Changeable hawk-eagle

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. Nonetheless, total lengths of up to 77 cm (30 in) 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 72 cm (28 in), N. c. limnaeetus at 70 cm (28 in) and N. c. andamanensis at 61 cm (24 in). 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). 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. 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. 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).

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. When perched, their wings reach only about one-third to halfway down tail. Pale morph adults are mainly dark brown above with very faint paler edges that are usually only conspicuous on the wing-coverts. They tend to have an evenly black-streaked and somewhat rufous-tinged head and neck, with a blackish crest (if present). The tail tends to be paler brown than the back with a thin whitish tip, a broad, blackish subterminal band (both the whitish tip and the subterminal band are also visible from tail underside) and 3 to 4 narrower, brown and often rather obscure bars. On the pale morph hawk-eagle's underside, the base colour is white to buff overlaid with bold black to dark brown streaking; the streaking tends to be more subtle on abdomen but more obvious on the legs. Beyond the typical pale morph, some subspecies but especially N. c. liminaeetus tend to have a further intermediate and a dark morph. The intermediate morph is somewhat similar to pale morph adults but is heavily grey-brown below with little to no paler base colour showing and more obscure streaking, with the area from belly down to the crissum usually being unpatterned. Meanwhile, the dark morph adult can range from all dark chocolate brown to almost pure black with variable browner edges, relieved only by the greyish inner half of tail as well as some greyish tail bars. Most juvenile changeable hawk-eagles are dark brown above but with far more conspicuous white edges on mantle and wings than the adults, in some cases, the median coverts are largely white and greater and even lesser coverts are largely scaled with white. The juvenile has a light brown tail with about seven thin dark bars and a whitish tip. The young hawk-eagle's head varies from buff with white-tipped black crest (as is the case in peninsular India and Sri Lanka) through entirely whitish, but almost always the young birds are spotted and streaked with black or dark brown about the rear crown and nape. As is the head, the underparts are variable with juveniles in much of India and in Sri Lanka showing thin brown streaks on chest or small spots on breast, with obscure tawny barring on thigh, legs and crissum. Juveniles elsewhere are often nearly all pure whitish below. By the time the young hawk-eagles reaches their 2nd to 3rd year, they tend to show less white above and more brown or black below. Their tail starts to molt to resemble that of adults in the 3rd year but in the 2nd year in N. c. limnaeetus (or at least in Philippines). Changeable hawk-eagles may attempt to breed at 3 years of age but full adult plumage is not obtained until the 4th year. Adults have yellow to orange yellow eyes, while those of the juvenile are grey-brown to pale greenish. Adult have a cere that's grey to pale greenish yellow and juveniles' ceres are dull greyish, while all ages have yellow feet.

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.

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).

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). 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. 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.