SPECIES

Bald Eagle Haliaeetus leucocephalus Scientific name definitions

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

Behavior

Locomotion

Walking, Running, Hopping, Climbing, etc.

When on the ground, typically walks in an awkward, rocking gait of alternating steps, and rarely hops. It uses its wings to power hops or when it jumps off the ground onto low-level perches (e.g., logs, rocks, stumps).

Flight

Capable of extensive soaring, gliding, and flapping flight. Soaring and gliding flight are used for food searching and migration. In winter on Missouri River, individuals soared when winds were 7.4–25.9 km/h (194). Adults (mean wing-loading of 0.895 g/cm2) are better adapted to chasing flights for the capture of live prey; immatures (mean wing-loading 0.755 g/cm2) are better suited for soaring flight to locate carrion (125).

Swimming and Diving

Capable of floating on the surface of the water, and its wings are used in a “rowing” motion when water is too deep for wading (210) or if an individual is too wet to fly off the water's surface.

Self-Maintenance

Preening, Head-Scratching, Stretching, Bathing, etc.

Preening occurs after feeding, while perching during the day, in roosts at nightfall, and by adults in nests. Head-scratching with foot/talons is common while perched. The Bald Eagle will repeatedly rub its bill on a perch using a side-to-side motion, presumably as a means of cleaning the bill. Bathing is observed year-round, although it is limited when temperatures are below freezing. Nestlings preen as new feathers emerge, and preening is a regular activity prior to fledging (209).

Sleeping, Roosting, Sunbathing

Communal roosting has been widely reported, especially during the nonbreeding period (18, 134). Communal roosting by nonbreeders during summer is less common. Communal roosting involves many social interactions among individuals, including vocalizations and physical displacement from favorite perches within the roost. Communal roosts may serve an information-exchange function related to the location of best foraging sites. There is likely little thermal benefit associated with the transfer of heat from one eagle to another at communal roosts. Some individuals may use daytime foraging perches as night roosts. In inclement weather and winter, Bald Eagles select protected roost sites (143). Departure from roost sites generally about 0.5 hours prior to sunrise but may be later on cloudy days. Bald Eagles return to communal roosts over a prolonged period, with some birds arriving several hours before sunset. As a whole, Bald Eagles may return to their roost earlier during cloudy days or on days with inclement weather. Bald Eagles on Chesapeake Bay remained in roosts through daytime hours during Hurricane Gloria when wind speeds reached almost 100 km/h (DAB). See Habitat: Roosting Habitat for more details on roost site. Sunning has been reported in nestlings, although shading was much more common in California nests where heat stress was a greater problem than that of cold (241). In cold climates, nestlings may share warmth by body contact.

Daily Time Budget

Daily activity budgets differ for nesting and wintering birds and for adults and immatures. In winter, the Bald Eagle is largely sedentary, conserving energy by roosting in sites protected from prevailing winds and by perching adjacent to potential foraging areas, waiting for foraging opportunities. Four immatures monitored in western Washington spent 68% of 24-h cycle roosting, 30% perched, and <2% in flight or feeding (142). In winter in Connecticut, Bald Eagles spent 56% of 24-h cycle roosting; furthermore, adults spent 40.8% of 24-h cycle perched and 2.4% in flight, while immatures spent 33.3% of a 24-h cycle perched and 7.2% in flight. A nonbreeding adult in Saskatchewan during summer spent 25% of a 24-h cycle roosting, 70.1% perched, and 4.5% in flight (242).

During the nesting period, adult males and females split time at the nest, with females spending more time there than males during each stage of the nesting cycle. A camera study in Alaska found that females incubated eggs during 53% of daylight hours whereas males incubated 42% of daylight hours (~18 hours of daylight, behavior not observable on camera after dark); nestlings were brooded 80% of the daylight hours during the first five days post-hatching (females 71% and males 29% of brooding). Brooding fell to 3% of the daylight hours after 50 days post-hatching (240).

Agonistic Behavior

Physical Interactions

Aggression occurs mostly when competing for food (especially during winter), when competing for foraging perches and roosting sites, among adults due to territorial disputes during the breeding period, and among nestlings fighting for food. Territorial adults chase other individuals from their territory, especially other adults. Aggressive interactions among nestlings is common, occasionally leading to the death of the smaller, younger nestling (termed siblicide; 151). Death of a chick may occur because of either food deprivation or actual physical harassment from an older chick (i.e., facultative siblicide), although parental intervention, such as pecking the aggressive chick, may mitigate the effects of the interaction (243).

Actual physical contact is rare among Bald Eagles, but it can be fatal (e.g., 244). In a foraging study, a pirating aggressor was struck by a defending eagle in only 10 of 541 (1.8%) contested foraging events (200). In Minnesota, a territorial individual struck an intruder seven times, although without apparent injury (245). Fatal injury from territorial encounters has been reported based on circumstantial evidence, such as talon puncture-wounds found on a dead eagle. At Alaskan communal feeding sites, interaction outcomes are governed by size, hunger level, and possibly position (air superior to ground), but not by age (200). No age advantage was observed during piracy attempts among individuals wintering on the Mississippi River (246). Studies in western Washington have had conflicting findings, with an advantage only for smaller adults in one study (247), but all immatures were subordinate to adults in another study in the same area (142).

Communicative Interactions

Avoids physical contact by using threat vocalizations (see Sounds and Vocal Behavior: Vocalizations) and visual displays (200). At a communal feeding site in Alaska, individual aggressors attempted to strike another individual with its talons in 13% of 467 interactions, but only once did a physical strike occur; most birds (74% of 39 observations) that were attacked retreated rather than retaliated (200). Aggressors use displays involving head motions (head and neck extended up and/or out), wing motions (wings raised above body at a 45° angle followed by a flap), and a crouch/squat body position to challenge feeding birds, with these physical signs usually accompanied by vocalizations (200). Feeding birds may use similar displays to fend off aggressors. Territorial birds in Minnesota circled and/or called (Peal and Hic calls, see Sounds and Vocal Behavior: Vocalizations) in response to the presence of a decoy adult in their territory during the nesting season (239). Roosting birds threaten by half-raising their wings, extending their neck and head, and and using a threat vocalization when approached by aggressors wishing to displace them from favored perches (196, 18).

Territoriality

The defense of a territory is common during the breeding season as it helps to ensure sufficient food for raising young and allows adults to breed and raise their young without interference. Defense can involve perching in prominent areas (showing occupation of territory), using threat vocalizations, and at the extreme, chasing intruders out of the area (18). Both sexes defend the nest, and responses may be sex specific (i.e., males chase males, females chase females; 248). Territorial defense is most pronounced during the breeding season. For estimates of territory size, see Demography and Populations: Population Spatial Metrics.

Sexual Behavior

Mating System and Operational Sex Ratio

The Bald Eagle is monogamous and thought to mate for life unless a mate dies, but this is not well documented by marked individuals (18).

The sex ratio is poorly known as distinguishing males from females in the field is difficult at best. Necropsy of 133 adults from Alaska revealed 64 males to 69 females, close to a 50:50 ratio (40). The sex of free-ranging birds is determinable with good accuracy via morphometric measurements, laparoscopic examination, or karyotypic analysis from blood samples (249). The sex of a nesting pair can be determined by the size differential between mates and copulatory behavior. In healthy populations, unmated adults are present, and the sex of these individuals is generally unknown (198, 250).

Courtship, Copulation, and Pair Bond

Undertakes spectacular courtship rituals, involving loud vocalizations and acrobatic flight displays. Perhaps the best known courtship act is the Cartwheel Display, in which a courting pair fly to a great altitude, lock talons, and tumble/cartwheel back toward earth, separating just before collision with the ground. Other courtship displays include the Chase Display, in which paired individuals pursue each other and occasionally lock talons, roll, and dive, and also the so-called Roller-Coaster Flight, in which eagle will fly to great altitude, fold wings, and dive directly to earth, swooping back up at the last instance to avoid collision with the ground (18).

Copulation seldom observed in the wild (18), although detailed description has been made from captive pairs (237). Copulation usually preceded by perching side by side. In half of the observations (n = 16), the female approached the male. The female readied herself on the perch with feet apart, head lowered and wings slightly apart. The male then mounted her back and vocalized, flapping wings for balance. The female lifted her tail up, the male put his tail down to enable cloacal contact (237). Copulation typically last 5‒15 s, though it may last longer (151). Male then dismounted and perched next to female (237).

The pair bond can last more than a year, but there are very few data because of the difficulty of capturing mates for color-marking or radio-tagging. In climates where adults overwinter in their breeding territories, pair bonds can remain intact, thus facilitating mate retention from year to year. In Oregon, resident pairs are observed “near” the nest year-round, suggesting that a pair bond lasts longer than one breeding season, although some radio-tagged adults left the area for several weeks in late summer–early fall to exploit alternate food sources (251). For migratory populations in northerly areas, the observation of courtship and copulatory behavior on the wintering grounds suggests that mates may winter together and that the pair bonds remain intact from year to year (125). The same female apparently occupied a territory in Ohio from 1924 through 1932 but mated with four different males because the males died from various causes (151). The pair bond may break up after failed nesting attempts, although data for this is circumstantial (252).

Extra-Pair Mating Behavior/Paternity

Rarely, three adults have been observed at a single nest, with reports from Alaska, Minnesota, Connecticut, California, and British Columbia (198, 253, 254, 255, 256). When three adults have been reported at a nest, extra-pair copulations have generally not been observed, although in most cases the three adults were noted together during or after incubation, so extra-pair copulations would not likely be observed. However, an apparent extra-pair copulation was observed (male and extra female) in British Columbia although females were not marked to allow for definitive identification and the outcome of the extra-pair copulation (i.e., fertilized egg being laid) was unknown (256).

Brood Parasitism of Conspecifics

Not reported.

Social and Interspecific Behavior

Degree of Sociality

The Bald Eagle exhibits a high degree of social interaction at communal roosts and communal feeding sites at which hundreds of individuals may congregate. Immature birds and nonbreeding adults are highly social throughout the year at such communal sites (e.g., fall migration—Chilkat River, Alaska; winter—Pacific Northwest rivers and along Mississippi River; summer—James River and lower Chesapeake Bay). Breeding adults are highly territorial when nesting, and social interaction then is largely with mates and offspring. However, adults that bred, or attempted to breed, associate freely with other Bald Eagles during the nonbreeding period. During migration, generally migrates alone (125), but will incidentally associate in small groups (< 10 birds) in flight, and may congregate with other Bald Eagles in small to very large groups at feeding and roost sites.

Play

Will sometimes engage in activity of no apparent short-term survival value and is thus suggestive of play. For example, 6 Bald Eagles observed were observed passing sticks in the air (257). Bald Eagle will also pick up and manipulate unusual objects, such as plastic bottles. Such behavior is common in nestlings and may prepare them for foraging, nest-building, etc. (209). Such play behavior in free-flying birds likely hones foraging and other skills needed ultimately for reproduction and survival.

Nonpredatory Interspecific Interactions

Interspecific interactions are most often related to competition for food. Bald Eagle sometimes steals food from Osprey (Pandion haliaetus)and it also harasses and is harassed by Golden Eagle (Aquila chrysaetos), other raptors, and corvids (Corvidae). Bald Eagles are sometimes chased and mobbed by birds that consider it a potential threat, such as blackbirds (Icteridae), , corvids, and gulls (Laridae). The defense of nest sites may involve driving off intruders without the intention of preying upon them, including other raptors (especially Osprey), ravens and crows, gulls, and mammals (especially Canidae, such as foxes and coyote [Canis latrans]). In Florida Bay, Osprey sometimes relocated their nests after a new Bald Eagle territory was established, and the Osprey that did not change nest sites experienced reduced nesting success; the reduced success may have been due to harassment by nesting eagles, though a reduction in hostilities occurred in successive years after Bald Eagles started nesting (258). Bald Eagle and Golden Eagle aggressively displaced each other at winter feeding sites in Utah, with no apparent dominance of either species (259). Bald Eagle was dominant over Black Vulture (Coragyps atratus) and Turkey Vulture (Cathartes aura) during interactions at communal feeding and roosting sites in Florida (260). Mammalian scavengers, including coyote, feral dog (Canis lupus familiaris), and bobcat (Lynx rufus), displaced Bald Eagle from carcasses during a supplemental feeding study in Maine (166).

Predation

Eggs, nestlings, and fledgling are most vulnerable to predators. Eggs in tree nests have been predated by Black-billed Magpie (Pica hudsonia), gulls (Larus spp.), ravens and crows (Corvus spp.), black bear (Ursus americanus), and raccoon (Procyon lotor) (261, 262, 263, 264, 265). Ground nests on Amchitka Island, Alaska, were vulnerable to the Arctic fox (Alopex lagopus) until the fox was removed from the island (198).

Nestlings have been reportedly taken by black bear, raccoon, hawks, owls, crows and ravens, bobcat (Lynx rufus), and wolverine (Gulo gulo) (171, 265, 266, 46, 267), although there is little actual documentation. Nestling mortality is more likely to be due to starvation and sibling attacks. Few nonhuman species are capable of, or likely to attempt, predation upon healthy immature or adult Bald Eagles. The exception is at the time of nest departure, when fledglings on the ground are vulnerable to mammalian predators. Immatures and adults that are in poor condition from starvation, injury, or disease may be vulnerable to mammalian predation.

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