Inaccessible Island rail

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Inaccessible Island rail
Inaccessible Island Rail (Atlantisia rogersi).jpg
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Gruiformes
Family: Rallidae
Genus: Atlantisia
Lowe, 1923
Species: A. rogersi
Binomial name
Atlantisia rogersi
Lowe, 1923
Tristan Map.png
Inaccessible Island in the Tristan Archipelago

The Inaccessible Island rail (Atlantisia rogersi) is a small bird of the rail family, Rallidae. It is the only species in the genus Atlantisia. It is found only on Inaccessible Island in the Tristan Archipelago in the isolated south Atlantic, and is notable for being the smallest extant flightless bird in the world. Unlike many other islands, Inaccessible Island has remained free from introduced predators, allowing this species to flourish while many other flightless birds, including the even smaller Lyall's wren, have perished.

Discovery and description[edit]

Although the Inaccessible Island rail was probably 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 1872–76. 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, much to his regret.[2]

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. 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.[3]

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,[2] but on examination Lowe felt "compelled to refer it to a new genus".[4] The generic name Atlantisia was named for the fabled island of Atlantis, destroyed by volcano. The specific name rogersi honours the Rev Rogers who collected and sent the first specimens of the species to Lowe.[5]

Taxonomy[edit]

Two extinct flightless species of rail were at one time placed in the same genus as the Inaccessible Island rail. The Ascension crake (Mundia elpenor) which disappeared some time before 1700 but was briefly mentioned and described by traveller and hobby naturalist Peter Mundy in 1656, and Aphanocrex podarces, the Saint Helena swamphen which disappeared before 1600 and has never been encountered by scientists were once considered congeners of A. rogersi. In 1973 American paleontologist Storrs Olson synonymised this genus with the genus Atlantisia with ''Aphanocrex, and described the Ascension crake as being congeneric. Today they are considered to have evolved independently (with A. podarces probably not even being closely related), 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 species became extinct due to predation by introduced species, mainly cats and rats.[6]

Morphology[edit]

Inaccessible Island rail, showing the small flightless wings

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 weigh more 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. 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,[3] and a red eye.[7] 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.[8] 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) long, and the uppertail coverts and undertail coverts are nearly as long as the tail rectrices.[3]

The Inaccessible Island rail has a low basal metabolic rate (BMR), measured in a 1989 at around 60-68% the rate expected for a bird of their weight. The scientists responsible for the study speculated that the low BMR was not as a result of flightlessness, as flightlessness does not result in this 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.[9] 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.[10]

Distribution and habitat[edit]

Tussock-grass lowlands and steep cliffs from the shore of Inaccessible Island

The Inaccessible rail is endemic to the uninhabited Inaccessible Island in the Tristan da Cunha group in the mid-Atlantic Ocean.[7] The island has a temperate wet oceanic climate with high rainfall, limited sunshine and persistent westerly winds.[11] The rail is found in almost all habitats on the island and at all altitudes, from 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.[12] This habitat is found close to the shore and surrounds most of the island on the steep cliffs.[11] The Inaccessible Island rail can also be found in upland fern-bush heath, dominated by wind-stunted Blechnum palmiforme and in the island forest in the central plateau which is dominated by Phylica arborea (which can reach 5m where sheltered) and Blechnum palmiforme.[11] In both these habitats the population is estimated to be 2 birds per hectare.[12] It will also forage among boulders on the beaches, but has not been found in the short dry grasses on the cinder cones (but the scientists making the observations cautioned that this does not mean that they never use the habitat).[12] It frequently uses natural cavities among boulders or tunnels through grasses created by frequent use to move around while concealed.[7]

Behaviour[edit]

The Inaccessible Island rail is territorial, although given the high densities of the birds the boundaries may be loose and flexible. The territories are tiny, with territories in the tussock-grass habitats around Blenden Hall, where the population densities are highest, extending to 0.01–0.04 ha (0.025–0.099 acres). The small size of the territories make encounters between families and individuals frequent and encounters 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.[12]

Diet and feeding[edit]

Inaccessible Island rails forage under tussock-grass

The foraging method used by the Inaccessible Island rail is slow and deliberate and has been compared to that of a mouse. In fact the species has actually been compared ecologically to a mouse as well.[7] 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 don't feed on carrion or dead fish.[7]

Calls[edit]

The Inaccessible Island rail is a highly vocal species, calling frequently. This may be because of the dense vegetation the species lives in, making calls the best way to communicate, and pairs and families contact call frequently while feeding. Calls used include a long trill used when pairs meet and when confronting a rival.[7][12] Rivals also make a long twitter "keekeekeekeekee" which can be long and short and end in a "keekeechitrrrr". After skirmishes between rivals the victorious bird may make a "weechup weechup" call. Birds may make a monotonous "tchik tchik tchok tchik" while hunting for prey, and a the alarm call when predators are around is a short and hard "chip". They also make a variety of calls while incubating, including trilling calls and particularly when pairs swapped placed during incubation. Before changing places the incubating bird may make a "chip chip chip", but they fall silent when Tristan thrushes approach the nest.[12]

Breeding[edit]

Inaccessible Island rails often nest in tussock grass

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. From the entrance of the nest an entrance track or tunnel can go up to half a metre from the nest. The nests are typically built entirely of the same material the nest is found in; for example tussock leaves or sedges. Where tussock leaves are used larger leaves are used on the outside and finer leaves are used to line the nest. There are a few reports of other material being used to line the nest, for example the leaves of introduced Malus domestica (apple) or Salix babylonica (willow).[12]

The clutch size is two eggs, which is low for comparative sized rails.[10] 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.[13]

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.[12]

The eggs hatch between 23-32 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.[12] Eggs and chicks are vulnerable to predation by the Tristan thrush.

Predation[edit]

Tristan thrushes prey on rail chicks

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.[3] A study of the diet of brown skuas on Inaccessible Island confirmed this, but found that while 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.[14] After hearing another rail make an alarm call, adult Inaccessible Island rails become alert, while chicks become silent.[12] While the adults are rarely preyed upon, chicks mortality is high and predation by Tristan thrushes is a major cause.[7]

Threats[edit]

This species is rated as Vulnerable by the IUCN Red List, due to its small range. While it remains common within that range, if an invasive species were to come in to Inaccessible Island, it could wipe out these birds entirely.

References[edit]

  1. ^ BirdLife International (2012). "Atlantisia rogersi". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 26 November 2013. 
  2. ^ a b Thomson, Sir Charles Wyville (1877). The Voyage of the "Challenger.": The Atlantic; a Preliminary Account of the General Results of the Exploring Voyage of H. M. S. "Challenger" During the Year 1873 and the Early Part of the Year 1876, Volume 2. London: Macmillan. pp. 184–185. Retrieved 16 March 2017. 
  3. ^ a b c d Lowe, Percy Roychoft (1928). "VII.-A Description of Atlantisia rogersi, the Diminutive and Flightless Rail of Inaccessible Island (Southern Atlantic), with some Notes on Flightless Rails.". Ibis. 70 (1): 99–131. doi:10.1111/j.1474-919X.1928.tb08711.x. 
  4. ^ Lowe (1928) p. 103.
  5. ^ Jobling, J. A. (2017). "Key to Scientific Names in Ornithology | HBW Alive". Handbook of the Birds of the World. Lynx Edicions, Barcelona. Retrieved 13 March 2017. 
  6. ^ Bourne, W. R. P.; Ashmole, N. P.; Simmons, K. E. L. (2003). "A New Subfossil Night Heron and A New Genus for the Extinct Rail Ascension Island, Central Tropical Atlantic Ocean" (PDF). Ardea. 91 (1): 45–51. 
  7. ^ a b c d e f g Taylor, B. & Sharpe, C.J. (2017). Inaccessible Rail (Atlantisia rogersi). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from http://www.hbw.com/node/53636 on 13 March 2017).
  8. ^ McGowan, C. (1989). "Feather structure in flightless birds and its bearing on the question of the origin of feathers". Journal of Zoology. 218 (4): 537–547. doi:10.1111/j.1469-7998.1989.tb04997.x. 
  9. ^ Ryan, Peter G.; Watkins, Barry P.; Siegfried, W. Roy (1989). "Morphometrics, Metabolic Rate and Body Temperature of the Smallest Flightless Bird: The Inaccessible Island Rail" (PDF). The Condor. 91 (2): 465–467. doi:10.2307/1368325. 
  10. ^ a b McNab, Brian K.; Ellis, Hugh I. (November 2006). "Flightless rails endemic to islands have lower energy expenditures and clutch sizes than flighted rails on islands and continents" (PDF). Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology. 145 (3): 295–311. doi:10.1016/j.cbpa.2006.02.025. 
  11. ^ a b c Roux, J. P.; Ryan, P. G.; Milton, S. J.; Moloney, C. L. (15 December 1992). "Vegetation and checklist of Inaccessible Island, central South Atlantic Ocean, with notes on Nightingale Island". Bothalia. 22 (1). doi:10.4102/abc.v22i1.828. 
  12. ^ a b c d e f g h i j Fraser, M. W.; Dean, W. R. J.; Best, I. C. (1992). "Observations on the Inaccessible Island Rail Atlantisia rogersi: the world's smallest flightless bird". Bulletin of the British Ornithologists' Club. 112: 12–22. Retrieved 14 March 2017. 
  13. ^ Rothschild, L. W. (1928). "On the eggs of Atlantisia rogersi". Bulletin of the British Ornithologists Club. 48: 121–124. 
  14. ^ Fraser, M. W. (1984). "Foods of Subantarctic Skuas on Inaccessible Island". Ostrich. 55 (4): 192–195. doi:10.1080/00306525.1984.9634487. 

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