Inaccessible Island rail
The Inaccessible Island rail, or Inaccessible rail, is a small bird species of the rail family, Rallidae. Endemic to Inaccessible Island in the Tristan Archipelago in the isolated south Atlantic, it is the smallest extant flightless bird in the world. The species was formally described by physician Percy Lowe in 1923 but had first come to the attention of scientists 50 years earlier. The Inaccessible Island rail's taxonomic affinities and origin were a long-standing mystery; in 2018 its closest relative was identified as the South American dot-winged crake, and it was decided that both species are best classified in the genus Laterallus.
The Inaccessible Island rail has brown plumage, black bill and feet, and adults have a red eye. It occupies most habitats on Inaccessible Island, from the beaches to the central plateau, feeding on a variety of small invertebrates and also some plant matter. Pairs are territorial and monogamous, with both parents being responsible for incubating the eggs and raising the chicks. Its adaptations to living on a tiny island at high densities include low base metabolic rates, small clutch sizes, and flightlessness.
Unlike many other oceanic islands, Inaccessible Island has remained free from introduced predators, allowing this species to flourish while many other flightless birds, particularly flightless rails, have gone extinct. The species is nevertheless considered vulnerable by the International Union for Conservation of Nature, due to its single small population, which would be threatened by the accidental introduction of mammalian predators such as rats or cats.
Discovery
Although the Inaccessible Island rail may have been known to the Tristan Islanders who visited the island annually to hunt seals, the species first came to the attention of scientists during the Challenger expedition of 1872–1876. When the expedition visited the island in October 1873, Sir Charles Wyville Thomson learned of the species and recorded observations made by two German brothers, the Stoltenhoffs, who had been living on the island for the last two years. Thomson was unable to collect a specimen.Another attempt was made to collect a specimen by Lord Crawford on his yacht Valhalla in 1905. A final attempt was made during the Shackleton–Rowett Expedition, which passed by in April 1922 on its way back to Britain. This visit also failed, but members of the expedition left collecting material with a Rev. H. M. C. Rogers, then chaplain on Tristan da Cunha. The following year two study skins arrived in the Natural History Museum, London, followed soon after by another skin and a specimen in spirits. Physician Percy Lowe was then able to use the skins to describe the species. He did so, briefly, at a meeting of the British Ornithologists' Club, in 1923.
Evolution and taxonomy
Prior to its collection it had been assumed by Thomson that the species was close to the other "island hens" known in the Atlantic, possibly a gallinule, but on examination Lowe felt "compelled to refer it to a new genus". The generic name Atlantisia was named for the mythical island of Atlantis, destroyed by a volcano. The specific name rogersi honours the Rev. Rogers, who collected and sent the first specimens of the species to Lowe.In his 1928 paper on the species, Lowe thought the Inaccessible Island rail was the descendant of flightless ancestors which had reached Inaccessible Island via a land bridge or sunken continent such as Lemuria. Land bridges were commonly invoked to explain biogeographical distribution patterns before the development and acceptance of plate tectonics. By 1955 it was understood that the rail was the descendant of ancestors that had flown to Inaccessible Island. It was also presumed that, like most other land birds of the Tristan Archipelago, it had probably reached the island from ancestors in South America.
The position of the Inaccessible Island rail in the larger rail family has long been a source of uncertainty. Lowe thought that its closest relatives may have been black crake of Africa, or perhaps an early offshoot of the genus Porphyrio, a conclusion based mostly on similarities of plumage. He did concede that it was difficult to assign the rail to any relatives. American paleontologist Storrs Olson suggested in 1973 that it was related to the "Rallus assemblage", which included the genera Rallus and Hypotaenidia, based on the structure of the skeleton. In particular he suggested it was part of a pro-Rallus group that included the Indian Ocean genus Dryolimnas, and the Australasian Lewinia. Olson suggested the pro-Rallus rails have a relict distribution and would have been present in Africa and South America, where the ancestors of the Inaccessible Island rail came from. The most recent comparative morphological study to include the genus, in 1998, placed it in a subtribe Crecina of the crakes. Its exact position could not be determined, but it was suggested to perhaps be the sister taxon to the genus Laterallus, a genus of small crakes found mostly in South America.
Two extinct flightless species of rail were at one time placed in the genus Atlantisia with the Inaccessible Island rail. The Ascension crake and the Saint Helena swamphen were once considered congeners of A. rogersi. The Ascension crake disappeared some time before 1700 but was briefly mentioned and described by traveller and hobby naturalist Peter Mundy in 1656. The Saint Helena swamphen disappeared before 1600 and has never been encountered alive by scientists. In 1973 Olson synonymised the genus Aphanocrex with the genus Atlantisia, and described the Ascension crake as being congeneric. Today they are considered to have evolved independently, and in 2003 the genus Mundia was erected and the Saint Helena swamphen moved back to Aphanocrex, leaving the Inaccessible Island rail the only species in the genus Atlantisia. Both the Ascension crake and the Saint Helena swamphen became extinct due to predation by introduced species, mainly cats and rats.
Stervander et al. resolved the taxonomic affinity and evolutionary history of the Inaccessible Island rail by phylogenetic analyses of the DNA sequence of its full mitochondrial genome and a handful mitochondrial and nuclear genetic markers. According to this study, the Inaccessible Island rail belongs to a clade comprising its sister species, the dot-winged crake, the black rail in America, and most likely the Galápagos crake, therefore suggested referring the Inaccessible Island rail to the genus Laterallus. It colonized Inaccessible Island from South America c. 1.5 million years ago.
Morphology
The Inaccessible Island rail is the smallest living flightless bird in the world, measuring. Males are larger and heavier than females, weighing, average, compared to, average, in females. It is dark chestnut-brown above and dark grey on the head and below, with degraded white barring on the flanks and belly, and adults have red eyes. The female is similar to the male but with paler grey and a faint brown wash on the underparts. It has a black bill, which is shorter than the head. The feathers of the Inaccessible Island rail are almost hair-like, and in particular the flight feathers are degenerate, as the barbules on many of the feathers fail to interlock, giving the feathers a ragged appearance. The wings are reduced and weak, and smaller than same-sized flying relatives, as is the sternum. The tail is short, in length, and the and are nearly as long as the tail.The Inaccessible Island rail has a low basal metabolic rate, measured in 1989 at around 60–68% the rate expected for a bird of its weight. The scientists responsible for the study speculated that the low BMR was not as a result of flightlessness, which does not have this effect in other bird species, but was instead the result of the rail's island lifestyle. The island lacks predators and other competitors, and as such can be expected to be at full carrying capacity for rails. This in turn would favour energy conservation by the rails, resulting in small body size, low BMR and flightlessness. A comparison of flighted and flightless rails, including the Inaccessible Island rail, found that rails that lose the ability to fly also have low BMRs.