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Kagu
The kagu or cagou (''Rhynochetos jubatus'') is a crested, long-legged, and bluish-grey bird endemic to the dense mountain forests of New Caledonia. It is the only surviving member of the genus ''Rhynochetos'' and the family Rhynochetidae, although a second species has been described from the fossil record. Measuring in length, it has pale grey plumage and bright red legs. Its 'nasal corns' are a unique feature not shared with any other bird. Almost flightless, it spends its time on or near the ground, where it hunts its invertebrate prey, and builds a nest of sticks on the forest floor. Both parents share incubation of a single egg, as well as rearing the chick. It has proven vulnerable to introduced predators, and is threatened with extinction.
==Taxonomy and systematics==
The kagu's affinities are not too well resolved. It was long one of the most enigmatic birds and in more recent times usually affiliated with the Gruiformes. It was initially classed as a member of the clade Ardeidae because of the presence of powder down, similarities in plumage colour and internal anatomy, the colour of the chicks and eggs, an the change in colouration of the chick as it grows.〔del Hoyo, J. Elliott, A. & Sargatal, J. (editors). (1996) ''Handbook of the Birds of the World. Volume 3: Hoatzin to Auks''. Lynx Edicions. ISBN 84-87334-20-2〕
When seen as a gruiform, the kagu is generally considered related to the extinct adzebills from New Zealand and the sunbittern from Central and South America. Recent studies do indicate that the sunbittern is the closest living relative of the kagu. For example, Fain & Houde found these to be certainly sister taxa. They and the mesites did not group with traditional Gruiformes in their study, but instead with their proposed clade Metaves, which also includes the hoatzin, pigeons, Caprimulgiformes, flamingos, tropicbirds, Apodiformes, sandgrouse and grebes. The internal structure of this group was not well resolvable by their data, although later studies confirmed a close relationship between the kagu and sunbittern.〔Hackett, S.J. ''et al.'' (2008) A Phylogenomic Study of Birds Reveals Their Evolutionary History. ''Science'', 320(5884):1763–1768.〕 The kagu and sunbittern, and possibly the adzebills, seem to form a distinct Gondwanan lineage of birds, possibly one order, possibly more, though the relationships between them and groups previously considered related, such as the mesites and the "core Gruiformes," are not yet resolved. It is notable, however, that the sunbittern and the mesites possess powder down too, whereas the "core Gruiformes" do not.
While the kagu is the only living species in the clade Rhynochetidae, a larger species, the lowland kagu (''Rhynochetos orarius''), has been described from Holocene subfossil remains. The measurements of this species were 15% bigger than ''Rhynochetos jubatus'', with no overlap in measurements except those of the forelimbs. Given that the sites from which ''R. orarius'' remains have been recovered are all lowland sites, and that no fossils of ''R. jubatus'' have been found in these sites, the scientists that described the fossils have suggested that they represent highland and lowland species respectively. ''R. orarius'' is one of many species to have become extinct in New Caledonia after the arrival of humans. The validity of the species has been questioned by some authors,〔 but accepted by others.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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