38), presumably because of a differential in nutrition. Hatching asynchrony and differential growth leads to a mass differential between siblings, facilitating competition and fratricide (298). Sibling competition and mortality is greatest early in the nestling period, when the size differences are at their greatest (298). Third-hatched chicks in Saskatchewan nests received little food and usually starved. In one nest, the masses of 6-day-old, 8-day-old, and 9-day-old siblings were 80 g, 260 g, and 477 g, respectively (266).

Feather development is not complete at the time of nest departure. Bortolotti (22) used equations from Dunn (23) to calculate that thermoregulation should occur at 14.7 days of age on average, coincident with the emergence of second down. Equations developed by Bortolotti (24) are useful for aging/sexing northern nestlings.

Sex Ratios

Long-term nesting studies on Besnard Lake, Saskatchewan documented primary sex ratios in Bald Eagle nests at hatching (299). Sex ratio from all years combined in the study for chicks that survived long enough to be sexed were essentially 50:50 (53 M:50 F). Sex ratio was linked to hatching order; in 2-egg clutches, females represented 63% of first eggs hatched and 31% of second egg hatched (n = 56 eggs, 28 2-egg clutches; 299). Relative food abundance was linked to sex ratio, with a female bias (61% F) in 1-chick nest in good years but sex ratio did not differ from 50:50 in poor years (300). In 2-chick nests, sex ratio tended to be male-biased in poor years (300).

Parental Care

During the first 2 to 3 weeks of the nestling period, the female is present at the nest about 90% of the time, the male is present about 50% of time, and at least one adult is at the nest nearly 100% of the time (245, 301). Nest attendance declines sharply at 5 to 6 weeks after hatching, and adults roost away from nest, usually in adjacent trees (301, 241). At one Virginia nest, the male was present 25% and the female 38% of daylight time from hatching to fledging; at least one adult was present 54% of daylight time. Adults shade young to avoid heat stress (241) and shield young from inclement weather (209).

Brooding

An adult (usually the female) broods constantly during inclement and otherwise cool weather until the chicks are about 4 weeks of age (18, 266). In Alaska, nestlings were brooded 80% of the daylight hours during the first 5 days post-hatching (71% of brooding by females, 29% by males). Brooding fell to 3% of daylight hours after 50 days post-hatching (240). In Minnesota, during 4 days of observation, the young were brooded, on average, 85% of the daylight period during the first week after hatching, with the female brooding 65% of time and the male 35% of the time (245).

Feeding

Both sexes hunt and feed the young. Adults bring food to the nest, tear off small pieces, and deliver them to the young at an early age. The male provides most of the food in the first 2 weeks, while the female tends to the young in the nest (301, 266). After 3 to 4 weeks, the female delivers as much prey as the male (302, 301). After 3 to 4 weeks, the young are able to peck at food but not tear off food and feed themselves until 6 weeks of age (1). The adult female did the majority of feeding in captivity (275), and in a Virginia nest (301), and in 3 nests in Alaska (240) monitored by video camera. In the 3 Alaska nests, adults delivered prey 2‒3 times per day on average and young were fed 8 times per day on average during the first 2 weeks after hatching (240). The frequency of prey deliveries and feedings declined over the 8-wk nestling (240). During the late nestling phase in two nests in Ohio, female provided about twice as much food as male; adults delivered food 2–8 times/d (mean = 4) (209). Early-morning period (05:00–09:00) accounted for 36 of 105 visits to nest (209). The largest young get the majority of the food; in one 10-minute feeding bout, the largest young was fed 39 times, the second largest young 8 times, and the smallest 11 times (225). In a nest where the hatching asynchrony was 4 days, in one feeding bout when the nestlings were 4 weeks old, the oldest nestling got 76 pieces of fish while the youngest nestling got two pieces; the youngest nestling died 1 day later during rain and hail storm (151). There are few data on relationship between the clutch size and the number of young fledged; unless food is abundant, it appears that most three-egg clutches fail to produce three young because youngest chick starves (266). Prey (97% fish) delivery rates at Wisconsin nests averaged 5.2 items/nest/d and 3.0 items/nestling/d (303).

Nest Sanitation

Food uneaten by nestlings is often buried in the nest under grass and moss rather than tossed out of the nest, perhaps to avoid attracting mammalian predators (151, 266). At the end of the nesting cycle, however, numerous prey remains can be retrieved from under the nest. Nestlings usually defecate over the side of the nest, leaving a pungent spray of “whitewash” on shrubs and saplings in the understory below (266).

Carrying of Eggs or Young

Not known to occur.

Cooperative Breeding

Cooperative breeding has been rarely reported. Dawe (256) summarized 8 instances and commented that cooperative breeding could be more common than previously reported. On Amchitka Island, Alaska, 2 different nests had 3 adults in attendance in 2 separate years, and 3 adults were reported at 3 nests in one year (198). Fraser et al. (253) reported 3 adults at the same nest in Minnesota over a 3-year period; the sex of the third individual was not known. In Connecticut, an extra male delivered food to nestlings, but generally avoided interactions with the nesting pair (254). In California, an extra female assisted with incubation and food provisioning of young and was tolerated by the nesting pair; low numbers of adult males may have provided incentive to female for cooperative venture (255). On Vancouver Island, British Columbia, an extra female was reported at a nest during two successive years, but circumstantial evidence suggested that the trio may have formed 10 years earlier (256); all three adults were observed defending the nesting territory, maintaining the nest, and delivering prey to the nest.

Brood Parasitism by Other Species

Not reported.

Fledgling Stage

Departure from the Nest

For several weeks prior to the first true flight from the nest (i.e., fledging), nestlings flap their wings to cross the nest or to cross to adjacent tree limbs to practice flight; in doing so, they develop muscle strength, flight coordination, and (importantly) landing ability (209). Up to half of nest departures are unsuccessful and such young may remain on the ground for weeks before regaining flight ability; in most cases, the parents will continue to feed these grounded young. Grounded birds are more vulnerable to predators, and they may not be fed if trapped in dense vegetation (234, 245).

Nest departure can occur from 8 to 14 weeks of age. This variability in departure date relates to effects of sex and hatching order on growth and development (299). In Saskatchewan, males tended to leave the nest earlier than females, regardless of hatching order (mean = 78 d, n = 29; female mean = 82 d, n = 21) (299). Saskatchewan nestlings usually leave 1–2 days apart, although at one nest where the male hatched before the female, the male left the nest 10 days prior to the female (299). California nestlings leave, on average, at 12 weeks of age (158), Florida nestlings at about 11 weeks of age (173), and Maine nestlings at 11–13 weeks of age (170). Adults may “encourage” nest departure by flying around the nest with a prey item and vocalizing when the young are hungry. One first flight observed by Herrick (148) was about 1.6 km in length. Humans climbing to nests with eaglets that are more than 6 weeks of age can cause premature departure; young that are returned to nests continue to be fed by adults and may not fly again for 1 to 3 weeks (DAB). Fledglings may continue to use the nest as a feeding platform for several weeks as they gain flight and foraging skills.

Growth

Juveniles continue to grow and develop after fledging. Mass may actually decrease or remain stable after fledging, depending on food availability and the foraging ability of an individual. Skeletal growth and development are minimal post-fledging, but muscle mass continues to develop and flight feathers fully emerge. There are few data on when growth is complete in immatures.

Association with Parents or Other Young

Young eagles associate with other young eagles and adults for 4–10 weeks before dispersing in Montana (123), 4–11 weeks in Florida (304), 5–10 weeks in Maine (170), 2–5 weeks in California (158), 3–10 weeks in Arizona (180), and 7–8 weeks in Saskatchewan (161). Fledglings follow adults and may be fed by adults up to 6 weeks after leaving the nest (234). In Florida, most fledglings (> 80% of 292 observations) were located within 232 m of the nest during the post-fledging period (304).

Ability to Get Around, Feed, and Care for Self

The degree of independence increases regularly until dispersal. After leaving the nest, young are dependent on adults for all food, and often follow adults to foraging sites. In Minnesota, no fledglings were observed catching live prey during the first 6 weeks after nest departure (234). During this period, young spend progressively less time with their parents, increasingly hunting on their own. Fledglings develop hunting skill by trial-and-error rather than by learning from adults, at first scavenging fish carcasses along shorelines and then picking up floating dead fish (234). Fledglings in Minnesota during this period fed almost exclusively on fish (234).

Immature Stage

Fledgling Bald Eagles leave their natal area and break ties with their parents at 17 to 23 weeks of age (161, 234), and embark on a 4 to 5 year period of seemingly nomadic exploration during which they acquire a progression of plumages from Juvenile to Definitive Basic (see Plumages, Molts, and Structure: Plumages). Study of this immature phase of their life history has been difficult, because movements may cover thousands of kilometers (e.g., 174, 180) and keeping track of individuals can best be accomplished with satellite telemetry (e.g., 305). Initial dispersal in Montana was either west to the Pacific Coast or south to Intermountain winter sites (123). Initial dispersal was northward for VHF-radio tagged eaglets in Arizona (n = 11), with juveniles tracked to the Pacific Northwest (Oregon, Washington, British Columbia), the northern Rocky Mountains (Idaho, Montana, Wyoming) and Manitoba and South Dakota in the northern Great Plains (180). Maine juveniles moved to coastal areas and generally dispersed southwards (170). Florida juveniles dispersed northwards (171, 173, 174). California juveniles dispersed northward along the coast as far as Canada and southeastern Alaska (158, 306). Saskatchewan juveniles dispersed to the south (161). Texas juveniles generally dispersed northwards, although there are several band recoveries to the east along the Atlantic Coast and to the west, west of Rockies (46). Exploratory movements during the immature phase likely are driven by food availability and by innate tendencies to migrate seasonally (18). Kentucky juveniles dispersed northward to the Great Lakes region and used known eagle concentration areas below dams along major rivers as stopover sites (307). Immatures migrating northward from Florida in spring‒summer spent 1‒4 weeks at a given site (mean = 14 d) before moving on (174). A male and a female eaglet from a nest in Oklahoma and tagged with satellite transmitters moved regularly within a triangle bordered by Kansas and Nebraska (during spring and summer) and Texas (during winter) with the most time spent in Oklahoma; the male never established a breeding territory and died at 4 years of age in Oklahoma, 41 km from its natal site; the female nested at 5 years of age in Oklahoma, 100 km southwest of its natal site (305). In spite of these large-scale nomadic movements, median natal dispersal distance (from where they were hatched to where they ultimately bred) for Bald Eagles banded as nestlings and recovered as adults was only 68.2 km (n = 878), although female natal dispersal was considerably larger than male dispersal (137.6 km versus 58.7 km; 145). There appears to be little site fidelity among immatures, with movements largely opportunistic related to heritable tendencies (i.e., to migrate), local food availability, and weather. These patterns can be repeated year after year, such that the same immatures may repeatedly use the same areas in summer and winter, as well as migratory stopover sites (174).

Recommended Citation

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