The changeable hawk-eagle (Nisaetus cirrhatus) or crested hawk-eagle is a large bird of prey species of the family Accipitridae. More informal or antiquated English common names include the marsh hawk-eagle or Indian crested hawk-eagle. It is a member of the subfamily Aquilinae, with signature feathers, absent in tropical raptors from outside this subfamily, covering the tarsus. It was formerly placed in the genus Spizaetus, but studies pointed to the group being paraphyletic resulting in the Old World members being placed in Nisaetus and separated from the New World species. It is a typical "hawk-eagle" in that it is an agile forest-dwelling predator and like many such eagles readily varies its prey selection between birds, mammals or reptiles as well as other vertebrates. Among the members of its genus, the changeable hawk-eagle stands out as the most widely distributed, adaptable and abundant species.

Taxonomy

The changeable hawk-eagle was formally described in 1788 by the German naturalist Johann Friedrich Gmelin in his revised and expanded edition of Carl Linnaeus's Systema Naturae. He placed it with the eagles, hawks and relatives in the genus Falco and coined the binomial name Falco cirrhatus. Gmelin's description was based on the "crested Indian falcon" or the Falco Indicus cirratus that had been first described in 1676 by the English naturalists Francis Willughby and John Ray from a live bird kept in the Royal Aviary in St James's Park, London. The changeable hawk-eagle was formerly placed in the genus Spizaetus but it and nine other Old World hawk-eagles were moved to the resurrected genus Nisaetus following the publication in 2005 and 2007 of two molecular phylogentic studies. The genus Nisaetus had been introduced in 1836 by the English naturalist Brian Houghton Hodgson. The genus name Nisaetus combines the Medieval Latin nisus for a sparrowhawk with the Ancient Greek aetos meaning "eagle". The specific epithet cirrhatus is from Latin cirratus meaning "curly-headed".

The taxonomy of the wide-ranging changeable hawk-eagle is complex and confusing, with few authorities agreeing on whether the species in fact houses a species complex. Gamauf et al. (2005) analyzed mtDNA cytochrome b and control region sequence data of a considerable number of specimens of the crested hawk-eagle and some relatives. Despite the large sample, even the most conspicuous dichotomy - that between the crested and crestless groups - was not as well resolved as it might have been expected to be. The Flores and changeable hawk-eagles are regarded as sister species. The three small-island taxa (N. c. andamanensis, N. c. vanheurni, and N. floris) as a whole each appear as monophyletic lineages. Their placement is even more unresolved, with N. floris being apparently a very ancient lineage. The other two seem quite certainly to derive from N. c. limnaeetus. The latter taxon has a confusing phylogeny. Different lineages exist that are apparently not stable in space and time, are best described as polytomy, from which the similar island taxa derive. Dark morphs exist for some populations.

Subspecies

thumb|In flight, changeable hawk-eagles have distinctly short but broad wings with dark barring on flight feathers and a long tail.

Changeable hawk-eagle group

  • N. c. cirrhatus (Gmelin, JF, 1788) : The nominate subspecies is found in the peninsular India, from the Gangetic plain southwards throughout the remainder of the country. It is monomorphic in colour with no dark morph. The nominate subspecies is relatively pale above compared to other races, with a slightly more rufous hue to the head. It tends to be less extensively streaked below down to the lower breast below compared to other races, with dark wing lings and a strongly barred tail. In the juvenile plumage, the nominate race has a strongly buffy headed and has underparts that are flecked and spotted. The crest tends to be longest in this race, with common measurements of in length, therefore the race is commonly referred to as the crested hawk-eagle. Furthermore, in both sexes tail length is from and tarsus length from .

Crestless changeable hawk-eagle group

thumb|A crestless changeable hawk-eagle (N. c. limnaeetus) in [[Singapore.]]

  • N. c. limnaeetus (Horsfield, 1821): This race is found through the Himalayan foothills from Nepal, northeast India, via Burma to much of southeast Asia including the Malay Peninsula along Wallace Line to Philippines and the Greater Sunda Islands. This race can appear similar in size and build to the nominate race but unlike it is rather polymorphic and this is the most widely found of subspecies. This race's polymorphism is the source for the species being referred to as the changeable hawk-eagle. A sample of 6 females averaged in the length of the rear hallux-claw, the main killing apparatus of most accipitrids, while 7 males averaged .
  • N. c. andamanensis (Tytler, 1865): This race is found in the Andaman Islands. The subspecies is darker than the nominate subspecies and more similar to N. c. limnaeetus in hue but does not appear to have a full dark morph as is known in that race. Andamanian juveniles tend to have a rather white head and underpart colour. A crest may variably be present but is usually only up to . This race is small, a little smaller even than N. c. ceylanensis. Wing chord length ranges from in males and from in females. Nonetheless, total lengths of up to have been listed for N. c. limnaeetus in Nepal. Ali & Ripley (1978) estimated these average total lengths for the following subspecies: N. c. cirrhatus at , N. c. limnaeetus at and N. c. andamanensis at . Weights in this species have been reported from but the source of this is unclear and it probably underrepresents the size variation known to occur in the species. The only precise body masses known for the species are derived from the Philippine population, where males average and females average but they could weigh in excess of .

right|thumb|A crestless rare dark morph in flight in [[Sunderbans National Park]]

thumb|A juvenile in [[Bundala National Park]]

Adult changeable hawk-eagles are typically dark brown above and boldly streaked below with a strong bill, a variably sized, often floppy crest or no crest, rather short wings, a quite long, thinly-barred tail and long feathered legs. This species tends to perch bolt upright, and may perch in various places from somewhat secluded spots to also quite open area as well.

Voice

The changeable hawk-eagle is often largely silent but in breeding season it may readily call, both from their perch or on the wing. Like many diurnal raptors, their calls are a form of high-pitched scream. The shrill ringing and loud call of the changeable hawk-eagle is various described in pattern of yeep-yip-yip-yip, rising kwip-kwip-kwip-kwee-ah and penetrating klee-leeuw (for birds from the Sunda islands). Ascending kri-kri-kri-kree-ah and kreeee-krit with much stress on the elongated first syllable are similar. From western birds from India and Sri Lanka, the call is a slightly different ki-ki-ki-ki-ki-ki-ki-keee, beginning short, rising in crescendo and ending in long, drawn-out scream. In northern India and Malaysia, the calls of this species have variously been compared to those of the Eurasian curlew (Numenius arquatus) and the crested serpent eagle (Spilornis cheela).

Confusion species

thumb|The formidable talons of the changeable hawk-eagle

The changeable hawk-eagle is almost always the most common and/or most widely spread Nisaetus species anywhere in their range. Their distribution overlaps with the entirety of the distribution of the (newly recognized) Legge's hawk-eagle (N. kelaarti), most of the range of the Javan (N. bartelsi), Blyth's (N. alboniger) and Wallace's hawk-eagle (N. nanus) and partially overlaps with the ranges of the mountain hawk-eagle (N. nipalensis), the Philippine hawk-eagle (N. philippinus) (on Mindoro and possibly Palawan) and Pinsker's hawk-eagle (N. pinskeri) (on Mindanao). Only two species of the genus Nisaetus are outside the changeable hawk-eagle's normal distribution (including their own former subspecies, the Flores hawk-eagle). As many honey buzzards are thought to mimic more powerful raptors to protect themselves from predation, the crested honey buzzard (Pernis ptilorhynchus) is thought to mimic the general appearance of Nisaetus hawk-eagles but has a distinctly smaller head and longer and narrower wings than changeable hawk-eagles. Adult crested honey buzzards are barred but the juvenile is streaked rather like the changeable hawk-eagle, however if seen well the honey buzzard generally looks much more solidly orange-buffy as a base colour rather than whitish below. The flight actions of the honey buzzard are also distinct, with a more robotic even flap during flights.

Juvenile crested serpent eagles, which are unlikely to be mistaken for the changeable other than at a distance and in flight, appear chunkier and less rangy with a bigger head, slightly longer wings and a substantially shorter tailed with fewer bars (these differences in proportions are generally applicable to various island serpent-eagle species that may be found with changeable hawk-eagles as well). Juvenile rufous-bellied eagles (Lophotriorchis kienerii) are rather smaller and more compact with a relatively longer winged and shorter tailed appearance. The rufous-bellied juvenile when compared to the juvenile changeable is generally purer white looking below which contrasts more strongly with their sparse blackish streaks. Dark morph changeable hawk-eagles may be confused with the similarly sized but even more slender black eagle (Ictinaetus malaiensis). However, the latter is much longer winged with distinctly pinched-in bases, a uniformly dark tail and has small light feather bases only to primaries. Also dark morph can be told from dark morph booted eagles (Hieraeetus pennatus) by the latter being rather smaller, much shorter tailed and having relatively longer and more rectangular wings. Dark morph booted eagles are also grey-brown or cinnamon from below on the tail and have pale wedges on the underside of the primaries.

Behaviour and ecology

thumb|Changeable hawk-eagle eating [[grey junglefowl in Bandipur National Park]]

thumb|Changeable hawk-eagle eating an egret in Sri Lanka

thumb|Changeable hawk-eagle with Indian garden lizard in [[Satpura National Park]]

Changeable hawk-eagles are at home in a variety of wooded and semi-open habitats. Their physical form and flight style is typical of forest-dwelling raptors in general and is often compared to the features of true hawks or Accipiters in particular larger species such as goshawks. Like most other forest raptors, changeable 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. Even compared to the variable prey of large goshawks, the prey selection of changeable hawk-eagles appears to be somewhat indiscriminate and opportunistic. Unfortunately, compared to tropical raptors from the neotropics and especially Africa, the life histories of raptors from tropical Asia are generally quite poorly-known, even in the case of easily observed raptors such as changeable hawk-eagles. Therefore, what is known of changeable hawk-eagles dietary biology is largely drawn together based on reliable eye-witness and anecdotal accounts, photographic evidence and wide-ranging bird census surveys, rather than direct, extensive study. Like many tropical forest raptors, they are primarily ambush predators who use concealing foliage to still-hunt from hidden branch or open branch with a leafy background, pouncing fast to take most of their prey on the ground. Like goshawks, changeable hawk-eagles often perch-hunts which are short, low-level flights from perch to perch interspersed with brief pauses, during which they scan for potential prey. Perch-hunting lends them the greatest success when hunting birds that they will capture in the trees. Primary prey by class differed in West Java, where 62% of prey selected was reptiles, 24% birds and 12% mammals. Elsewhere in India, prey selection at the nest showed what prey species were selected by changeable hawk-eagles but lacked any quantitative data or studies on prey biomass. One study in Gujarat showed several rather small prey species largely being taken such as the Indian chameleon (Chamaeleo zeylanicus), Oriental garden lizard (Calotes versicolor), other lizards, common myna (Acridotheres tristis), red-vented bulbul (Pycnonotus cafer), other birds (including unidentified parakeets and gamebirds), Indian bush rat (Golunda ellioti) and a palm squirrel. The identified prey would vary in size only from for a garden lizard to for a myna with the small lizards and chameleons reportedly the most often delivered prey. In Mudumalai National Park, only three prey types were specified to genera which consisted of black-hooded oriole (Orolus xanthornus), common bronzeback snake (Dendrelaphis tristis) and an unidentified giant squirrel.

The upper size limit for changeable hawk-eagle prey seems to be fairly liquid and the species is quite the equal of various other booted eagles in making bold attacks on prey of their own size or larger. Even more impressive accounts and photos show that changeable hawk-eagle can hunt and kill adult mammals with formidable defenses such as felids and primates, although it cannot be ruled out that they will usually attack infirm or injured specimens rather than healthy ones. Furthermore, most accounts of attacks on Old World monkeys show juveniles are by far the most vulnerable members of their troops to hawk-eagle attacks. Cases of either verified, likely or potential predation have been reported on northern pig-tailed macaque (Macaca leonina), crab-eating macaque (Macaca fascicularis), toque macaque (Macaca sinica) and lion-tailed macaque (Macaca silenus), including specimens potentially to weigh up to an estimated . A case of predation was photographed by a changeable hawk-eagle on an estimated juvenile banded langur (Presbytis femoralis) while possible cases have been reported of predation on juveniles of other langurs as well as additional larger primates such as gibbons and proboscis monkeys (Nasalis larvatus). While their predator-prey relationship is even more nebulous, the peculiar, smaller but toxic nocturnal primates known as slow lorises are known to fall prey as well to changeable hawk-eagles. There are several accounts of predation by changeable hawk-eagles on domestic cats (Felis catus), though largely kittens, in addition to a case of a large adult jungle cat (Felis chaus), with an estimated median weight of , being attacked and killed by one of these hawk-eagles. Apparently, the changeable hawk-eagle is also counted among the predators of calves of Indian gazelles (Gazella bennetti). Adults of large monitor lizards of a few species may be also included in their prey spectrum. Avian kills include adult Indian peafowl (Pavo cristatus), of an estimated weight of up to , and the species is considered a threat as well to the similar green peafowl (Pavo muticus). Attempted predation on both adult and young giant ibis (Thaumatibis gigantea) have been recorded but all known attacks were unsuccessful. Cases of scavenging on carcasses of various deer and monkey have been reported as well, in one case a hawk-eagle was filmed chasing an adult yellow-throated marten (Martes flavigula) from a carcass. Changeable hawk-eagles were also observing preying on predating on Oriental pied hornbills (Anthracoceros albirostris).

Food and feeding

The habitat selection and overall distribution of the changeable hawk-eagle is largely concurrent with other largish raptors such as the crested serpent eagle and the crested honey buzzard, apparently the three species are largely tolerant and non-aggressive towards each other, perhaps surprisingly given the otherwise aggressive habits of hawk-eagles. Unsurprisingly, these species have quite discrepant ecological niches, especially in terms of their dietary habits. They are also found living alongside the much larger Philippine eagle (Pithecophaga jefferyi) and are likely to similarly take smaller prey and avoid direct encounters with the much less numerous giant eagle. Apparently, changeable hawk-eagle are considered likely predators of the slightly smaller Indian spotted eagles (Clanga hastata). However, on the contrary, one case of predation was reported in captivity when a barred eagle-owl (Bubo sumatranus), which is found in the wild with changeable hawk-eagles in southeast Asia, killed a changeable hawk-eagle in an aviary.

Breeding

The changeable hawk-eagle is, like many raptorial birds, a largely solitary bird otherwise but during breeding, stays in a dedicated pairs that often mate for life. Each pair engages in a territorial display over the fringes of their home range. This aerial display is usually engaged in by a male but sometimes the female or both members of the pair will engage in displays, often starting with their wings and tail arched upwards in exaggerated poises. During the display, the shoot up vertically and nose-diving or stooping, if the rival hawk-eagle continues to engage the defending individual, they will fly at each other at "lightening speed" doing a complete loop-the-loop turn in air. They often call loudly throughout the display. The parents tend to allow closer approach by humans when living in vicinity of villages. In Java, the changeable hawk-eagle was found to persist in all seven habitat types available on the islands, whereas the Javan hawk-eagle was confined to only four habitat types: the deeper, primary forested areas.