Is Hawk Eagle

[Lorin Searing]

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.[8] 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.[9][10][11] 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.[12][13][14] The genus Nisaetus had been introduced in 1836 by the English naturalist Brian Houghton Hodgson.[15] 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".[16]

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.[17] 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.[17] At least one widely accepted genetic study has resulted in a recent revision to the recognition of a new species, as the Flores hawk-eagle has traditionally been treated as a subspecies of the changeable hawk-eagle, but it is now often treated as a separate species, N. floris.[18] The Flores and changeable hawk-eagles are regarded as sister species.[19] 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.[17][19] Obviously, N. c. limnaeetus does not represent a monophyletic lineage. Neither the biological nor the phylogenetic species concepts, nor phylogenetic systematics can be applied to satisfaction. The crested group apparently is close to becoming a distinct species. The island taxa derived from N. c. limnaeetus appear to have undergone founder effects, which has restricted their genetic diversity. In the continental population, genetic diversity is considerable, and the evolutionary pattern of the two studied genes did not agree, and neither did the origin of specimens show clear structures. N. c. limnaeetus thus is best considered a metapopulation.[17]

Gamauf et al. (2005) therefore suggest the island taxa which are obviously at higher risk of extinction are, for conservation considered evolutionary significant units regardless of their systematic status. This case also demonstrates that a too-rigid interpretation of cladistics and the desire for monophyletic taxa, as well as universal application of single-species concept to all birds will undermine correct understanding of evolutionary relationships. It would even not be inconceivable to find mainland lineages to group closely with the western island taxa, if little genetic drift had occurred in the initial population. nonetheless, the divergence of this species' lineages seems to have taken place too recently to award them species status, as compared to the level of genetic divergence at which clades are usually considered distinct species.[17] N. c. limnaeetus appears for all that can be said with reasonable certainty basal pool of lineages in the crestless group that, despite not being monophyletic, should be considered a valid taxon as long as gene flow is possible through its range. In addition, as ancient DNA from museum specimens was used extensively, the possibility of ghost lineages must be considered. If it is assumed that all or most of the ancient lineages still exist today, considerable recombination must have taken place as the two genes' phylogenies do not agree much, indicating a healthy level of gene flow. Whether this still holds true today remains to be determined.[17]

Two distinct groups exist in the changeable hawk-eagle; one with crests and one without or with hardly visible crests. A 2020 study found that the reproductive isolation between the two groups was weak and recommended treating the two groups as conspecific.[20] Dark morphs exist for some populations.[21]

The changeable hawk-eagle is a largish but slender eagle. They fall near the middle of sizes among the currently accepted species in the genus Nisaetus. As in most birds of prey, females are larger than males with an average overall size difference of 7% but this can individually range to an 18-22% difference, with island races apparently thought to be less dimorphic on average. Size is quite variable and total length has been reported in the past to vary from 51 to 82 cm (20 to 32 in) and wingspan from 100 to 160 cm (39 to 63 in), however these figures appear to include the much more massive hawk-eagles from Flores that are currently considered their own separate species by modern authorities.[3][26] Nonetheless, total lengths of up to 77 cm (30 in) have been listed for N. c. limnaeetus in Nepal.[27] Ali & Ripley (1978) estimated these average total lengths for the following subspecies: N. c. cirrhatus at 72 cm (28 in), N. c. limnaeetus at 70 cm (28 in) and N. c. andamanensis at 61 cm (24 in).[23] Legge (1880) measured the length of Sri Lankan changeable hawk-eagles (N. c. ceylanensis) without including the beak as 55 to 60 cm (22 to 24 in).[22] The average length of birds from the Philippines (N. c. limnaeetus) was measured as 58.4 cm (23.0 in) in males and 64.9 cm (25.6 in) in females.[28] Weights in this species have been reported from 1.2 to 1.9 kg (2.6 to 4.2 lb) but the source of this is unclear and it probably underrepresents the size variation known to occur in the species.[5][26][29] The only precise body masses known for the species are derived from the Philippine population, where males average 1.36 kg (3.0 lb) and females average 1.6 kg (3.5 lb) but they could weigh in excess of 1.81 kg (4.0 lb).[28][30]

In flight, the changeable hawk-eagles is a large raptor with a prominent head, rather short rounded and broad wings, longish squarish or rounded tail, but has somewhat slenderer wings and straighter trailing edges than sympatric species of hawk-eagles. The species tends to fly with a fast agile flight, showing powerful shallow beats interspersed with glides on flat or bowed wings with their carpals well forward (above level of bill) and primaries swept back. When soaring, the wings are gently lowered or sometimes held level, with the carpals again well forward. In pale morph adults in flight, their hand in flight may be variously dark brownish buff (as in peninsular India) to a much paler buff or whitish. On the flight feathers, the area from wingtips extending to the primary and secondary feathers often have blackish barring, forming ragged lines from the carpals to rear axillaries with variable other dark marks elsewhere. Adult intermediate morph hawk-eagles have similar flight feathers but have grey-brown underparts with a less distinct, contrasting under-pattern. Meanwhile, in both juvenile and adult dark morph, the blackish-brown colour of the body extends to the hand but the base of their tail, their primaries and, less so, their secondaries are a much paler, contrasting grey with streaking similar to other changeable hawk-eagles. Typical juveniles show large areas of whitish streaking or mottling seen from above in flight. Juveniles from much of India and Sri Lanka show extensive darker tawny but obscure barring above and below, while other races are much whiter. Much like adult, the juvenile has dark tips to primary and greater coverts produce thin ragged diagonal bars but the barring tends to be thinner.[3][31]

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).[3][5][32]

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).[1] In range with most other Nisaetus species (such as the islands or mainland of southeast Asia), the changeable hawk-eagle is more likely to be almost crestless. Other species tend to have proportionately broader wings with more bulging secondaries, relatively shorter tails and, as adults, more barred underbody and unique tail patterns. Adult colour patterns can range from somewhat different to boldly distinct (especially in the black-and-white Blyth's hawk-eagle). Juveniles tend to be more difficult to distinguish but usually most other Nisaetus have less white showing than relevant changeable hawk-eagles and the changeable tends to show a slightly stronger V while in flight. The mountain hawk-eagle, Flores hawk-eagle (which is the only hawk-eagle in its small-island range) and Legge's hawk-eagle, in decreasing magnitude of size, are all are larger and bulkier than the changeable hawk-eagle whereas other Nisaetus species are smaller to varying degrees, distinctly so in the Wallace's hawk-eagle and Blyth's hawk-eagle.[3][33][34] 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.[3][33][35] 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.[3][5][31][33]

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