Working for birds in Africa

Birds of Aldabra

p 42-47
Comoro (Aldabra) Blue Pigeon
A. Skerrett
Aldabra Fody
A. Skerrett

Aldabra, un atoll éloigné dans le sud-ouest de l'archipel des Seychelles, constitue un écosystème quasiment intact. Pour visiter l'île, plusieurs options se présentent, quoique aucune ne soit peu coûteuse. L'atoll est riche en flore et faune endémiques, bien que des deux espèces d'oiseaux, la Fauvette d'Aldabra Nesillas aldabranus et le Drongo d'Aldabra Dicrurus aldabranus, la première ait probablement disparu. La population du drongo est actuellement estimée à 1,500 individus. L'élévation de plusieurs autres taxons au rang d'espèce pourrait se justifier, comme par exemple dans les cas du Râle d'Aldabra Dryolimnas cuvieri aldabranus, du Foudi de forêt d'Aldabra Fondia eminentissima aldabrana et de l'Ibis sacré Threskiornis aethiopicus abbotti. Huit autres espèces terrestres sont présenter, mais elles ont été peu etudées jusqu'ici. Plusieurs espèces d'oiseaux de mer nichent en nombres important sur l'atoll, le Flamant rose Phoenicopterus nibery niche également, comme prouvé récemment, et un certain nombre d'espèces occasionnelles observées nulle part ailleurs aux Seychelles y ont été notées. Les espèces endémiques et les oiseaux de mer nicheurs sont menacés par des prédateurs, tels que chèvres, chats et rats, le braconnage et l'introduction d'oiseaux exotiques par le personnel de la mine de phosphate de l'île voisine d'Assomption, tels que le Bulbul orphée Pycnonotus jocusus et le Foudi de forêt malgache Foudia eminentissima. Ces espèces pourraient supplanter les taxons endémiques ou, dans le cas du foudi, s'apparier au foudi local et former une population hybride. Toutefois, le gouvernement seychellois adoptant une attitude positive envers la conservation de la nature, I'on pent espérer qu'Aldabra demeurera une île sans trop d'espèces introduites.

Adrian Skerett

Due to its remote location, lack of freshwater, dense scrub, difficult terrain and no small degree of good fortune, Aldabra's ecosystem has survived relatively intact. Turtles, tortoises, some birds and other resources were once heavily exploited. Aldabra's darkest hour came in the mid-1960s when plans were hatched to turn the atoll into a gigantic aircraft carrier. The campaign to save Aldabra, spear-headed by the UK's Royal Society, was probably the first successful, major international conservation campaign. The proponents of the military option were not used to this and attempted to fight back using ridicule. Denis Healey of Britain's Labour Government declared "As I understand, the island of Aldabra is inhabited - like Her Majesty's Opposition Front Bench - by giant turtles, frigate birds and boobies. Nevertheless, it may well provide useful facilities for aircraft'(1). However, Sir Julian Huxley offered a wider vision: 'the animals and plants of Aldabra...can fairly claim to have international value and as such, the owner into whose hand they happen to have fallen, surely has a responsibility to exercise some degree of limitation of his activities in their favour"(1).

The 1967 devaluation of sterling was used as the reason to back down. Magnanimous in defeat, the UK government (then colonial masters of Seychelles), passed the lease of Aldabra to the Royal Society. A scientific research station was established on the atoll. Dubbed 'the land that time forgot' during the media campaign, the Royal Society's work made Aldabra one of the best researched atoll ecosystems in the world. Subsequently, management responsibility passed to the Seychelles Islands Foundation, founded by presidential decree in 1979. The atoll which comprises approximately one-third of Seychelles' total land area, became a nature reserve. In 1982, it became Seychelles' first World Heritage Site.

Location

Aldabra is situated in the extreme south-west of the Seychelles archipelago, 600 km east of Africa, 400 km north-west of Madagascar and 1,100 km south-west of Seychelles main population centre, Mahé in the granitic group of islands. The atoll of Aldabra comprises four main islands - Grande Terre, Malabar, Polymnie and Picard - and a number of smaller lagoon islands and islets. The Aldabra group comprises Aldabra atoll and with the neighbouring islands of similar age and structure - Assumption, Astove and Cosmoledo atolls.

Getting there

For many years, Royal Society staff on Aldabra needed to employ devious techniques, such as hitching a lift on supertankers from the Middle East, to reach Aldabra. For others the arrival of cruiseships in the western Indian Ocean were an easier, if more expensive, alternative. This is an excellent way to make a short visit to Aldabra provided you chose the right ship. The best cruiseship for expedition travel in the region is Caledonian Star with its team of expert Zodiac drivers and leaders who know the atoll and its wildlife intimately.

For a longer stay, it was formerly necessary to charter a yacht from Mahé for upwards of US$12,000, allowing c6 days at sea for the return journey. Today, there is one live-aboard motor yacht - the Indian Ocean Explorer - based at Aldabra for part of the year, which takes visitors to and from neighbouring Assumption for the flight to Mahé. The price per person in 1998 on a double occupancy basis was US$300 per day plus flight costs. Another live-aboard - the catamaran Aldabra - takes the scenic route between Mahé and Aldabra.

For landlubbers, there is still hope. In 1997, through funding from the World Bank and Seychelles Island Foundation, the old research station, constructed by the Royal Society almost 30 years earlier, was replaced with an attractive log-frame research facility which includes an accommodation block of six twin rooms, each with en suite toilet and shower. This is principally for the use of visiting scientists, but if there is space it is available for private hire. The cost at SR2,000 (cUS$400) per twin or SR1,500 (US$300) per single room per day, which includes guiding services of the resident warden and research officer, is still not cheap, but this is not an average package deal.

The birds

Endemics are rarely found on remote coral atolls due to their relatively short geological history. Just 4,000 years ago, sea-levels in the western Indian Ocean were several metres higher than at present. At this time, there was just one point of land between Seychelles granitic islands - 1,000 km to the north-eastand the Aldabra group; the raised coral platform island of St. Pierre. Subsequently, a change in ocean currents led to a localised fall in sea-levels and the emergence of three coral island groups: the Amirantes, the Alphonse group and the Farquhar group.

Given their short history, it is unsurprising that none of the Amirantes, Alphonse group or Farquhar group have any endemic birds. A race of Madagascar Turtle Dove Streptopelia picturata saturata (synonym aldabrana, a misnomer) once occupied some of the Amirantes and a blue pigeon (possibly a race of Comoro Blue Pigeon Alectroenas sganzini) was found in the Farquhar group (29), but both are now extinct. Today, the only landbirds are introduced species.

Aldabra is the world's largest raised coral atoll. Whereas most coral islands, including those of Amirantes, Alphonse and Farquhar do not rise above 3 m, Aldabra is higher, with two platforms at 4 m and 8 m. It has undergone several emergences and submergences during its history, but due to its height Aldabra's last submergence was probably c125,000 years ago.

Given the antiquity of Aldabra, it is perhaps unsurprising that levels of endemism among its flora and fauna are high. Almost all of the landbird species belong to an endemic taxon. Of 176 flowering plants, c40 (22%), are confined to the Aldabra group. By contrast, the coral cay of Bird Island, close to the granitic group, has just 30 flowering plants in total, none of which are endemic (2). Endemic insects too, are well represented: c38% of the estimated 1,000 species. Aldabra's most famous species is the endemic Aldabra Giant Tortoise. The estimated population in 1997 was 100,000 (11), perhaps 90% of the world population of all giant tortoise species.

Sadly, one endemic bird species - Aldabra Warbler Nesillas aldabranus - is probably now extinct. Undiscovered until 1967, except for one record of a bird heard singing, it was only ever known from a 10 ha coastal strip (2 km x 50 m) at the west end of Malabar. A 1974-75 study located five birds: three males and two females (18) and the last sighting, of a single male, was in 1983 (13). At this time it was described as '...almost certainly the rarest, most restricted and most highly threatened species of bird in the world' (5), but by 1994 it was considered extinct (6). It is possible the species might still survive in little-visited and impenetrable south-west Grande Terre, but the chances of this appear increasingly remote. Extinction may have been natural, but rats are also prime suspects given the high level of predation experienced by other species (22).

Aldabra Drongo Dicrurus aldabranus has the distinction of being the sole undisputed endemic still certainly extant. It is relatively common throughout the atoll with an estimated population of 1,500. Related to the Crested Drongo Dicrurus forficatus of Madagascar (3) (the original source of all Aldabra's landbirds), it is accorded species status on the basis of differences in plumage and call. Aldabra awaits a full genetic study of its avifauna. Arguments exist for raising other taxa, currently considered races, to species level. One such is White-throated Rail Dryolimnas cuvieri aldabranus, which has been considered specifically distinct, as Aldabra Rail Dryolimnas aldabranus (25). The Aldabra Rail's differences to Madagascar stock clearly go beyond habitat. It has evolved in geographic isolation and its wings are defunct, giving it the dubious distinction of being the only surviving flightless bird of the Indian Ocean region. Morphologically it is not obviously distinct, but specific status would give this remarkable taxon a higher conservation profile.

A second candidate is Forest Fody Foudia eminentissima aldabrana, which was treated as Aldabra Fody Foudia aldabrana by Sinclair & Langrand (25). Fodies are confined to the western Indian Ocean. The genus comprises 5-6 sexually dimorphic species which vary considerably in plumage and in the size and the shape of the bill (according to diet) (12). Aldabra Fody is particularly distinctive, males being the most attractive of all fodies, possessing a bright scarlet head, neck and upper breast, which is sharply demarcated from the sulphur yellow belly. Its huge bill is remarkable, even relative to its large body size. Forest Fody F. eminentissima of Madagascar is smaller, has a proportionately smaller bill, a grey belly and flanks, and is thus considerably less striking than Aldabra Fody. The taxon confined to Aldabra has a catholic diet, including insects, seeds and nectar (its tongue exhibits a degree of adaptation to a nectar diet). Its large bill is probably the result of adaptive radiation to exploit all available food sources on Aldabra in competition with other landbirds.

Sacred Ibis Threskiornis aethiopicus is currently considered to comprise three subspecies: nominate aethiopicus in Africa, bernieri in Madagascar and abbotti in Aldabra. The ibis of Aldabra and Madagascar are very similar. Both are smaller than the nominate race, the bills are more slender and the tips to the primaries and secondaries are white; these are black in the nominate race (14). The iris of abbotti is blue and that of bernieri white, both considerably paler than the brown iris of the nominate race. It is possible that Madagascan and Aldabran birds merit specific status. If so, the name bernieri has priority and Madagascar Sacred Ibis T. bernieri has two races: nominate bernieri in Madagascar and abbotti on Aldabra. This would have immediate conservation implications. Madagascar Sacred Ibis is probably one of the most threatened species in the Afrotropics. Indeed, Aldabra with 100-250 pairs may prove a vital stronghold.

The case for elevating other taxa to species is less obvious. Souimanga Sunbird Nectarina souimanga aldabrensis, Aldbra's most common landbird, is little different morphologically to the nominate race of Madagascar, although this is not true for the sunbirds on other islands in the Aldabra group. These are larger and have much darker underparts (almost black in buchenorumof Cosmoledo and Astove, and dark brown in abbotti of Assumption); birds on Madagascar and Aldabra have a pale yellow belly. If this is a separate species, abbotti has priority, and the taxon could be known as Abbott's Sunbird Nectarina abbotti.

Aldabra atoll boasts a further seven endemic races of landbird: Madagascar Kestrel Falco newtoni aldabranus, Madagascar Turtle Dove Streptopelia picturata coppingeri, Comoro Blue Pigeon Alectroenas sganzini minor, Madagascar Bulbul Hypsipetes madagascariensis rostratus, Madagascar Nightjar Caprimulgus madagascariensis aldabrensis, Madagascar CoucalCentropus toulou insularis and Madagascar White-eye Zosterops maderaspatana aldabrensis. The case for subspecific recognition of the kestrel is probably weak, there being no obvious morphological differences between birds on Aldabra and Madagascar. However, it is the rarest surviving landbird of the Aldabra group, with probably fewer than 50 pairs. Comparative genetic analysis and a taxonomic review of all the endemic subspecies of the Aldabra group, taking into account voice, behaviour and ecology, is certainly required.

Elsewhere in the group, Madagascar Turtle Dove was rediscovered Cosmoledo in small numbers on in 1982, having been considered extinct (16). Cosmoledo has one other endemic subspecies - Madagascar White-eye Zosterops maderaspatana menaiensis. However, many species on Cosmoledo, Astove and Assumption failed to survive long after the arrival of man. The only non-endemic landbird on Aldabra is Pied Crow Corvus alba. Crows have been fingered as a menace by some who claim they arrived at Aldabra in the wake of man. Crows have had a bad press in Seychelles, with a well-publicised and successful campaign to eradicate the introduced Indian House Crow C. splendens. However, written accounts of Aldabra mention Pied Crow in the group as early as 1878 (7) and in 1836, long before human settlement, at Astove (28). The species' habit of soaring at altitude to locate food sources may have led to continued genetic inflow along the chain of islands from Madagascar to the Comoros, explaining the lack of subspeciation.

Aldabra's capacity to surprise was vividly demonstrated in 1995 when evidence of breeding Greater Flamingo Phoenicopterus ruber was discovered (4). Flocks of up to 500 birds had been seen in the past (smaller numbers are more common) and breeding had been suspected by some authors, although others considered the species migratory. Breeding was proven again in 1996. Aldabra is the world's only coral atoll where the species breeds, and one of only two oceanic sites in the world: the other is the Galápagos. Aldabra is also the only oceanic breeding site for Caspian Tern Sterna caspia which survives in a tiny colony of 6-10 pairs. Two other species - Swift S. bergii and Black-naped Terns S. sumatrana - breed in greater numbers here than anywhere else in Seychelles, but numbers of Fairy Tern Gygis alba and Brown Noddy Anous stolidus are considerably smaller than further east in Seychelles (8).

The logo of Seychelles Islands Foundation is a frigatebird. Two species - Great FrigatebirdFregata minor and Lesser Frigatebird F. ariel - breed in colonies of 4,000 pairs and 6,000 pairs (21). With non-breeding birds (frigatebirds take seven months to fledge followed by up to 18 months of post-fledging parental care), the total population of frigatebirds exceeds 30,000. This is the world's second largest population after McKean Island in the Pacific Ocean. Red-footed Booby Sula sula breeds alongside the frigatebirds, nesting in mangroves. Boobies are harried by frigatebirds at sea in order to steal nesting material and food. In some parts of the world robbing boobies may be an important food source, but this does not appear to be the case at Aldabra, where frigatebirds greatly outnumber boobies (9).

On rat-free islets, Red-tailed Phaethon rubricauda and White-tailed Tropicbirds P. lepturus breed with c2,000 pairs of each (10). Some islets also host Audubon's Shearwater Puffinus lherminieri colstoni; a race described in 1996 (and apparently confined to Aldabra, although it may breed elsewhere, perhaps on Cosmoledo) (24). The eastern islands and the atoll and neighbouring lagoon coast of Grand Terre is the stronghold of the globally threatened Madagascar Pond HeronArdeola idae. Like the ibis, it is threatened in Madagascar, which adds importance to the protected status of its habitat in Aldabra despite the relatively small number of breeding pairs (20-50). The lagoon dries extensively at low tide and is an important feeding area for migrant waders. Crab Plover Dromas ardeola are commonly reported in flocks of up to 1,000 birds. Dimorphic Egret Egretta (garzetta) dimorpha, which breeds on the atoll is also common along the shoreline.

Several vagrants accepted by Seychelles Bird Records Committee (SBRC) have been recorded in the Aldabra group and nowhere else in the islands, including White-faced Whistling DuckDendrocygna viduata, Whitethroat Sylvia communis, and Red-backed Lanius collurio and Lesser Grey Shrikes L. minor (27). Some species which are vagrants elsewhere in Seychelles are more regular in the Aldabra group and may prove to be annual in occurrence, notably Broad-billed Roller Eurystomus glaucurus, Northern Wheatear Oenanthe oenanthe and Spotted FlycatcherMuscicapa striata. Bird recording in Seychelles is still in its infancy. However, in one study Phillips et al (17) analysed 300 records of 52 species of migrant landbirds accepted by SBRC up to 1995. They found that, as expected, there was October-November peak in records throughout Seychelles, but, more surprisingly, a large March peak in the Aldabra group not evident elsewhere. It is possible Aldabra lies on the normal migration route of some species.

Threats
Human presence is now restricted to a small population at the research station on Picard, but earlier residents brought with them the usual introduced predators. Goats, introduced to provide food for passing ships, have now been eradicated except on Grand Terre. However, the size of this island makes total eradication extremely difficult. Cats are present, although not on islands supporting populations of Aldabra Rail. The recent elimination of cats from Picard hopefully will pave the way for the re-introduction of rails to the island. Rats are a major threat, with some species restricted to tiny rat-free islets. They take a heavy toll on landbirds such as Aldabra Fody. Eradication from some islands is desirable, but the logistics will make it difficult.

Poaching is a problem, although probably not of major concern to Aldabra's avifauna at present. This threat was certainly greater in the past, when birds were seen as another exploitable asset. With the advent of tourism, disturbance could create difficulties. Seychelles Islands Foundation have designated areas where tourism is permitted and can be controlled. A greater problem comes from unauthorised access, with boats sometimes arriving illegally from Comoros. Possibly the biggest threat to Aldabra's avifauna is on Assumption. It was here that, in the 1970s, Mauritian staff of a phosphate mining company based on the island introduced a number of exotic bird species from their home country (20). These include Red-whiskered Bulbul Pycnotus jocususwhich, based on transect data, numbered 1,250-1,500 in 1997 (G. Rocamora unpubl.). Its spread on Assumption suggests it would do well on Aldabra, competing with the endemic avifauna (23).

Madagascar Fody, another introduction, could threaten the Aldabra Fody through hybridisation. This was demonstrated recently, when a lone Seychelles Fody Foudia sechellarum arrived on Aride Island in the granitic islands, became established within a population of Madagascar FodyFoudia madagascariensis and produced two hybrid offspring (15). Two other species introduced on Assumption in the 1970s - Barred Ground Dove Geopelia striata and Yellow-fronted CanarySerinus mozambicus - are also established, although their numbers appear low. To compound the problem a pair of Feral Pigeon Columba livia introduced in c1990-91 had increased to 69 birds by 1994 (26). Aldabra is unusual for a tropical island of its size in having no introduced avifauna. It would be as well to keep it that way. Indeed, a proposal to effect the removal of introduced avifauna to be followed by the reintroduction of Aldabran avifauna to Assumption has been made19. Sadly, despite the support of the Royal Society and Seychelles Islands Foundation, the plan has failed to convince the Seychelles Government of the seriousness of the problem except to gain support for the eradication of feral pigeons on the island (completed in 1996).

Nonetheless, the Seychelles Government has taken its custodianship of Aldabra very seriously. It deserves credit for setting aside such a large portion of its small land mass as a nature reserve. Over 40% of Seychelles has been designated as reserves, National Parks or protected areas. In particular, Seychelles deserves credit for securing the financial future of Seychelles Islands Foundation by handing over management of the country's second World Heritage Site - Vallée de Mai on Praslin - to the Foundation. Unlike Aldabra, Vallée de Mai has low overheads and high tourism income. Vallée de Mai's profits fund the huge expenses of running Aldabra. Thus, the prospects for the future of the birds of Aldabra and the rest of its fauna and flora are considerably brighter than in many equally fragile ecosystems elsewhere in the world.

Acknowledgements
Thanks are due to Ian Sinclair, whose observations first alerted me to the case to elevate Madagascar Sacred Ibis, Aldabra Rail, Aldabra Fody and Abbott's Sunbird to species status, and to Gérard Rocamora for valuable comments on the first draft of this article.

References

  1. Beamish, T. 1970. Aldabra Alone. London, UK: George Allen & Unwin.
  2. Beaver K. and Chong Seng, L. 1995. Aldabra the green. In Amin, M., Willetts, D. and Skerrett, A. (eds) Aldabra World Heritage Site. Nairobi: Camerapix.
  3. Benson, C.W. 1984. Origins of Seychelles land birds. In Stoddart, D.R. (ed) Biogeography and ecology of the Seychelles islands. The Hague: Junk.
  4. Bergeson, M. and Rainbolt, R. 1995. Flamingos nesting. Birdwatch: 15: 7-10.
  5. Collar, N.J. and Stuart, S.N. 1985. Threatened birds of Africa and related islands. Cambridge, UK: International Council for Bird Preservation.
  6. Collar, N.J., Crosby, M.J. and Stattersfield, A. 1994. Birds to watch 2: the world list of threatened birds. Cambridge, UK: BirdLife International.
  7. Diamond, E.P. 1981. An early report of the flora and fauna of the Aldabra group. Atoll Res. Bull. 255: 1-10.
  8. Diamond, A.W. 1971. The ecology of the seabirds of Aldabra. Phil. Trans. R. Soc. London (B) 260: 561-571.
  9. Diamond, A.W. 1974. The Red-footed Booby on Aldabra Atoll, Indian Ocean. Ardea 62: 196-218.
  10. Diamond, A.W. 1975. The biology of tropicbirds at Aldabra Atoll, Indian Ocean. Auk 92: 16-39.
  11. Environmental Research Group Oxford Limited. 1997. Final Report of the 1997 Aldabran Giant Tortoise and vegetation study. Oxford: ERGO.
  12. Frith, C.B. A twelve month field study of the Aldabran Fody Foudia eminentissima aldabrana. Ibis 118: 155-178.
  13. Hambler, C., Hambler, K. and Newing, J.M. 1985. Some observations on Nesillas aldabranus the endangered Brush Warbler of Aldabra Atoll, with hypotheses on its distribution. Atoll Res. Bull. 290.
  14. Lowe, K.W. and Richards G.C. 1991. Morphological variation in the Sacred Ibis Threskiornis aethiopicus superspecies complex. Emu 91: 41-45.
  15. Lucking, R.S. 1997. Hybridization between Madagascan Red Fody Foudia madagascariensis and Seychelles Fody Foudia sechellarum on Aride Island, Seychelles.Bird Conserv. International 7: 1-6.
  16. Mortimer, J. 1984. Rediscovery of the Turtle Dove Streptopelia picturata on Cosmoledo Atoll in the Seychelles. Ibis 126: 81-82.
  17. Phillips N.J. and the Seychelles Bird Records Committee. 1997. Migrant landbirds in Seychelles. Phelsuma 5: 13-26.
  18. Prys-Jones, R.P. 1979. The ecology and conservation of the Aldabra brush warbler (Nesillas aldabranus). Phil. Trans. R. Soc. London (B) 286: 211-224.
  19. Prys-Jones, R.P. 1993. The conservation of avian biodiversity on a unique atoll ecosystem. Unpubl. proposal.
  20. Prys-Jones, R., Prys-Jones, M. and Lawley, J.C. 1981. The birds of Assumption Island, Indian Ocean: past and future. Atoll Res. Bull. 248: 1-16.
  21. Reville, B.J. 1983. Numbers of nesting frigatebirds, Fregata minor and F. ariel on Aldabra Atoll Nature Reserve, Seychelles. Biological Conserv. 27: 59-76.
  22. Roberts. P. 1987. Is the Aldabra brush warbler extinct? Oryx 21 : 209-210.
  23. Roberts, P. 1988. Introduced birds on Assumption Island - a threat to Aldabra. Oryx 22: 15-17.
  24. Shirihai, H. and Christie, D.A. 1996. A new taxon of small shearwater from the Indian Ocean. Bull. Br. Ornithol. Cl. 116: 180-186.
  25. Sinclair, I. and Langrand, O. 1998. Birds of the Indian Ocean islands. Cape Town: Struik.
  26. Skerrett, A. 1994. The introduced birds of Assumption. Birdwatch 10: 4-8.
  27. Skerrett, A. 1996. The first report of the Seychelles Bird Records Committee. Bull. ABC 3: 45-50.
  28. Stirling, W. 1843. Narrative of the wreck of the Ship Tiger, of Liverpool... on the desert island of Astova. Exeter: privately printed.
  29. Stoddart, D.R. and Benson, C.W. 1970. An old record of blue pigeon Alectroenas species and seabirds on Farquhar and Providence. Atoll Res. Bull. 136: 35-36.

Adrian Skerrett

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