Changeable hawk-eagle
The changeable hawk-eagle 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 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. 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 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.
Gamauf et al. 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. 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.
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. Dark morphs exist for some populations.
Subspecies
Changeable hawk-eagle group- N. c. cirrhatus : 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. It is one of the two larger subspecies. Wing chord length ranges from in males and from in females. In both sexes, the tail commonly measures and tarsus from.
- N. c. ceylanensis : This race is endemic to Sri Lanka but some authorities also include Travancore in the southern tip of India as part of this race's range. This race has a proportionally longer crest on average than the nominate race, with a similar crest length to it at about. It is monomorphic as is the nominate race, averaging paler and less streaky, but a similar appearance has been noted to birds from southernmost India are rather similar in appearance. The differences represented in this race from the nominate race are largely clinal. It is rather smaller than the nominate race. Wing chord length ranges from in males and from in females. Furthermore, in both sexes tail length is from and tarsus length from.
- N. c. limnaeetus : 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. The pale morph of this race is still darker than the nominate race found further south in India. However, pale morphs from the Philippines tend to have a paler head and neck. Characteristically, this race is heavily streaked below, while juveniles look quite different as their head and underparts are largely white. Dark morph individuals tend to be chocolate-brown all over, although the tail base might appear lighter in flight. Some birds of this race do have a vestigial crest of merely in length but many appear to completely lack the crest. This race is generally similar in size to the nominate race but is more variable in size over its large distribution within the southernmost island birds averaging smallest while those found in the Himalayan foothills are arguably the largest hawk-eagles known in the species. Wing chord length ranges from in males and from in females. In limited samples, tail length was reported as and tarsus length from. 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 : 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.
- N. c. vanheurni : This race is native to Simeulue Island. It is similar in appearance to N. c. limnaeetus, but apparently lacks a dark morph. The subspecies is also much less heavily blotched black on its whiter looking breast and never bears a crest. It is the smallest accepted subspecies of the changeable hawk-eagle, considerably smaller than N. c. limnaeetus. Wing chord was measured as in a single male and in a small sample of females.