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
thumb|Mountain hawk-eagles are the largest and most robust of the hawk-eagles in the genus Nisaetus.
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.
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. 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 .
thumb|Drawing of the Japanese race by Yūshi Ishizaki, from the 1830s.
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. 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.
thumb|Close-up of a mountain hawk-eagle from China.
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 ptilorhynchus), beyond the latter being polymorphic, as even most similarly plumaged individual honey buzzards have bare legs, much smaller and slimmer head and bill with a longer neck and deeper wing beats on relatively longer, more slender wings.
Vocalisation
Mountain hawk-eagles are silent apart from their breeding season. Their call is a shrill treble note, with a quality often compared to a penny whistle. Their typical call is often likened to the klu-weet-weet of a green sandpiper (Tringa ochropus) or the kee-kikik of the common green magpie (Cissa chinensis). The calls of mountain hawk-eagles are said to be expertly mimicked by drongos in some parts of the range.
Distribution and habitat
thumb|left|Mountain hawk-eagles often live in cooler, more temperate habitats including deciduous forest than other hawk-eagles.
Both the northern and southern limits of this widely found raptor are surprisingly poorly known to this day, with historic records suggesting that the species may take up residence hundreds of kilometers north of its accepted range and year-around reports of this species from areas formerly considered only to be visited by wintering migrant or vagrant hawk-eagles. The mountain hawk-eagle has been recorded under the status of "rare breeder" in areas much farther north than is conventionally accepted as part of their range, such as far eastern Mongolia and the landlocked, extreme southern part of the Russian Far East such as in Primorsky Krai. To this date, the IUCN has not updated the range maps for mountain hawk-eagles to reflect the species' presence in these areas, although their status as continual breeders here may still need confirmation. Through much of their range, mountain hawk-eagles are typically sedentary but both adults and young hawk-eagles sometimes also disperse in descent from higher grounds in winter and it may be characterized as a partial migrant. There are several recorded cases of the species wandering in north India down into Indo-Gangetic plains. In Bhutan, fragmentary information suggest short-distance altitudinal movements are not infrequent. Relatively low volumes of migrant mountain hawk-eagles were detected in Nepal, however. With a fair amount of consistency, the hawk-eagles found in the northern part of southeast Asia range into more lowland areas of Burma, eastern Thailand and peninsular Malaysia. There are also similar movements to lowlands in Japan with some Japanese ones moving to the Korean peninsula. In some of the areas above such as Thailand and Malaysia (mainly far northern part of country), year-around reports of mountain hawk-eagles may suggest small, isolated pockets of residency and/or breeding occurring. Mountain hawk-eagles have been recorded as a vagrant in Hong Kong and Cambodia. Broader vagrancy has been reported in the case of a mountain hawk-eagle that turned up in the island of Borneo.
The mountain hawk-eagle tends to reside in dense hill and montane forests at any point up to the tree line. They are mainly found in various wooded foothills. Typically, primary evergreen or mixed forests are preferred, with the presence of nearby streams a plus. The central part of the range falls in subtropical broadleaf forest made cooler by their high elevations but they can range anywhere from temperate mixed forests to tropical rainforests. However, in some areas the species can range into second growth. In non-breeding times, mountain hawk-eagles may sometimes wander through wooded plains and briefly near fairly developed villages (though usually more secluded and primitive one) and even cities. Elevations the species has been known to live at in the Himalayas are mostly above sea-level. However, they've been recorded at elevations of up to in northern Yunnan. In Japan, they usually reside somewhat lower than in their mainland haunts, typical at between . Mostly only during winter, the species has been recorded (albeit seldom) down to or even lower as vagrant.
Behaviour and ecology
thumb|A mountain hawk-eagle stooping down among the trees in [[Uttarakhand.]]
Mountain hawk-eagles are well adapted to living in forests. As is the case for all Nisaetus species, their physical form and flight style is typical of forest-dwelling raptors in general and is often found to be roughly comparable to the features of true hawks or Accipiters in particular larger species such as the occasionally sympatric Eurasian Goshawk (Accipiter Gentilis). Like most other forest raptors, mountain hawk-eagles (and Nisaetus species in general) have a long tail, short broad wings and relatively long but powerful legs, all of which impart greater maneuverability and quicker strike times in denser wooded hunting grounds than other raptorial body plans. The common name hawk-eagle is apparently in reference to their similar adaptations to true hawks. Contrary to the suggestion that, based on their physiology, especially their longer wings and tarsus but shorter talon and bill length, when physically compared to the Legge's hawk-eagle implies that the mountain hawk-eagle is morphologically adapted to hunting birds more so than mammals, dietary studies indicate that the mountain hawk-eagle is not necessarily a specialized bird predator but rather a generalist and opportunist like many predators. In fact the small handful of dietary studies of the species show that the mountain hawk-eagle somewhat prefer small mammals as prey but readily takes both birds and reptiles given the opportunity. Typically, the mountain hawk-eagle tends to still-hunt from a concealed perch in foliage, stooping down to take prey. Most prey is taken on the ground. A study in the Hyōgo Prefecture of Japan reviewed 142 prey items of a single breeding pair. Quantitatively, most prey deliveries by this pair were rather (almost surprisingly) small in body size, whether this is typical of Japanese hawk-eagles is not clear given the lack of comprehensive study. Quantitatively rare prey items that have been recorded have including amphibians and, recorded only once, fish.
thumb|left|Small to medium-sized mammals such as the [[Japanese hare seem to form the bulk of the mountain hawk-eagle's diet.]]
While most of the prey mentioned above is of relatively modest size, the mountain hawk-eagle is not infrequently reported to attack prey of quite large sizes, including prey equal to their own size or larger. Mountain hawk-eagles have been reported to attack young ungulates but often relatively very young and small ones, probably close to a neonatal state. In Taiwan, they took the young of Formosan serow (Capricornis swinhoei) that were estimated to weigh on average under . Scavenging of sika deer killed by human hunters has also been reported. Larger avian prey has been taken by mountain hawk-eagles, including adult Indian peafowl (Pavo cristatus) weighing up to an estimated .
thumb|left|A mountain hawk-eagle in the little-known (and cartographically undescribed) population in [[Primorsky Krai in Russia.]]
Carnivorans taken by mountain hawk-eagles can also be relatively large as well as potentially dangerous. An estimated kit of a Japanese badger (Meles anakuma) was preyed upon by a female. Meanwhile, four adult Chinese ferret-badgers (Melogale moschata), weighing on average about , were taken in Taiwan, while a Japanese marten (Martes melampus) of the same estimated weight was taken there by a female hawk-eagle. More impressive carnivoran prey dispatched by this species included an adult yellow-throated marten (Martes flavigula) weighing an estimated , an adult red panda (Ailurus fulgens) weighing an estimated and reportedly adult raccoon dogs (Nyctereutes procyonoides), which weigh an average of , not to mention (in a similar size range) an occasional domestic cat (Felis silvestris catus) taken by this species. Of a similarly impressive nature in size and defensive temperament are primates, of which the mountain hawk-eagle is an occasional predator. However, a rather large portion of primate prey, such as monkeys, are taken as infants or juveniles, and most but not all adults killed by them are perhaps are likely to be previously injured or sickly. Taking even infant monkeys can be provide some risk for hunting hawk-eagles due to the protective nature of mothers as well as the overall monkey troops. For example, Formosan rock macaques (Macaca cyclopis) recorded to be taken in Taiwan were infants, weighing only an estimated . The mountain hawk-eagle is also considered a potential or confirmed threat to some larger primates (though largely or entirely younger, more vulnerable members of their troops) including: the François' langur (Trachypithecus francoisi), the black snub-nosed monkey (Rhinopithecus bieti), the lar gibbon (Hylobates lar) and the eastern hoolock gibbon (Hoolock leuconedys). However, the most impressive primate kill was an adult Japanese macaque (Macaca fuscata), estimated to weigh somewhere between , that was taken alive and subsequently dismantled by a large (presumed female) mountain hawk-eagle.
Interspecies predatory relationships
thumb|right|A mountain hawk-eagle in flight.
The mountain hawk-eagle overlaps in distribution with several other eagles, including about three species of Aquila and three species of Haliaeetus that are broadly similar in size to them as well as one slightly (in Japan) to notably (mainland) larger Aquila, the golden eagle (Aquila chrysaetos), and two much larger Haliaeetus species. However, the mountain hawk-eagle is the largest eagle in its range to live mostly within the confines of forest habitats, thus habitat differences against other larger eagles would normally provide ample partitioning and lessen competition. In some of its range, the mountain hawk-eagle overlaps and shares forests with three other Nisaetus species and, often, with various species of Accipiter, the latter of which lead a comparable lifestyle but are far smaller and more agile. However, usually these other forest raptors can co-exist with the larger raptor by focusing on more generalized and usually smaller prey, largely birds but also reptiles and amphibians, than the mammals seemingly preferred by the mountain hawk-eagle. Larger owls sometimes also occur in the mountain hawk-eagles range and are a potential source of competition (excluding the fish owls, which are more restricted by diet) despite the temporal partitioning implied in their nocturnal habits. Although the Eurasian eagle-owl (Bubo bubo) usually prefers more open and rockier environments, the little-known spot-bellied eagle owl (Bubo nipalensis) has a very similar central distribution and habitat preferences as the mountain hawk-eagle, and despite being slightly smaller, it goes after exceptionally large prey with perhaps even more aplomb.
In Japan, mountain hawk-eagles are regarded as the fourth largest eagle after the Steller's sea eagle (Haliaeetus pelagicus), the white-tailed eagle (Haliaeetus albicilla) and the golden eagle. Despite the golden eagle race in Japan (A. c. japonica) being much smaller than other races and the larger size of Japanese mountain hawk-eagles, the golden eagle here still has a slight size advantage (about 7% larger) and much larger wings which gives them an advantage at fighting and hunting in open air but less maneuverability. While the golden is more a bird of open and rocky environments, the two species prey selection overlaps here (probably more so than mainland populations of the two species), with both taking Japanese hares supplemented by pheasants whenever possible, and this can cause a level of direct competition despite their different preferred habitats. On the other hand, a mountain hawk-eagle may have preyed on the young of the black eagle (Ictinaetus malaiensis) in Taiwan. The mountain hawk-eagle is an occasional predator of a wide diversity of owls. Owl prey known to have been taken has included Asian barred owlet (Glaucidium cuculoides), jungle owlet (Glaucidium radiatum), brown boobook (Ninox scutulata), Eastern barn owl (Tyto javanica), Ural owl (Strix uralensis) and, once reportedly, even a Eurasian eagle-owl, a species of similar size and power to the hawk-eagle itself.
Breeding
thumb|left|A female mountain hawk-eagle in [[Japan dismantles prey on her nest.]]
The mountain hawk-eagle maintain their home range with a rather spectacular aerial display. Display activities tend to peak within the time period prior to breeding. Their aerial display includes conspicuous and often noisy high circling, both single and mutual, and undulating sky dance of steep dives and climbs with bubbling call uttered at each peak. Like many raptors, the display is likely largely to proclaim ownership to conspecifics but also probably has some function in reinforcing existing pair bonds. Clutch size is usually 1 or 2 but up to 3 eggs in a clutch have been reported in Japan. Cousins such as the Legge's and changeable hawk-eagle do not typically display any aggression or, if so, are very mild in protective behaviour towards humans while nesting. In one case, a local woman in the Kumaon division of northern India fell victim to a "particularly savage" attack by a female mountain hawk-eagle and subsequently died from the injuries sustained. In India, the minimum amount of time from hatching to leaving the nest was claimed as 53 days.
Status
thumb|right|A captive specimen of the threatened Japanese subspecies, N. n. orientalis.
Though it is not considered a globally threatened species, the mountain hawk-eagle is never more than uncommon to rare locally. It occurs over a rather large distributional range that apparently extends over 19 million square kilometers. However, in estimations from the late 1990s, it was considered doubtful that the density of mountain hawk-eagle was high enough to reach 10,000 individuals, even with the now-separated Legge's hawk-eagle included at that time and all juveniles. Likely more localized and minor threats from humans are persecution as occasional killers of domestic poultry. Despite their popularity in Asian falconry, it is unlikely gathering of hawk-eagles for this is a significant problem. Lead poisoning from consuming carcasses of sika deer, left there by human hunters using lead bullets, have resulted in the death of some mountain hawk-eagles. Similar threats are faced by all Nisaetus hawk-eagles, with only the changeable hawk-eagle shown to be resilient to human interferences and not in decline at the species level. As the species is a K-strategist like all eagles, it was feared that the ongoing population reduction of N. n. orientalis might lead to loss of genetic diversity, and consequently inbreeding depression. However, genetic diversity was shown to be still considerable at present.