Chapter 2 Chapter 2
2. Birds
The land birds, of which t h e r e a r e sixteen known resident species, a r e most numerous in the Mixed Scrub on the platin, and in Casuarina and coconut groves.
Benson (1967) considers that only one, the Drongo Dicrurus aldabranus, is a full species endemic to Aldabra, but at the s a m e time only two of the native Aldabra land birds cannot be distinguished from other m e m b e r s of the s a m e species in nearby a r e a s (the Grey Heron Ardea cinerea cinerea and the Barn Owl Tyto alba affinis). Endemic subspecies a r e t h e s a c r e d Ibis Threskiornis
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aethiopica
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abbotti, the r a i l Dryolimnas-
cuvieri aldabranus, the nightjarCaprimulgus madagascariensis aldabrensis, and the fody Foudia eminentissima -- aldabrana, the last of which commonly occurs a t tbe West Island settlement and in Casuarina woodland elsewhere. Good subspecies found on Aldabra and also on nearby reef islands a r e the little Green Heron Butorides
--
s t r i a t u s crawfordi,--
the Madagascar Turtledove Streptopelia picturata copp-i, - and the Soui- manga Sunbird Nectarinia sovimanga aldabrensis. Aldabra also h a s l e s s dis- tinct f o r m s of the k e s t r e l F a l c o newtoni aldabranus, the Comoro Blue Pigeon Alectroenas sganzini minor, the Madagascar Coucal Centropus e u insularis, the Madagascar Bulbul Hypsipetes madagascariensis r o s t r a t u s , and the
Madagascar White-eye Zosterops --- maderaspatana aldabrensis. Benson finds the land avifauna to be mainly of Madagascan origin, and suggests colonisation either via the Comoros o r via Gloriosa and the islands to the e a s t of Aldabra.
F o r detailed consideration of the native land birds, and also of r e c e n t a r r i v a l s such a s the Pied Crow Corvus albus, s e e the accompanying papers by Benson (1967) and Gaymer (1967): t h r e e species only a r e considered f u r t h e r here, on account of their susceptibility to human interference.
The White-throated Rail Dryolimnas cuvieri aldabranus (Gunther 1879) is almost flightless. Rails, many of them flightless, a r e found on islands through- out the world, though many of the insular populations have recently become ex- tinct following the introduction of p r e d a t o r s and increased human activity. Thus the r a i l s of Laysan and Wake Island in the Pacific have both recently become extinct. F r y e r (1911, 418) reported that the Aldabra r a i l ' 5 s generally distributed over the atoll, though i t is s c a r c e on Picard (West Island), and has generally been exterminated in the neighbourhood of Takamaka by the cats". Abbott (1893, 762) f e a r e d that the r a i l would soon be exterminated, "as their a r c h enemy, the cat, has already exterminated them f r o m Grande T e r r e (South Island), and must sooner o r l a t e r reach the other s m a l l islands of the group, where the r a i l s a s yet abound in great numbers". Voeltzkow (1897, 63) had found them plentiful and extremely tame. They have been recorded from Ile E s p r i t by F r y e r (1911, 418) and from Ile Michel (Anon. 1920, Ch.8, 9; Vesey-FitzGerald 1940, 487).
Rails now exist on Middle Island, where they w e r e seen in 1966, and on Polymnie (Bourne 19661, but they have disappeared f r o m West Island. Abbott (in Ridgway 1895, 528-529) g i v e s an account of their behaviour. Related b i r d s f o r m e r l y existed on Assumption, Cosmoledo and Astove, buthave a l l become extinct in this century, and the flightless r a i l of Aldabra is now the l a s t of the flightless birds of the Indian Ocean islands, a s e r i e s which once included the dodo and the solitaire (Lorenz 1908, Hutchinson 1953).
The S a c r e d Ibis Threskiornis aethiopica
w ,
is conspicuous on South Island, particularly round the m a j o r freshwater pools, each of which has one o r two b i r d s (Plates 28 and 32, and illustrations in Nicoll 1908). At Takamaka i t i s s t i l l extremely inquisitive and h a s to be kept away f r o m baggage, as described by Nicoll (1908, 121), but elsewhere i t i s less approachable. It i s r a r e a t the west end of the atoll, and was absent f r o m West Island n e a r thesettlement fifty y e a r s ago (Nicoll 1908, 119). F r y e r (1911, 417) reported i t on Ile Michel, and also described the destruction of eggs by the b i r d s in a nesting colony on South Island (1911, 417-418). Gaymer found the s a c r e d Ibis nesting at Takamaka (Plate 33). Because of itsinquisitiveness i t would undoubtedly suffer if the human population of Aldahra increased.
Another species in considerable danger is the flamingo, Phoenicopterus ruber roseus, which is not yet definitely known to breed, and which Benson
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(1967) does not consider to be distinct. Abhott in 1892 found a population of 500-1000 birds, in flocks of 20-60 individuals, on the south and e a s t s h o r e s of the lagoon (Ridgway 1895, 529). Dupont (1907, 21, 23) reported numerous flocks of s e v e r a l hundred birds along the south side of the atoll, and he and F r y e r (1911, 419) describe their flight and c r i e s . More recently, T r a v i s (1959, 202) noted "several small flocks" o n the north side of the atoll, but the Bristol Seychelles Expedition suggested that t h e r e may be only 50 left, and that the s u r v i v o r s breed in the B r a s Takamaka (Plate 36). It was not seen in 1966.
F r y e r collected the bird louse Esthiopterum subsignatum f r o m this species in 1908-9 (Scott 1914).
Sea and other s h o r e birds a r e numerous (Vesey-FitzGerald 1941; Benson 1967), and include both breeding and migrant species. F r i g a t e b i r d s (Fregata minor aldabrensis and F. a r i e l i r e d a l e i ) and Red-footed Boobies (Sula s u l a rubripes) nest in g r e a t numbers in the mangroves of Middle Island, where the f o r m e r a r e concentrated towards E a s t Channel and the l a t t e r n e a r Johnny Channel, though the nests of both s p e c i e s a r e intermingled. F r y e r (1911, 419) reported a nesting colony of f r i g a t e s on West Island which has now disappeared.
A s is usual with this species the f r i g a t e s p a r a s i t i s e the boobies, spending the day soaring on a i r c u r r e n t s to heights of a few thousand feet over the windward end of the atoll, awaiting the r e t u r n of the boobies with food. Though no ade- quate observations could be made of these vastcolonies in 1966, i t appeared that the number of frigates greatly exceed that of boobies. Several of the l a r g e f r e s h w a t e r pools on South Island a r e frequented by frigates, which dive con- tinuously to drink, scooping up water f r o m the s u r f a c e in their beaks while still on the wing (Plate 29). Similar diving behaviour of frigates (in this c a s e
F.
minor palmerstoni) h a s been reported f r o m a freshwater pool on Canton atoll in the Phoenix Islands (Degener and Gillaspy 1955, 6). It is thought that the Aldabra f r i g a t e colonies s e r v e as the m a j o r breeding ground f o r the f r i g a t e s of the western Indian Ocean, and that a considerable non-breeding population may be scattered over this a r e a (W. R. P. Bourne, personal communication). If an air- field w e r e to he built at Aldabra, t h e f r i g a t e s would clearly r e p r e s e n t a major aviation hazard, s i m i l a r to that of the a l b a t r o s s e s a t Midway Atoll in the Pacific ( F i s h e r 1966), and any control m e a s u r e s would have to take account of the fact that birds may continue to r e t u r n to Aldabra to breed f o r s e v e r a l years.
Red and White-tailed Tropicbirds (Phaethon ruhricauda rubricauda,
r.
lepturus lepturus) w e r e s e e n nesting on the ground on lagoon i s l e t s n e a r E a s t Channel in 1966. Other v e r y common s e a birds include the Noddy - Anous -- stoli- dus
-- pileatus, breeding on lagoon i s l e t s (Ridgway 1895, 527), and f a i r y t e r n s (Gygis alba monte). --
Shore and wading b i r d s a r e especially numerous, particularly in the lagoon a t low water. Dimorphic e g r e t s Egretta garzetta dimorpha, in both white and dark phases, a r e perhaps most striking; together with the C r a b Plover Dromas
ardeola, the Turnstone Arenaria