 = Invisible rail = 
 
 The invisible rail , Wallace 's rail , or drummer rail ( Habroptila <unk> ) is a large flightless rail that is endemic to the island of Halmahera in North <unk> , Indonesia , where it inhabits impenetrable sago swamps adjacent to forests . Its plumage is predominantly dark slate @-@ grey , and the bare skin around its eyes , the long , thick bill , and the legs are all bright red . Its call is a low drumming sound which is accompanied by wing @-@ beating . The difficulty of seeing this shy bird in its dense habitat means that information on its behaviour is limited . 
 Recorded dietary items include sago shoots and insects , and it also swallows small stones to help break up its food . It is apparently monogamous , but little else is known of its courtship behaviour . The only known nest was a shallow bowl in the top of a rotting tree stump that was lined with wood chips and dry leaves . The two young chicks were entirely covered in black down typical of precocial newly hatched rails . The estimated population of 3 @,@ 500 – 15 @,@ 000 birds and the restricted range mean that the invisible rail is classified as vulnerable by the International Union for Conservation of Nature ( IUCN ) . Habitat loss has occurred through the harvesting of sago and conversion of the wetlands to rice cultivation , and the rail is eaten by local people . The described nest was in an area frequented by local villagers , so the rail may be more adaptable to habitat changes than had been thought . 
 
 = = Taxonomy = = 
 
 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 . 
 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 <unk> , " delicate , pretty , splendid " and <unk> , " feather , wing " ; <unk> commemorates British zoologist Alfred <unk> Wallace . Local names include " <unk> " , " <unk> " and " <unk> " . 
 This rail is related to the New Guinea flightless rail , Megacrex <unk> , and the chestnut rail , <unk> <unk> , all three <unk> genera probably being derived from <unk> ancestors . <unk> <unk> 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 <unk> in Sidney Dillon Ripley 's 1977 <unk> of the <unk> ; he included Habroptila within the large genus <unk> . This suggestion was , however , not accepted by <unk> <unk> <unk> , who pointed out distinct differences in the shape and structure of the bill . A molecular phylogenetic analysis based on mitochondrial DNA sequence similarity found that Habroptila is part of evolutionary radiation within the broad genus <unk> that took place around 400 @,@ 000 years ago in the region . 
 
 = = Description = = 
 
 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 <unk> . 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 . 
 The invisible rail is superficially similar to the purple swamphen , <unk> <unk> , 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 <unk> rail , <unk> <unk> , in that it is larger and lacks the barred plumage of that species ; there is no overlap between the ranges of the two species . 
 The call is a low drumming , accompanied by a tuk , tuk , tuk made with the wings . The nature of the <unk> 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 " <unk> " , meaning drum , and described the call as being a subdued drumming purre – purre – purre – purre – purre which sometimes ends in a loud <unk> scream . The bird also produced a dull hum similar to the voice of the banded pig ( <unk> <unk> <unk> ) and reminiscent of the call of the <unk> rail ( <unk> <unk> ) . 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 ( <unk> <unk> ) . 
 
 = = Distribution and habitat = = 
 
 The invisible rail inhabits the dense , spiky sago swamps of Halmahera , particularly where forest <unk> the <unk> areas . Claims that the rail occurs in <unk> @-@ <unk> 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 <unk> <unk> , wrote of the sago swamp habitat in the 1930s : 
 I am <unk> confident no European has ever seen this rail alive , for that requires such a degree of <unk> 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 <unk> wilderness , I walked barefoot and half @-@ naked for weeks . 
 <unk> 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 <unk> , in the northwest . 
 
 = = Behaviour = = 
 
 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 <unk> . 
 The invisible rail is thought to be monogamous , but little else is known of its courtship behaviour prior to nesting . A report of 4 – 5 striped chicks was long thought to be incorrect , since such a plumage is not normal for rails . In this family , chicks are typically precocial , <unk> and black , with any ornamentation confined to the head , bare flesh , or specially modified plume feathers . 
 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 <unk> @-@ <unk> 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 <unk> ( the equivalent of the <unk> 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 . 
 
 = = Status = = 
 
 Bird species with a restricted range are especially vulnerable to human activities , and eight of the 26 bird species occurring only in the Northern <unk> <unk> Bird Area are threatened , including the invisible rail . Almost a quarter of all rail species have conservation concerns , and flightless island species are particularly at risk , at least 15 species having become extinct since 1600 . The estimated population of the invisible rail is 3 @,@ 500 – 15 @,@ 000 birds , and its restricted range and small population mean that the species is classified as vulnerable by the International Union for Conservation of Nature ( IUCN ) , although this rail is so poorly known that it may be more common than the estimates suggest . 
 Habitat loss has occurred through commercial harvesting of the sago , or conversion to rice cultivation and <unk> . The rail is a prized food for local people who catch it with traps made from strings of bark and hunt it with dogs . The only described nest was in an area well @-@ used by local villagers , and the rail may be more adaptable to habitat changes than had been thought . There were also several sightings in northeast Halmahera in 2008 and 2011 , extending the area in which this bird has been seen in recent years . 
 
 