The Eastern imperial eagle (Aquila heliaca) is a large graceful bird of prey that breeds in Eurasia. Like all eagles, the Eastern imperial eagle is a member of the family Accipitridae. Furthermore, its well-feathered legs mark it as a member of the subfamily Aquilinae.
The Eastern imperial eagle is a large eagle, but usually falls near the average size of the large-bodied genus Aquila. Adult total length can range from 68 to 90 cm (27 to 35 in) with a typical wingspan of 1.76 to 2.2 m (5 ft 9 in to 7 ft 3 in). Although otherwise outwardly similar, the species displays reverse sexual dimorphism as do most birds of prey, in which males are usually smaller than the females. For the Eastern imperial eagle, females are up to 10% larger linearly and 40% heavier in body mass in some cases. In terms of body mass, one survey found five males to weigh from 2.45 to 2.72 kg (5.4 to 6.0 lb) and five females to weigh from 3.26 to 4.54 kg (7.2 to 10.0 lb). Among standard measurements, males may range in wing chord length from 540 to 622 mm (21.3 to 24.5 in), in tail length from 260 to 308 mm (10.2 to 12.1 in), and in tarsus length from 91 to 98 mm (3.6 to 3.9 in). Meanwhile, females may range in wing chord length from 565 to 665 mm (22.2 to 26.2 in), in tail length from 270 to 330 mm (11 to 13 in), and in tarsus length from 97 to 107 mm (3.8 to 4.2 in). A sample of imperial eagles ranged in total bill length from 65.7 to 76.5 mm (2.59 to 3.01 in).
In general, compared to other species in their taxonomic group, the Eastern imperial eagle has a relatively long and thick neck, a big head and bill (with a gape line level with middle of eye), a longish square-tipped tail, somewhat long and well-feathered legs, and strong feet. The adult plumage is largely a tar-like blackish-brown but for a well-demarcated and highly contrasting creamy to golden buff color about the crown, hindneck, and neck sides. Furthermore, adults have bold white spots on their shoulder braces, which are usually fairly conspicuous on perched birds. The adult’s tail is narrowly dark barred over a greyish ground color and has a broad black subterminal band, while a white tail tip sometimes manifests in adults that are freshly molted. The undertail coverts are sometimes indistinctly paler, rust to creamy, combined with a grey tail base to give the appearance of a paler rear end. At rest, the wing tips tend to reach the tail tip. The juvenile Eastern imperial eagle is mostly pale tawny-buff to sandy yellow with fairly heavy dark brown streaks from the throat down to the breast, mantle, scapulars and forewing coverts. The juvenile’s scapulars and forewing coverts also have sometimes noticeable white-tipped feathers while the median coverts are perceptibly browner and greater coverts blackish both with broad creamy-yellow tips forming clear wing bars. The flight feathers and tail on juveniles are often blackish and tipped whitish, however, the white parts on the lower back to tail coverts are only lightly streaked in the center and often not visible when perched. Below the streaked breast, the remainder of the juvenile’s underparts are plain pale buff. By the 2nd year, the brown streaking on the underside fades to a plainer sandy hue and the pale bars also start to fade on the wings. Especially later into the 2nd year into the 3rd year, some young Eastern imperial eagles show an erratic blotching of blackish-brown feathers below. By about the 4th winter, as the birds enter their subadult plumage, the forefront of the eagle is often a rather patchy mix of sandy and darker adult-like feathers. During slow annual molts, the dark feathers expand initially from the throat and upper breast outward. Late into the subadult stage, the birds also start to develop a pale crown and nape but usually the rear body is still more juvenile-like, such as the pale rump-band and crissum despite the otherwise darkening tail and wing feathers. The full adult plumage is attained at 5-6 years of age but some subadults are already breeding before this.
In flight, the Eastern imperial eagle is a large raptor that has for an Aquila a very projecting neck and "huge" head and bill. The long wings may appear fairly broad when compared to other, smaller raptors, but are relatively narrow with even parallel edges when compared to other Aquila eagles. Their flight style is relatively heavy but steady with deep powerful beats but they are not uncommonly clumsy at first takeoff. They tend to soar with forward-pressed but rather flat wings, the outer wing feathers may sometimes curve up but as a rule they do not fly with a V as do some other Aquila. The species may also hold their wings flat while engaging in a glide but as it accelerates they may arch wings back. In flight, the adult from above shows dark brown with small white braces, greyish tail with fine dark bars, and a very broad, blackish subterminal band. The adult is essentially all dark colored when seen from below relieved only by some very dusky grey flight feathers on the primaries (against the black wing tips), a grey crissum, and thinly barred grey tail base. However, these features may be only obvious in good light and at reasonable distances. In flight, the juvenile is largely pale buff with brown streaking. The lower back, rump, tail coverts, and leg feathers are all whitish cream in color which contrasts noticeably with their white-tipped blackish greater coverts, primary coverts, and quills. On their mantle, juveniles manifest two white wing bars above and a narrow whitish lower trailing edge. Meanwhile, the juvenile’s inner primaries are much paler creamy-buff hue than their other flight feathers. The dusky brown wing linings of the juvenile plumage, when compared to the darker ones of adults, show more extensive coarse patterning. Some birds by 2nd to 3rd year are so worn in their flight feathers as to appear almost unstreaked sandy while, from the 3rd year on, darker feathers start to appear below with various untidy variations.
Eastern imperial eagles breed in southeastern Europe and extensively throughout West and Central Asia. Most populations are migratory and winter in northeastern Africa, the Middle East, and South and East Asia. These birds prefer to live in taiga forests, woodlands, grasslands, meadows, vast areas of steppe, river valleys, agricultural areas, semi-deserts, forested lower mountains and montane steppe, near marshes, lakes, and other wetlands.
Eastern imperial eagles usually spend time singly or in mated pairs but may sometimes form small groups at waterholes or food during migrant or winter. These powerful birds hunt by day using various hunting techniques but almost exclusively take their prey on the ground. They may also take some prey from low perches in ambushes or capture prey in water or from the edge of waterways and may even become waterlogged, especially when capturing birds. Imperial eagles typically still hunt, watching for prey at length from a moderately low perch, then often making a short stoop or dive to the ground once the prey is spotted. Alternately, they may make a longer dive onto prey from a low soaring flight, often using any vegetation available to obscure their approach. Some prey may be captured on foot, including insects and burrowing mammals, which birds may wait by the entrance of the animal’s burrow. Occasionally, they will even steal food from other eagles, especially during winter, and also tandem hunt in pairs as well. Eastern imperial eagles are quite vocal on their breeding ground but tend to be silent or nearly so elsewhere. Their main call is a deep, harsh bark ‘owk owk, gok gok or kraw-kraw’. The birds most frequently call during their aerial displays. Females may also call when a male arrives with prey. Alarm calls include a soft ‘ko-gok, wk wk wk’, uttered in response to a distant intruder, a closer approach also may cause a harder ‘gek’ call by the mother.
Eastern imperial eagles are carnivores and like most active predators, opportunists who take any prey they are capable of overpowering. Their prey spectrum is highly varied, including somewhere between 200 and 300 prey species. Small to medium-sized mammals are the most regularly selected prey, with a preference for hares, various rodents especially ground squirrels, hamsters, and voles, as well as insectivores. They also prey on various birds, especially the young or fledglings. Other prey includes reptiles, fish, and rarely insects.
Eastern imperial eagles are monogamous form pair-bonds that last for life. They breed between late March and September throughout the range. During this period pairs engage in aerial displays with loud calling and extensive high circling, which is likely for the purposes of the territorial proclamation to other eagles of their species. The display is often interspersed with mock dives and talon showing. One or both members of a pair may participate in the aerial display. The pair constructs a very large nest of sticks, which may average up to 1.2 to 1.5 m (3 ft 11 in to 4 ft 11 in) across and 60 to 70 cm (24 to 28 in) in depth. Nests are frequently lined with various materials, including twigs, grass, fur, debris, and greenery. Usually, Eastern imperial eagles build their nest in tall trees but may also construct it as low as 2 m (6.6 ft) in low scrub or, rarely, on a cliff or ground. The female lays 2 to 3 eggs at intervals of 2 or more days and incubates them for 43 days. The eaglets hatch at intervals of several days. Mostly the female broods the young and the male captures prey. From when the young are 40 days onward, the female typically resumes hunting and takes to a perch nearby. The young fledge at 63 to 77 days of age and after leaving the nest they usually linger near it for 2-3 days.
The Eastern imperial eagle has declined greatly throughout history. At one time, people of eastern Europe and adjacent parts of Russia used to admire and even revere this eagle as a "sacred bird". It was widely referred to as the "cross-bearing eagle" (because of the shape of the white spots on its wings). The global population of this powerful bird of prey is small and declining due to persecution, loss of habitat, depletion of prey species, and incidental consumption by the eagles of poison baits, left out to eliminate wolves.
According to IUCN Red List, the total population size of the Eastern imperial eagle is around 3,500-15,000 individuals or 2,500-9,999 mature individuals. In Europe, the breeding population consists of 1,300-1,900 breeding pairs, equating to 2,500-3,800 mature individuals. Currently, this species is classified as Vulnerable (VU) on the IUCN Red List and its numbers today are decreasing.
