Birds of Aldabra: 2

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 buchenorum of 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 Coucal Centropus 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 Frigatebird Fregata 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).

Red-footed Booby, (A. Skerrett)

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 Duck Dendrocygna 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 Flycatcher Muscicapa 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 jocusus which, 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 Fody Foudia madagascariensis and produced two hybrid offspring (15). Two other species introduced on Assumption in the 1970s - Barred Ground Dove Geopelia striata and Yellow-fronted Canary Serinus 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

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