Species of bird
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The mountain hawk-eagle (Nisaetus nipalensis) or Hodgson's hawk-eagle, is a large bird of prey native to Asia. The latter name is in reference to the naturalist, Brian Houghton Hodgson, who described the species after collecting one himself in the Himalayas. A less widely recognized common English name is the feather-toed eagle. Like all eagles, it is in the family Accipitridae. Its feathered tarsus marks this species as a member of the subfamily Aquilinae. It is a confirmed breeding species in the northern part of the Indian subcontinent, from India, Nepal (hence the epithet "nipalensis") through Bangladesh to Thailand, Taiwan, Vietnam and Japan, although its distribution could be wider still as breeding species. Like other Asian hawk-eagles, this species was earlier treated under the genera of "Spizaetus" but genetic studies have shown this group to be paraphyletic, resulting in the Old World members being placed in "Nisaetus" (Hodgson, 1836) and separated from the New World species. As is typical of hawk-eagles, the mountain hawk-eagle is a forest dwelling opportunistic predator who readily varies its prey selection between birds, mammals and reptiles along with other vertebrates. Although classified currently as a least-concern species due its persistence over a rather wide distribution, this species is often quite rare and scarce and seems to be decreasing, especially in response to large-scale habitat degradation and deforestation.
Description.
Size and taxonomy.
The mountain hawk-eagle is a large raptor and fairly large eagle. Although described not infrequently as "slim", it is usually perceptibly bulkier and more massive than most other members of its genus. It is seemingly the largest member of the 10 currently recognized species in the genus " Nisaetus", notwithstanding the recently recognized Flores hawk-eagle ("Nisaetus floris") (which was separated from the changeable hawk-eagle). The latter critically endangered island hawk-eagle seems to be of broadly similar size (weight is unknown), albeit with shorter wings, however the Flores species seems to be linearly outmatched by and slimmer in build the largest mountain hawk-eagle. The mountain hawk-eagle attains a total length of and a wingspan of . Like most birds of prey, females are larger on average than the male, with a typical size difference of 3-8%, though it can rarely range up to a 21% difference. Although its wings are relatively short compared to eagles of open country, it has the longest wings of any of the hawk-eagles, even relative to their size. Mountain hawk-eagles have a short but strong bill, long and often erect crest (though can also be very short), short wings, a longish three-banded tail, feathered legs and powerful feet. It is usually rather unobtrusive, perching rather upright inside of canopy, with its wing-tips coming to less than one-fifth down the tail. There are two currently recognized races of the mountain hawk-eagle: the nominate subspecies ("N. n. nipalensis") and the subspecies native to Japan ("N. n. orientalis"). The nominate race is found throughout mainland range and includes the likely dubious southeast Chinese races of "N. n. fokiensis" and "N. n. whiteheadi". The average total length of the nominate subspecies is estimated to be . Among standard measurements in the nominate race, the wing chord of males ranges from while the female's ranges from . In both sexes, the tail ranges from and the tarsus from . 9 males of the nominate race were found to average in wing length, in tail length, in hallux claw length (the large rear talon often utilized by accipitrids as a killing tool), in tarsus length and in bill length. 13 females of the nominate were found to average in wing length, in tail length, in hallux claw length, in tarsus length and in bill length. One unsexed mountain hawk-eagle from northern India was found to weigh . A single male from the Yangtze area of east-central China was found to have weighed while two females from there weighed , suggesting size increases further north in this subspecies in accordance with Bergmann's rule.
The Japanese race averages about 9% larger than mainland race, and also has a proportionately longer tail and longer wings. The populations from Taiwan and the possible ones in Hainan are also probably part of this race. "N. n. orientalis" is generally paler than the nominate race with less heavy markings below. The underside in this race has a paler ground colour against much browner and darker barring, often showing less of the warmer or rufous tones typical of mainland mountain hawk-eagles. The throat often has reduced blackish streaking compared to mainland birds and the upper chest can be whitish and nearly unmarked. "N. n. orientalis" has black mottled or light streaked wing-linings and a small, often vestigial crest compared to the rather ample one of the mainland birds. Sometimes, "N. n. orientalis" is hypothesized to be a separate species. Among standard measurements wing chord of males ranges from while the female's ranges from . In both sexes, the tail ranges from and the tarsus from . From a sample of unknown size from the Suzuka Mountains, males of "N. n. orientalis" were found to average and females in total length. From the same sample, males had a mean wing chord length of , tail length of , culmen length of and tarsus length of . Meanwhile, females had a mean wing chord length of , tail length of , culmen length of and tarsus length of . Perhaps most surprisingly, the Suzuka mountain birds were not noticeably discrepant in body mass from known weights of mainland mountain hawk-eagles, especially similar to that of the apparently larger hawk-eagles from east-central China. The Suzuka sample as above found males to weigh from , with an average of , while females were found to weigh from , with an average of . Another Japanese survey found the smallest male to weigh , the lightest known weight known anywhere for the species.
At one time largish hawk-eagles found in Sri Lanka and southwestern India was deemed to be part of the mountain hawk-eagle species under the subspecies "N. n. kelaarti". A 2008 study based on the geographic isolation and differences in call suggest that this be treated as a full species, "Nisaetus kelaarti", Legge's hawk-eagle. The full species status of Legge's hawk-eagle appears to be further supported by DNA studies, with an average difference in mitochondrial DNA of 4.3% (usually the minimum difference to differentiate species is considered to be 1.5%). Although extremely isolated in distribution from true mountain hawk-eagles, Legge's hawk-eagle is physically distinct as well, often being much paler and less marked below with the throat stripes characteristics of the mountain species often absent (occasionally faint stripes may manifest) being instead largely plain buff about the throat. Like mainland mountain hawk-eagles, Legge's hawk-eagles have a strong crest. The hand in flight on a Legge's is often plain buff in colour (or with some very faint streaking) and the banded wing feathers are rather faded. Legge's hawk-eagle appears to be about 10% smaller than mountain hawk-eagles and was found to differ in almost all bodily proportions from mountain hawk-eagle, with relatively smaller wings but the smaller species also has a larger bill and larger talons than the mountain hawk-eagle.
Colouring and confusion species.
Adult mountain hawk-eagles are dark brown above with slightly paler edges, which tend to be clearest on median and greater coverts. On adults, the head is fairly rusty above with strong black streaks, though the volume of streaks tends to decrease on the neck, which in turn may suggest a rufous collar. The crest is largely black with a small buffy tip. Their tail is grey-brown, with a whitish tip and rather obscure blackish banding above. The malar area and throat are marked with blackish, ragged and sparse but rather bold stripes which contrast with the rest of their underside which is predominantly barred with rufous over a whitish ground colour. The barring continues, though the white base colour narrows and the rufous becomes a somewhat browner hue, down to the crissum and the legs. In some cases, the colour about the legs has appeared variously chestnut or even blackish. The underside of the tail is boldly banded with blackish and grey. The juvenile mountain hawk-eagle is also dark brown above but usually has clear cream to whitish feather edges causing the wing coverts to have a scaled effect; meanwhile, the feather bases of median and greater coverts form tawnier mid-wing patches. The juvenile's tail is thinly banded alternately with lighter and darker brown but usually have a whitish tip like the tail of the adult. The juvenile mountain hawk-eagle's underside is all plain buffy to tawny. The underside colour also extends to the head and part of the neck flanks while the crown, cheek, nape and hind-neck all streaked with dark brown. The crest is black with a small white tip. The juvenile leg feathers are whitish. The markings on the underside begin to develop by the 2nd year, starting from the flanks and gradually increasing inward to the breast, but the young hawk-eagles are still quite paler below until their 3rd year, which is also when the tail starts to resemble the adults. Full adult plumage is attained at no later than the 4th year. Adults have golden or even yellowish-orange eyes, with juveniles having pale bluish-grey to pale yellow eyes. In the adult the cere is blackish-grey, while in juveniles it is dull-grey. In all ages, the feet range from dull yellow to yellowish white. In flight, it is notable for its rather prominent head and relatively short rounded wings, an effect emphasized by their broad hands and bulging secondaries, which tend to pinch in at the rear bases. The mountain hawk-eagle is capable of fast, agile flight "with astonishing maneuverability". They usually glide with powerful, shallow beats interspersed with glides on level wings, but soaring birds hold their wings in a shallow V, pressed slightly forward. The wing linings of adults are a rusty similar to flank coloring, becoming paler on forepart and marked with dusky mottling which becomes darker mid-wing. Blackish-brown bars are apparent over greyish ground colour on the secondaries, the feathers here relatively broadly barred with blackish while the primaries are whiter based and darker tipped. Flying juveniles are fairly heavily mottled with white above. Juveniles show more buff to tawny colour below extending to their wing linings while the wing-tips are black, in some cases extending to primary coverts form a vague carpal arc. Juvenile flight feathers are whitish grey with thin and rather faded looking dusky barring, with less white showing at the base of the primaries. Juveniles in flight usually evidence a less distinct subterminal band than do adults.
Confusion of mountain hawk-eagle in all plumages is possible with pale morph changeable hawk-eagles ("Nisaetus cirrhatus"). However, the latter species only has a vestigal crest in most areas of overlap from northern India to southeast Asia. Furthermore, the changeable is a slighter, more slender bird with a relatively longer tail. The latter species also has narrower wings with more even trailing edges. While soaring, changeable hawk-eagles tend to have flatter wing shape than mountain hawk-eagles. Changeable adults also have streaking rather than heavy rusty barring on their underside, apart from subtle parts of wing linings and flanks, and also have narrower tail bars. In flight, the changeable also has clear white base to their primaries and less whitish on the rump when seen from above. Juveniles of the two species are more easily mistaken but wing proportions always differ, the mountain juveniles usually appear perceptibly bulkier and changeable juveniles (of relevant races) are generally much paler, rather than warm buffy to tawny, on the head and underparts. The mountain hawk-eagle also overlaps somewhat in range, in southeast Asia, with Blyth's hawk-eagle ("Nisaetus alboniger") and Wallace's hawk-eagles ("Nisaetus nanus") but both are much smaller and different in multiple ways (especially the bold black-and-white of adult Blyth's). Another, albeit unlikely, potential source of confusion for the mountain hawk-eagle is with Jerdon's baza ("Aviceda jerdoni"), which is far smaller, and of a far more compact and chunky build. The baza is somewhat similar in marking to adult mountain hawk-eagles, but the baza lacks feathered legs and has relatively much longer and differently shaped wings. Mountain hawk-eagles can usually be told from the slighter, smaller crested honey buzzard ("Pernis