The eastern imperial eagle (Aquila heliaca) is a large bird of prey that breeds in southeastern Europe and extensively through West and Central Asia. Most populations are migratory and winter in northeastern Africa, the Middle East and South and East Asia. Like all eagles, the eastern imperial eagle is a member of the family Accipitridae. Furthermore, its feathered legs mark it as a member of the subfamily Aquilinae. It is a large, dark-colored eagle, with a resemblance to other members of the genus Aquila, but it is usually the darkest species in its range. It is an opportunistic predator that mostly selects smallish mammals as prey but also a fairly large proportion of birds, reptile and other prey types, including carrion. Compared to other Aquila eagles, it has a strong preference for the interface of tall woods with plains and other open, relatively flat habitats, including the wooded mosaics of the steppe. Normally, nests are located in large, mature trees and the parents raise around one or two fledglings. The global population is small and declining due to persecution, loss of habitat and prey. It has therefore been IUCN Red Listed as Vulnerable since 1994.
Taxonomy The eastern imperial eagle is a member of the Aquilinae or booted eagles, a rather monophyletic subfamily of the accipitrid family. At least 38 species are currently housed in the subfamily, all with signature well-feathered tarsi. This species is a member of the genus Aquila, which are mostly large, fairly dark colored eagles distributed largely through the more open habitats of Eurasia and Africa (with one in North America and a couple in Australasia). Studies on DNA have indicated that the imperial eagle is part of a subgroup with other moderately sized Aquila such as the steppe eagle (Aquila nipalensis) and the tawny eagle (Aquila rapax). Despite the outward resemblance to the imperial eagle, the 4 species in the golden eagle subgroup appear to be rather more closely related to the dissimilarly smallish and pale-bellied sister species, African hawk eagle (Aquila spilogaster) and Bonelli's eagle (Aquila fasicata).Väli, Ü. (2002). Mitochondrial pseudo‐control region in old world eagles (genus Aquila). Molecular Ecology, 11(10), 2189-2194. The Spanish imperial eagle, which is found in Spain and Portugal, was formerly lumped with this species, the name imperial eagle having been previously used in both circumstances. However, the two are now regarded as separate species due to significant differences in morphology, ecology and molecular characteristics.Collinson, M. (2006). Splitting headaches? British Birds, 99, 306-323.González, L. M., Hiraldo, F., Delibes, M., & Calderón, J. (1989). Zoogeographic support for the Spanish Imperial Eagle as a distinct species. Bulletin of the British Ornithologists' Club, 109, 86-93. It is likely that the eastern imperial eagle is the paraspecies for the Spanish imperial eagle and that the imperial eagle complex reached the Iberian peninsula sometimes between the late Pleistocene era and early Holocene. The Spanish imperial eagle may be considered an ice age relict due to its isolation.González, L. M. (2008). Origin and formation of the Spanish Imperial Eagle (Aquila adalberti). Journal of Ornithology, 149(2), 151-159.
Description
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 with a typical wingspan of . The average wingspan of a small sample showed males to average while a small sample of females averaged .Brown, Leslie and Amadon, Dean (1986) Eagles, Hawks and Falcons of the World. The Wellfleet Press. . 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.Katzner, T. E., Jackson, D. S., Ivy, J., Bragin, E. A., & DeWoody, A. (2014). Variation in offspring sex ratio of a long‐lived sexually dimorphic raptor, the Eastern Imperial Eagle Aquila heliaca. Ibis, 156(2), 395-403. In terms of body mass, one survey found five males to weigh from and five females to weigh from . The average weight of this sample of imperial eagles was reportedly in the males and in the females.CRC Handbook of Avian Body Masses, 2nd Edition by John B. Dunning Jr. (Editor). CRC Press (2008), . A sample of unknown size showed males to weigh an average while females reportedly weighed an average of .Kirmse, W. (1998). Morphometric features characterizing flight properties of palearctic eagles. Holarctic Birds of Prey. Merida and Berlin: ADENEX and World Working Group on Birds of Prey, 339-348.Weick, F., & Brown, L. (1980). Birds of prey of the world: a colored guide to identification of all the diurnal species order Falconiformes. Verlag Paul Parey. A further two mature females weighed an average of . Among standard measurements, males may range in wing chord length from , in tail length from and in tarsus length from . Meanwhile, females may range in wing chord length from , in tail length from and in tarsus length from . A sample of imperial eagles ranged in total bill length from .Catalogue of the Birds in the British Museum Volume 1 (1974).
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 species tends to perch in a fairly upright position often on rather exposed tree branch or low mound, rock, haystack or similar convenient site. For an Aquila eagle, it is seemingly relatively less shy and bolder in the presence of humans. The adult plumage is largely a tar-like blackish-brown but for a well-demarcated and highly contrasting creamy to golden buff colour about the crown, hindneck and neck sides. Furthermore, adults have bold white "epaulettes" on their shoulder braces, which are usually fairly conspicuous on perched birds. The adult's tail is narrowly dark barred over a greyish ground colour and has a broad black subterminal band, while a white tail tip sometimes manifest in adults that are freshly molted. The undertail coverts are sometimes indistinctly paler, rust to creamy, combined with grey tail base to give the appearance of a paler rear end.Clark, W. S. (1999). A field guide to the raptors of Europe, the Middle East, and North Africa. Oxford University Press, USA.Forsman, D. (2005). Eastern Imperial Eagle plumages. Alula, 11: 146-152. At rest, the wing tips tend to reach the tail tip.Naoroji, R., & Schmitt, N. J. (2007). Birds of prey of the Indian subcontinent. Om Books International. 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 centre 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 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 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.Bijleveld, M. (1974). Birds of prey in Europe. Macmillan International Higher Education.
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.Porter, R. F. (1981). Flight identification of European raptors. A&C Black.Forsman, D. (2016). Flight Identification of Raptors of Europe, North Africa and the Middle East. Bloomsbury Publishing. 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 colour 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.
Confusion of species