Inaccessible Island rail

Inaccessible Island rail

Inaccessible Island rail

2 languages
Kingdom
Phylum
Class
Subclass
Infraclass
Superorder
Order
Family
Genus
SPECIES
Laterallus rogersi

The Inaccessible Island rail (Laterallus rogersi ) is a small bird 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 described by physician Percy Lowe in 1923 but had first come to the attention of scientists 50 years earlier. The Inaccessible Island rail's affinities and origin were a long-standing mystery; in 2018 its closest relative was identified as the South American dot-winged crake (Porzana spiloptera ), and it was proposed that both species should be nested within the genus Laterallus.

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A small species, 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 (IUCN), due to its single small population, which would be threatened by the accidental introduction of mammalian predators such as rats or cats.

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Te

Terrestrial

Co

Congregatory

Fl

Flightless bird

Is

Island endemic

So

Social

No

Not a migrant

I

starts with

Appearance

The Inaccessible Island rail is the smallest living flightless bird in the world, measuring 13 to 15.5 cm (5.1–6.1 in). Males are larger and heavier than females, weighing 35–49 g (1.2–1.7 oz), average 40.5 g (1.43 oz), compared to 34–42 g (1.2–1.5 oz), average 37 g (1.3 oz), 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 a red eye. 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 (but not all, as has sometimes been reported) 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, 3.5 cm (1.4 in) in length, and the uppertail coverts and undertail coverts are nearly as long as the tail rectrices.

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The Inaccessible Island rail has a low basal metabolic rate (BMR), 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.

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Distribution

Geography

The Inaccessible rail is endemic to the uninhabited Inaccessible Island in the Tristan da Cunha group in the mid-Atlantic Ocean. The island is 14 km2 (5.4 sq mi) in area and has a temperate wet oceanic climate with high rainfall, limited sunshine and persistent westerly winds. The rail is found in almost all habitats on the island and at all altitudes, from sea-level to 449 m (1,473 ft). It reaches its highest densities in fields of tussock grass (Spartina arundinacea ), with 10 birds per hectare, and in tussock grass mixed with ferns (Blechnum penna-marina ) and sedges, with 15 birds per hectare. This habitat is found close to the shore and surrounds most of the island on the steep cliffs. The Inaccessible Island rail can also be found in upland fern-bush heath, dominated by wind-stunted tree-ferns (Blechnum palmiforme ) and in the island forest in the central plateau which is dominated by Island Cape myrtle (Phylica arborea )—which can reach 5 m (16 ft) where sheltered—and Blechnum palmiforme. In both these habitats the population is estimated to be two birds per hectare. It will also forage among boulders on the beaches, but has not been found in the short dry grasses on the cinder cones (the scientists making the observations cautioned that this does not mean that they never use the habitat). It frequently uses natural cavities among boulders or tunnels through grasses created by frequent use to move around while concealed.

Inaccessible Island rail habitat map
Inaccessible Island rail habitat map

Habits and Lifestyle

The Inaccessible Island rail is territorial, and the territories they defend are tiny. The territories in the tussock grass habitats around Blenden Hall, where the population densities are highest, extend to 100–400 m2 (1,100–4,300 sq ft). The small size of the territories makes encounters between families and individuals frequent, and confrontations and territorial calling are common. On meeting, confrontations start with loud trills or twittering, then birds may face off, standing very close to each other and displaying ritually with their heads lowered and their bills pointed towards the ground. They may circle, and continue displaying until one bird slowly retreats or a quick skirmish ensues and one bird is driven off.

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Lowe speculated in his 1927 paper that, in the absence of mammalian predators on the island, the brown skua would be the Inaccessible Island rail's only predator. A study of the diet of brown skuas on Inaccessible Island confirmed this, but found that although skuas do eat adults of this species, the rail and other landbirds formed only a small part of the diet of that seabird, especially compared to their abundance on the island. They noted that the landbirds alarm-called when brown skuas were seen. After hearing another rail make an alarm call, adult Inaccessible Island rails become alert, while chicks fall silent. Adults are rarely preyed upon, but the mortality of chicks is high and predation by Tristan thrushes is a major cause of death.

Two species of chewing lice have been found on Inaccessible Island rails, Pscudomenopon scopulacorne and Rallicola (Parricola) zumpti. R. zumpti has not been described on any other species of bird from Inaccessible Island. P. scopulacorne found on the Inaccessible Island rail were originally described as a new species, P. rowani (Keler, 1951), but were later lumped into the widespread species in 1974.

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Seasonal behavior

Diet and Nutrition

The foraging method used by the Inaccessible Island rail is slow and deliberate and has been compared to that of a mouse, and the bird occupies a similar ecological niche. They feed on a range of invertebrates, including earthworms, amphipods, isopods, mites and a range of insects such as beetles, flies, moths and caterpillars. Centipedes are taken as well, and an introduced species of centipede forms an important part of their diet. Along with animal prey they will take the berries of Empetrum and Nertera as well as the seeds of the dock Rumex. Unlike the Tristan thrush they do not feed on carrion or dead fish.

Mating Habits

The Inaccessible Island rail is a seasonal breeder, laying between October and January. They are monogamous, forming permanent pair-bonds. The nests are situated at the base of ferns with tussock grass, tussock grass clumps, or in tufts of sedges. The nests are domed and oval or pear shaped, with the entrances near the narrow end of the nest and linked by a track or tunnel that can go up to half a metre away. The nests are typically built entirely of the same material the nest is found in; for example, tussock grass or sedges. Where the construction material is tussock grass, larger leaves are used on the outside and finer material lines the nest. There are a few reports of other material being used as lining, such as the leaves of introduced Malus domestica (apple) or Salix babylonica (willow).

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The clutch size is two eggs, which is low for such small rails. The eggs are greyish milk white dotted with brown-rufous spots and lavender-mauve spots that are concentrated around the apex of the egg. They are large for the size of the mother compared to other rails, and resemble the eggs of the corn crake.

The incubation period for the species is not known, but both sexes incubate the clutch, although the males incubated for longer in the observations that have been made. Both sexes bring food to their partner that is incubating, which is either consumed on the nest or close to the nest. Changeovers of incubation details are preceded by "chip chip chip" calls, which become louder and more frequent the longer its takes the partner to respond.

The eggs hatch within between 23 and 32 hours of each other and can be preceded by the chick in the egg calling for up to 45 hours before hatching. One hatching was recorded as taking 15 hours to complete. Newly hatched chicks are covered by downy black plumage, the legs, feet and bill are black, and the mouth is silvery.

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Population

References

1. Inaccessible Island rail Wikipedia article - https://en.wikipedia.org/wiki/Inaccessible_Island_rail
2. Inaccessible Island rail on The IUCN Red List site - https://www.iucnredlist.org/species/22692556/93358821

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