Eastern imperial eagle
Eastern imperial eagle | |
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Subadult of typical transitional plumage at the Little Rann of Kutch | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Aves |
Order: | Accipitriformes |
Family: | Accipitridae |
Genus: | Aquila |
Species: | A. heliaca
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Binomial name | |
Aquila heliaca Savigny, 1809
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Distribution of Aquila heliaca:
Breeding Resident Passage Non-breeding
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Synonyms | |
Aquila heliaca heliaca |
The eastern imperial eagle (Aquila heliaca) is a large
Taxonomy
The eastern imperial eagle is a member of the
Description
The eastern imperial eagle is a large
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.[3] 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.[3][4][25][26] At rest, the wing tips tend to reach the tail tip.[27] 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.[3][4][25][26][28]
In flight, the eastern imperial eagle is a large raptor that has for an Aquila a very projecting neck and "huge" head and bill.[3][25] 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.[4][26] Their flight style is relatively heavy but steady with deep powerful beats but they are not uncommonly clumsy at first takeoff.[3] 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.[3][26][29][30] 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.[3][29][26]
Confusion of species
The eastern imperial eagle is typically smaller, being more slender and less bulky and powerful in appearance despite its proportionately larger head and longer neck, than the
The subadult eastern imperial eagle may be confused with older immature greater spotted eagles (Clanga clanga) but the latter is less contrastingly patterned, without a paler shawl and has greater covert band below (despite the rare hint of one on some imperials) and densely bared flight feathers. Adult greater spotted eagles have similar underwing contrast as juvenile eastern imperial eagles but, beyond plumage characteristics, are distinct for their much smaller head, slightly smaller overall size and more compact frame with a relatively broader and shorter wings. More similar to the juvenile eastern imperial eagle is the fulvescens morph of the greater spotted eagle which is fairly rare (more so in the west) but is similarly or even as pale buffy. However this spotted eagle morph lacks the brownish streaks below of the juvenile imperial and further has contrasting much darker feathering on the wing coverts (heavily spotted in juvenile greater spotteds) and mantle as well as sometimes the facial feathers, and can further be distinguished by the same aforementioned differences in form.[4][27] Indian spotted eagles (Clanga hastata) tend to show paler lesser under-wing coverts like juvenile eastern imperial eagles but are considerably smaller and differ in all other proportions and plumage features.[27] Although at times described as “very similar”, the eastern imperial eagles are fairly easily told from the golden eagle by being much darker with less broad wings that are held much flatter. The golden eagle, unlike imperial eagles, tends to fly with its wings pointed upwards and have a gradual tapering wing shape (somewhat narrower at the base, broader at the primaries). In golden eagles, the tail generally appears narrower and squarer in shape. At closer range, the pale area on the back of the head and neck in eastern imperial eagle may be suggestive to some of the golden eagle's golden nape but is always much paler and chalkier in colour, as well as more strongly contrasted by the otherwise dark feathers and is more extensive.[3][25][27] Against the somewhat similarly sized but more compact juvenile steppe eagle (Aquila nipalensis), the juvenile eastern imperial eagle does not show a white band on underwing like the steppe. More obviously, the steppe eagle lacks the paler overall colours and contrasting brownish streaking of juvenile imperials. Feather wear can make the wing shape of the two resemble the other but the larger head and less compact frame of the imperial species render its flight profile distinctive.[27] The only darker large booted eagle encountered by the eastern imperial eagle, in its African winter quarters, is the jet-black Verreaux's eagle (Aquila verreauxii) which is more suggestive in size and proportions of the golden eagle. An unlikely source of confusion, the Verreaux's differs in almost all plumage characteristics and has far more tapered wings that pinch in at the base and, like the golden eagle, tends to fly in a fairly strong dihedral.[3] In Asia, its proportions and size may be suggestive of the perhaps even rarer Pallas's fish eagle (Haliaeetus leucoryphus) and the two can potentially be mistaken in strongly backlit conditions which obscure their obviously distinct plumages.[27] The large headed and flat winged shape of the imperial eagle may too suggest in silhouette the white-tailed eagle (Haliaeetus albicilla) but that species is usually rather larger bodied and even larger headed with much broader wings and a proportionately shorter tail.[3][30] Cinereous vulture (Aegypius monachus), being similarly dusky overall below, have been suggested as confusion species, but are much larger than imperial eagles with differing proportions (far broader wings, proportionately smaller head and shorter tail) and lack any of the contrasting pale parts of the imperial eagle's plumage.[3]
Vocalizations
The eastern imperial eagle is quite vocal on their breeding ground but tends to be silent or nearly so elsewhere. The main call of the species is a deep, harsh bark owk owk, gok gok or kraw-kraw. The call tends to be repeated rapidly up to 8-10 times. Their call is perceptibly deeper and harsher than that of the golden eagle, somewhat incongruously also being more resonant and commanding. Sometimes when extending to a prolonged version, its call is sometimes considered reminiscent of the croak of a large frog.[3][33][34] The eastern imperial eagle most frequently calls during their aerial displays. In extreme cases, the call may be repeated up to 13 times during an aerial display. Furthermore, they may repeat the call from a perch. Females may also call when a male arrives with prey. Alarm calls recorded for the species 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. A rising and falling trill has been additionally recorded in captivity is also probably an alarm call.[3][35]
Distribution and habitat
Breeding range
The eastern imperial eagle is found as far west as east-central and southeastern Europe. The European part of its breeding range includes eastern Austria, eastern Czech Republic, Slovakia, Hungary, eastern Croatia, Serbia, northeastern Bosnia and Herzegovina, Macedonia, Bulgaria, Romania, Moldova and northern, western and much of the eastern part of the Ukraine.[1][3][10][36][37] The species distribution continues across central Russia, where it is found through most of the Central Federal District, essentially all of the North Caucasian Federal District, most of the Volga and Ural Federal Districts (excluding the northern parts), and the southern part of Siberia past Lake Baikal to the Transbaikalia in the landlocked southwest of the Russian Far East.[1][37][38][39] Out of Russia, their breeding extends south to mostly the northern portions of the following nations: Georgia, Armenia, Azerbaijan, Turkmenistan, Uzbekistan, much of Kazakhstan, northwestern China (Xinjiang) and northern Mongolia.[1][3][5][37][40] Isolated populations also persist in northwestern, central and eastern Turkey, Cyprus, and northern Iran. As a breeding species, they are probably extirpated from Afghanistan and Pakistan.[1][3][37][41]
Migration and wintering range
Unlike the
Habitat
The eastern imperial eagle is distributed as a breeding species largely along the southern edge of the vast
Behaviour and ecology
The eastern imperial eagle is somewhat varied in hunting techniques but almost exclusively takes its prey on the ground. It is possible some prey, such as fledgling birds, are taken from low perches in ambushes but this is seemingly unverified. They are also known to capture prey in water or from the edge of waterways and may even become waterlogged, especially when capturing water birds.[3][27][66] Imperial eagles typically still hunt, watching for prey at length from a moderately low perch (usually a tree branch but virtually any perch from rocks to bushes to power poles), then often making a short stoop or dive to the ground once 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.[3][27] Some prey are known to be captured on foot, including insects and ground squirrels, the latter reportedly by waiting by the entrance of the animal's burrow.[3][67] Occasionally, this species pirates foods from other eagles and other birds of prey, especially during winter, and also tandem hunts in pairs as well.[3] The eastern Imperial eagle is, like most active predators, an opportunist who exploits any prey they are capable of overpowering.[27] Their prey spectrum is highly varied, including somewhere between 200 and 300 prey species, a total number of prey species only a bit short of the occasionally sympatric common buzzard (Buteo buteo) which may be 500 times more numerous overall.[3][66][68] In the nations of Kazakhstan and Hungary alone, the total recorded prey spectrum is 154 and 126 species, respectively.[66][68] 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. Furthermore, various birds are taken, at times as much as or more so than mammals, especially the young or fledglings of various medium-sized to larger birds. Birds may locally become the primary foods in some parts of the winter range. Reptiles are taken secondarily in most of the range but can be locally somewhat important and fish and invertebrates, including insects, may be taken rarely.[3][27][66]
The prey type historically most often associated with the eastern imperial eagle, at least in Europe, has been ground squirrels and hamsters.[3][18][69] While these are significant, the primary prey type can vary and often hares or hedgehogs appear to take the primary position in recent studies.[66] The largest European dietary study to date, a multi-year analysis from Hungary, showed that European hare (Lepus europaeus) were the primary foods, making up 27.4% of a total of 8,543 prey items. The second best represented prey in Hungary was the European hamster (Cricetus cricetus), at 12.71% of the diet.[66][70] Similarly, in Slovakia and the Czech Republic, the European hare was the main prey, comprising 40.2% of 562 prey items and 41.3% of 109 prey items, respectively. The European hamster was the 4th most frequent prey in Slovakia but second most common prey species in the Czech Republic.[71][72] As presented in the multi-year studies from Hungary, a seeming decline of the European ground squirrel (Spermophilus citellus) population is the cause of their reduced importance in the imperial eagle's diet, with this species being the primary prey in 1975-1991 (51% of 606 prey items from 1975 to 1985) to contributing almost nothing in 2005-2017.[66][70][73] It is possible with reintroductions of the European ground squirrel underway in central Europe, that this prey species may again become more significant in the eastern imperial eagle's diet again here.[74] In Tyva Republic, the long-tailed ground squirrel (Spermophilus undulatus) still dominates the food of imperial eagles, making up 60.1% of 168 prey items.[75] In studies of two different areas of Bulgaria, one showed European hares as the main prey (25%) in the Dervent heights and southern white-breasted hedgehog (Erinaceus concolor) (32.5%) in the other, Saker mountain.[76] In East Thrace, Turkey, the same hedgehog was the most important prey, comprising 23.1% of 582 prey items and 21.2% of the prey biomass.[77]
In warmer, southerly areas, the primary prey species seem to generally be smaller. In
Assorted bird species may factor heavily into the diet of eastern imperial eagles. More than 120 bird species are known to be taken by this eagle.
European studies of the eastern imperial eagle's diet rarely reflect prey outside of the main preferred classes of mammals and birds, however studies from somewhat outside Europe show respectable numbers of
The size of prey taken by eastern imperial eagles can be fairly variable.[7][68] Most live prey taken by eastern imperial eagles weighs less than 2 kg (4.4 lb).[3] According to Watson (2010), the prey of imperial eagles is fairly evenly spread from 63 g (2.2 oz) up to 2,000 g (4.4 lb), with a peak focus (at around 25%) on prey weighing 500 and 1,000 g (1.1 and 2.2 lb) and a mean estimated prey size of 565 g (1.246 lb).[7] The prefferd sized of prey can be slightly varied throughout the region. In Hungary, staple prey size was estimated at between 250 g (8.8 oz) and 2.5 kg (5.5 lb).[66] By contrast, in Slovakia, maximum weight of live-caught prey was estimated at 1.4 kg (3.1 lb).[71]
Many of the prey species taken by imperial eagles are relatively large as adults such as
Interspecies predatory relationships
The eastern imperial eagle is a powerful bird of prey but frequently overlaps in range with multiple other eagle species and that may require them to share both habitats and prey.[3] The golden eagle is generally a larger, more powerful bird. It also tends to be a bolder, more aggressive predator than the imperial eagle and may be able to attack much larger prey. While the mean prey body mass relative to the eagle's weight is probably similar between the two species, an estimated 15% of golden eagle prey will weigh over 5 kg (11 lb).[7][102][103][104] In its very extensive range, the golden eagle's distribution includes nearly all areas occupied by breeding eastern imperial eagles. Furthermore, there is considerable overlap in prey species selected by these species.[7][105][106] There is a natural partitioning between the two Aquila species and that comes in the form of habitat preferences. The golden eagle takes to, usually but not always, rocky and uneven terrain, so favors mountainous areas with alpine meadows to access for prey. This is quite different from the eastern imperial eagle's preference for a flat or somewhat rolling interface between wood stands and fields at low elevations.[107][108] However, in some areas, especially eastern Europe, eastern imperial eagles have been driven to higher elevations and more montane habitats that are typically the haunts of golden eagles by persecution, habitat destruction, and other interferences by humans, usually with mixed to minor success as the golden species is scarce at best locally and unlikely to produce competition.[3][65] There is surprisingly little information on conflicts between these two eagle species.[18] In the Naurzum Nature Reserve of Kazakhstan the golden and eastern imperial eagle, and to some extent also both white-tailed eagles and steppe eagles, were recorded nesting with fairly close proximity to each other. The eagle species here would even use nests built by the other species and seemed to have similar or broadly overlapping food habits, but no interspecies conflicts were detected.[105] On the contrary, in the Altai Republic, it appears that golden and eastern imperial eagles are considered to fill a largely similar ecological niche in abutting areas and do compete for nesting sites.[91][106] In some cases in Europe, golden and eastern imperial eagles will engage in a territorial display if prompted against one another.[109]
Of a similar distribution to eastern imperial eagles, both in their mid-Eurasian breeding ground and southerly Indo-African wintering grounds, are the
As its preferred habitat seldom overlaps with larger eagles such as golden eagles and
The eastern imperial eagle may be characterized as an occasional and opportunistic predator of other birds of prey. The following raptorial birds have been known to fall prey this eagle: the lesser spotted eagle (Clanga pomarina), European honey buzzard (Pernis apivorus), black kite (Milvus migrans), hen harrier (Circus cyaneus), Montagu's harrier (Circus pygargus), western marsh harrier, Eurasian sparrowhawk, northern goshawk (Accipiter gentilis), common buzzard (Buteo buteo), long-legged buzzard, rough-legged buzzard (Buteo lagopus), Ural owl (Strix uralensis), tawny owl (Strix alucco), little owl (Athene noctua), long-eared owl (Asio otus), short-eared owl (Asio flammeus), Eurasian hobby (Falco subbuteo), common kestrel (Falco tinnunculus), lesser kestrel (Falco naumanni), merlin (Falco columbarius), red-footed falcon (Falco vespertinus), saker falcon and peregrine falcon.[66][68][71][77][79][83][118][120][121][122] Although it may be classed properly as an apex predator, eastern imperial eagles have fallen prey to other birds of prey on rare occasions. An instance of predation was reportedly committed by a white-tailed eagle.[123] Furthermore, imperial eagles may be vulnerable at their nest to nighttime ambushes by Eurasian eagle-owls (Bubo bubo).[7][124]
Breeding
The eastern imperial eagle is a fairly solitary bird, with rarely more than a pair occurring, although some small gatherings recorded at waterholes or food during migrant or winter. Pairs on the breeding grounds engage in aerial displays with loud calling and extensive high circling, which like most of accipitrids is likely for the purposes 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. If an interloper does not leave during the first part of the display, the territorial skirmish then may become physical and it sometimes leads to cartwheeling with interlocked talons, falling until they nearly hit the ground. Despite the fairly impressive display, it is generally of slightly shorter duration than the similar one done by
The mother eastern imperial eagle lays 2 to 3 eggs at intervals of 2 or more days. The average clutch size in Georgia was 2.09.[78] In the Czech Republic, the average clutch size was 2.27.[72] 81 nests from Russia had a mean clutch size of 2.1.[39] Exceptional clutch sizes of up to four eggs have been recorded in Kazakhstan.[132] The eggs are a dull buffish white colour overall and are sparsely marked with grey, purplish or, occasionally, brown spots. A sample of 150 eggs were found to have measured from 63 to 82.5 mm (2.48 to 3.25 in) in height by 52.5 to 62.5 mm (2.07 to 2.46 in), with an average of 73.3 mm × 56.5 mm (2.89 in × 2.22 in).[18] In Georgia, the average dimensions of eggs was measured in a sample of 20 as 72.3 mm × 57.5 mm (2.85 in × 2.26 in) with a mean weight of 136.8 g (4.83 oz).[78] As was recorded in 13 cases in Slovakia, if eggs are lost early into incubation, around March or April, a replacement clutch may be laid, though always the parent eagles used an alternate nest when this occurred.[133] In southeast Europe egg laying peaks at about mid-February to March while in central Asia, it is in first half of April shifting late April further north.[18][78] The incubation starts with the 1st egg, with males only infrequently taking a shift. The incubation stage lasts for 43 days.[18] In Bulgaria, two females were recorded to do 90.8-94.1% of the incubating.[134] The eaglets hatch at intervals of several days, with one usually being distinctly larger than the rest. However, cases of siblicide are infrequent compared to the golden eagle and, when not interrupted by human interference, prey population crashes and nest collapses, nest frequently bear two fledglings.[18][135] At 14 days of age, the first feathers through down, while feathers on the back starting at 21 days. Feathers cover the down by 35–40 days but sometimes the down persists about head and neck for up to 45 days. Feathering is complete at 55 days and first flight attempts by 60 days onward. Mostly the female broods the young and male captures prey in this species (as is often the case in booted eagles in which males rarely engage in direct brooding).[136][137] From when the young are 40 days onward, the female eastern imperial eagle typically resumes hunting and takes to a perch nearby. Male does not appear to bring prey directly to the nest instead to nearby branch for female to dismantle.[18][134] One or two large prey items last 1–2 days but small prey may require up to 5-6 daily prey deliveries.[18][134] Fledgling may variously occur at anywhere from 63 to 77 days.[3] After leaving the nest the young linger near it for 2–3 days. Breeding success was once considerable in Russia at about 1.5 young per pair.[18]
Breeding success and survivorship
Breeding success is somewhat variable in the eastern imperial eagle. In the Czech Republic the 1-3 pairs found to be nesting in 1998-2009 had an average number of 1.53 fledglings.
Status
The eastern imperial eagle has declined greatly through history. At one time, people of
Today, dedicated conservationists and biologists are working extensively to remediate and reverse the decline of eastern imperial eagle. The species was uplisted to
In some areas of western Russia and in Kazakhstan, the eastern imperial eagle has maintained the nearest thing to a population stronghold. In 2011, the total estimated number of pairs was estimated at 3000-3500 in Russia and 3500-4000 in Kazakhstan. Though only 1534 breeding territories found firsthand, only some parts of the range were surveyed. According to Russian and Kazakh studies, cattle grazing in
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External links
- BirdLife species factsheet for Aquila heliaca
- "Aquila heliaca". Avibase.
- "Eastern imperial eagle media". Internet Bird Collection.
- Imperial eagle photo gallery at VIREO (Drexel University)
- Interactive range map of Aquila heliaca at IUCN Red List maps
- Audio recordings of Eastern imperial eagle on Xeno-canto.