Aldabra rail (Dryolimnas aldabranus)

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Aldabra rail
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Aldabra rail fact file

Aldabra rail description

KingdomAnimalia
PhylumChordata
ClassAves
OrderGruiformes
FamilyRallidae
GenusDryolimnas (1)

The Aldabra rail is the last surviving flightless bird in the western Indian Ocean (3). It has a slender build, with a long, fairly slender neck, legs and feet (2). The plumage is well-defined, being largely bright chestnut except for the striking white throat (2) (3). The fairly long, straight bill is dark with, in females, a bright pink base, and in males, a dull or dark red base. Juveniles generally have duller plumage than adults (2). Being a flightless bird, the wings are short and are often held close to the body where they blend in with the rest of the plumage (4). Many consider the Aldabra rail a subspecies of the white-throated rail (Dryolimnas cuvieri) (1).

Synonyms
Dryolimnas cuvieri aldabranus.
Size
Length: 30 – 33 cm (2)
Male weight: 145 – 218 g (2)
Female weight: 138 – 223 g (2)
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Aldabra rail biology

The Aldabra rail is an unfussy eater, with a varied diet and a tendency to investigate any strange object as potential food (3). It searches among leaf litter, from where it plucks small beetles and other invertebrates, and will follow the tortoises of Aldabra to feed in the litter disturbed by their passing. Flies and mosquitoes are pecked from the shells of tortoises, and the carcass of a dead tortoise is a good source of small beetles and flies for the rail. The Aldabra rail does not eat the flesh of a dead tortoise, but it will consume the flesh of dead crabs. Small molluscs found just above the tide line are cracked open with the bill and consumed, and the rail forages among the roots of mangroves for snails and small crabs (3). When feeding in water, the Aldabra rail submerges its head and vigorously rakes the mud with its bill. These birds are able to drink both fresh and saltwater (2).

Aldabra rails are monogamous birds that form permanent pair-bonds and together defend a territory. Females without a male partner also hold territories. A clutch of three or four eggs are usually laid in December, into a flimsy nest constructed of twigs and leaves, situated in a depression among rocks (2), or into a more substantial nest shaped like a deep cup and well-concealed in foliage (3). The chicks are fed by the parents until they become independent at 12 to 15 weeks of age, after which the parents become aggressive toward their offspring (2). Often only one young in each clutch of eggs survives, possibly due to predation. Surviving chicks begin breeding at nine months, and Aldabra rails live for up to six years (2).

Land crabs (Cardisoma carnifex) and introduced rats (Rattus rattus) are thought to take eggs and possible kill some chicks, but there are no natural predators of adult Aldabra rails on Aldabra Atoll (3), the most likely reason why the rail has evolved to be flightless (6). The lack of predators means that the Aldabra rail shows no fear of humans, or the cats that occur on some of the Aldabra islands (5).

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Aldabra rail range

Occurs on Aldabra Atoll in the Indian Ocean (2). Aldabra Atoll is a large, slightly-raised coral platform comprised of four large islands, Grande Terre, Malabar, Picard and Polymnie, and numerous smaller islets, situated around a large, shallow lagoon (4) (5).

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Aldabra rail habitat

The Aldabra rail can be found in all habitats of Aldabra Atoll, which include mangroves, dense scrub dominated by Pemphis acidula, more open bush, and pebble and sand beaches (3).

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Aldabra rail status

Dryolimnas aldabranus is currently awaiting assessment by the IUCN.

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Aldabra rail threats

Historically, the Aldabra rail existed on all islands within the Aldabra Atoll as well as other nearby islands (7). The arrival of humans on these remote islands had devastating effects for this flightless bird, with the introduction of domestic cats (introduced to Aldabra as early as 1892) thought be responsible for the rail’s extinction on many of these islands. This reduction in the rail’s already limited distribution leaves the species very vulnerable to catastrophic events such as cyclones or disease (5). The dispersion of cats to the other islands of Aldabra poses a constant and serious threat to the existence of the Aldabra rail (3) (5). Aldabra rails are occasionally also caught and eaten by humans, but the rail population seems to withstand the pressure of human, rat and land crab predation (3).

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Aldabra rail conservation

For many years, the Aldabra rail continued to survive on just Polymnie and Malabar islands, and on Île aux Cèdres, a smaller islet. In an attempt to improve the long-term survival of this species, a re-introduction programme to Picard island was launched in 1999 (5) (7). Picard Island was selected as cats had not been seen on the island since 1970 (7), probably because feral cats could not survive on an island which has no year-round freshwater source (4). The programme appears to have been a success; 20 rails were captured from Malabar Island and taken to Picard in 1999, and by 2001 there were at least 51 on the island (5). In addition to this specific conservation action, the general conservation attention received by the Aldabra Atoll will benefit this species. The atoll is a World Heritage Site, and the Seychelles Islands Foundation was established to safeguard the treasures of Aldabra and promote its use purely for research and education. With support, the Foundation can preserve the unique system of Aldabra (8), and the irreplaceable Aldabra rail.

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Find out more

For further information on conservation on Aldabra see:

For more information on this and other bird species please see:

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Authentication

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact: arkive@wildscreen.org.uk
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Glossary

Invertebrates
Animals with no backbone.
Molluscs
A diverse group of invertebrates, mainly marine, that have one or all of the following; a horny, toothed ribbon in the mouth (the radula), a shell covering the upper surface of the body, and a mantle or mantle cavity with a type of gill. Includes snails, slugs, shellfish, octopuses and squid.
Monogamous
Having only one mate during a breeding season, or throughout the breeding life of a pair.
Subspecies
A population usually restricted to a geographical area that differs from other populations of the same species, but not to the extent of being classified as a separate species.
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References

  1. IUCN Red List (November, 2007)
    http://www.iucnredlist.org/
  2. del Hoyo, J., Elliott, A. and Sargatal, J. (1996) Handbook of the Birds of the World. Vol. 3: Hoatzin to Auks. Lynx Edicions, Barcelona.
  3. Penny, M.J. and Diamond, A.W. (1971) The White-Throated Rail Dryolimnas cuvieri on Aldabra. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, 260(836): 529 - 548.
  4. Ministry of Environment, Seychelles (March, 2008)
    http://www.env.gov.sc
  5. Wanless, R.M., Cunningham, J., Hockey, P.A.R., Wanless, J., White, R.W. and Wiseman, R. (2002) The success of a soft-release reintroduction of the flightless Aldabra rail (Dryolimnas [cuvieri] aldabranus) on Aldabra Atoll, Seychelles. Biological Conservation, 107(2): 203 - 210.
  6. Benson, C.W. and Penny, M.J. (1971) The land birds of Aldabra. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, 260(836): 417 - 527.
  7. Percy Fitzpatrick Institute. (2000) Aldabra Rails – A Runaway Success. Birds and Birding, 5(5): 11 - .
  8. Seychelles Islands Foundation (March, 2008)
    http://www.sif.sc
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Image credit

Aldabra rail  
Aldabra rail

© Dr. Justin Gerlach

Dr. Justin Gerlach
Nature Protection Trust of Seychelles
Seychelles
gerlachs@btinternet.com
http://islandbiodiversity.com

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