Kagu

识别特征

The kagu is a ground-living bird, 55 cm (21+1⁄2 in) in length. The weight can vary considerably by individual and by season, ranging from 700 to 1,100 g (25–39 oz). Its plumage is unusually bright for a bird of the forest floor; ash-grey and white coloured. There is little sexual dimorphism beyond a difference in the amount of barring in the primary feathers. It possesses powder down which helps keep it dry and insulates it in the extremes of New Caledonia's tropical climate. The crest, which is used to display to other members of the species, is barely noticeable when at rest but can be erected and fanned out.<p>It is nearly flightless, using its wings for displays (its primary wing feathers are patterned), and for moving quickly through the forest. It can also use them to glide when fleeing danger. The wings are not reduced in size like some other flightless birds, and have a span of around 77.5 cm (30+1⁄2 in), but they lack the musculature for flight. These wings are also used for a 'broken-wing' display, a behaviour shared with their relative sunbittern, used to fake an injury and draw the attention of a predator away from their chick. It possesses bright red legs which are long and strong, enabling the bird to travel long distances on foot and run quickly.<p>It has large eyes, positioned so that they give good binocular vision which is helpful in finding prey in the leaf litter and seeing in the gloom of the forest. It possesses 'nasal corns', structures covering its nostrils, which are a feature not shared by any other bird. These are presumed to prevent particles entering the nostrils when probing in soil during feeding. Another unique characteristic of the species is that it has only one-third as many red blood cells and three times more hemoglobin per red blood cell than is usual in birds.

分类信息

The generic name Rhynochetos, and the clade name Rhynochetidae, are derived from the Greek ῥίς rhis meaning "nose" and χαίτη khaitē meaning "long hair", referring to its long stiff hairs over the nostrils. The specific name jubatus is derived from the Latin iubātus meaning crested.<p>The kagu's affinities are not well resolved. It was long one of the most enigmatic birds and in more recent times is usually affiliated with the Gruiformes. It was initially classed as a member of the clade Ardeae because of the presence of powder down, similarities in plumage colour and internal anatomy, the colour of the chicks and eggs, and the change in colouration of the chick as it grows.<p>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 &amp; 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, nightjars, 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. <p>The kagu and sunbittern, and possibly the adzebills, seem to form a distinct Gondwanan lineage of birds, either as one order or possibly more. Although 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. The ancestors of the kagu are believed to have diverged from the sunbittern in the Oligocene, 45 to 17 million years ago, and colonized New Caledonia 60 to 25 million years ago. In the absence of terrestrial predators, it eventually became flightless.<p>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.

学名

Rhynochetos jubatus

命名信息

Verreaux, J & des Murs, 1860

目

Eurypygiformes

科

Rhynochetidae

属

Rhynochetos

分布与栖息地

The kagu is endemic to the forests and shrubland of New Caledonia. Within that island group it is restricted to the main island of Grande Terre. There is no evidence that it occurred on the Loyalty Islands, although fossil remains of the extinct lowland form R. orarius have been found on the Ile des Pines. The kagu is a habitat generalist and able to exist in a range of different forest types if sufficient prey is present, from rain forest to drier lowland forest. They are also able to feed in some drier shrubland associated with the island's ultrabasic rocks, although not in the poorest, low-prey shrubland of this type. They are also absent from areas where extensive ground cover makes foraging difficult, such as grassland or areas with high fern cover, but may pass through such areas to reach other foraging areas. The species has undergone some range contraction due to hunting and predation by introduced species. Its original, pre-human distribution, and the extent to which it and its sister species R. orarius coexisted in lowland areas of New Caledonia, is still not fully understood and awaits further research into the subfossil record.

迁徙

Resident species

行为

Migration pattern: Resident species.

保护状况

Population trend: declining.

种群趋势

declining