The changeable hawk-eagle or crested hawk-eagle (Nisaetus cirrhatus) 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 (Hodgson, 1836) and separated from the New World species.
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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. Birds show a wide range of variation in plumage from pale to dark, varying with moult and age giving rise to the name "changeable".
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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 underpattern. 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.
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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 were found to average 1.36 kg (3.0 lb) while females averaged 1.6 kg (3.5 lb) but they could weigh in excess of 1.81 kg (4.0 lb).
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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 (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. 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.
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