Rallo invisibile

Identificazione

The invisible rail is a large, 33 to 40 cm (13–16 in) long, flightless bird. The adult has a mainly dark slate-grey body, dark brown plumage on the lower back, rump and wings, and a black uppertail. Its underparts are slightly paler slate-grey than the back, and the bare skin around the eye, the long, thick bill and the strong legs are bright red. It has a small spine at the bend of the wings. The sexes are identical in appearance; the plumage of fledged immature birds has not been described.<p>The invisible rail is superficially similar to the purple swamphen, Porphyrio porphyrio, which has recently been found in Halmahera, but that species is larger, with a short, thick red bill and a red forehead shield; it also has purple underparts and a white undertail. The invisible rail is different from the Calayan rail, Gallirallus calayanensis, in that it is larger and lacks the barred plumage of that species; there is no overlap between the ranges of the two species.<p>The call is a low drumming, accompanied by a tuk, tuk, tuk made with the wings. The nature of the vocalisation led to a local legend that the sound is made by the bird beating on a hollow tree or branch with its feet. Gerd Heinrich noted the local name "soisa", meaning drum, and described the call as being a subdued drumming purre – purre – purre – purre – purre which sometimes ends in a loud shrill scream. The bird also produced a dull hum similar to the voice of the banded pig (Sus scrofa vittatus) and reminiscent of the call of the snoring rail (Aramidopsis plateni). Calling is most frequent in the early morning or late evening, and a human tapping a sago stem with a machete may elicit a response from the bird. A quieter version of the call is given at the nest.Other sounds attributed to this rail, such as loud screams, may be incorrect, since they are like those produced by the pale-vented bush-hen (Amaurornis moluccana).

Tassonomia

The rails are a large and very widespread family, with nearly 150 species. They are small to medium-sized, terrestrial or wetland birds, and their short bodies are often flattened laterally to help them move through dense vegetation. Island species readily become flightless; of 53 extant or recently extinct taxa restricted to islands, 32 have lost the ability to fly.<p>The invisible rail, first classified by English zoologist George Robert Gray in 1860, is the only member of the monotypic genus Habroptila. The genus name Habroptila derives from the Greek habros, "delicate, pretty, splendid" and ptilon, "feather, wing"; wallacii commemorates British zoologist Alfred Russel Wallace. Local names include "soisa", "tibiales" and "rèie".<p>This rail is related to the New Guinea flightless rail, Megacrex inepta, and the chestnut rail, Eulabeornis castaneoventris, all three Australasian genera probably being derived from Amaurornis ancestors. Storrs Olson argued that the genus Megacrex was so similar to Habroptila that Megacrex should be considered a junior synonym of Habroptila, resulting in two species in the genus. This was further lumped in Sidney Dillon Ripley's 1977 monograph of the Rallidae; he included Habroptila within the large genus Rallus. This suggestion was not accepted by Gerlof Fokko Mees, who pointed out distinct differences in the shape and structure of the bill. A 2012 molecular phylogenetic analysis based on mitochondrial DNA sequence similarity found that Habroptila is part of evolutionary radiation within the broad genus Gallirallus that took place around 400,000 years ago in the region. A 2014 genetic study found that it formed a clade with the extinct Hawkins's rail of the Chatham Islands, New Zealand, the divergence was estimated to have taken place around 10 million years ago, this clade was in turn sister to Gallirallus.

Nome scientifico

Gallirallus wallacii

Autorità

Gray, GR, 1861

Ordine

Gruiformi

Famiglia

Rallidae

Genere

Gallirallus

Areale e habitat

The invisible rail inhabits the dense, spiky sago swamps of Halmahera, particularly where forest adjoins the boggy areas. Claims that the rail occurs in alang-alang grass are thought to have arisen from confusion with the pale-vented bush-hen. German ornithologist Gerd Heinrich, who prepared for his Halmahera trip by rolling in stinging nettles, wrote of the sago swamp habitat in the 1930s:<p>I am solidly confident no European has ever seen this rail alive, for that requires such a degree of toughening and such demands on oneself as I cannot so easily attribute to others. Habroptila is shielded by the awful thorns of the sago swamps... In this thorn wilderness, I walked barefoot and half-naked for weeks.<p>Sightings of the rail from 1950 to 2003 were from a restricted area of West Halmahera Regency, at the base of the western peninsula of the island, but it was recorded prior to 1950 as far as the southern point of Halmahera. More recent records showed that it is still present in a significantly larger area, including the northeast of the island, and locals claim that it also occurs in the swamps near Kao, in the northwest.

Migrazione

Resident species

Comportamento

The difficult habitat and retiring nature of the invisible rail mean that information on its lifestyle is sparse, and there are few confirmed sightings. Recorded food items include sago shoots and insects. It also feeds at cut sago plants, although it is unclear whether it is eating the decaying plant or searching for other edible items. It swallows small stones, as do all rails, to help break up its food in the gizzard.<p>The invisible rail is thought to be monogamous, but little else is known of its courtship behaviour prior to nesting. A report of four or five striped chicks was long thought to be incorrect, since such a plumage is not normal for rails. In this family, chicks are typically precocial, downy and black, with any ornamentation confined to the head, bare flesh, or specially modified plume feathers.<p>The issue was resolved in November 2010 when a nest was found in the top of a rotting tree stump, 1 m (39 in) above ground level and 46 m (151 ft) in from the edge of a dry swamp forest in Aketajawe-Lolobata National Park. The nest depression was 15 cm (5.9 in) in depth, with a lower layer of small wood chips at its base and a lining of dead leaves. The egg shells were brownish-white with dark brown and black markings of different sizes. The two very young chicks were entirely covered in black down, contrasting with a white pollux (the equivalent of the thumbnail on a human hand) and pink index nail. The bill was black with a white tip, and the legs were black-streaked brown. The eyes had grey irises and blue pupils. Rail chicks leave the nest soon after hatching, so the chicks were assumed to be only a day or two old.

Conservazione

Tendenza della popolazione

declining