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. Individuals show a wide range of variation in plumage from pale to dark, varying with moult and age giving rise to the name "changeable".
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 hawks 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. 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. 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. 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.