The land birds of Aldabra

Abstract

An account is given of the investigations on the land birds made during phases I, II and III of the Royal Society Aldabra Expedition, from 30 August to 31 March 1968. Some further information collected during phase IV, from April to July 1968, when officially there was no ornithologist on the atoll, is also included. These birds are divided into three categories:— (a) Seven species in the order Ciconiiformes, five of them in the family Ardeidae (herons and egrets), an ibis Threskiornis aethiopica and a flamingo Phoenicopterus ruber . The ibis is an endemic subspecies, T. a. abbotti , and that of the Green-backed Heron, Butorides striatus crawfordi , is only otherwise so far known from Assumption and the Amirante Islands. (b) Fourteen species of land birds proper, ranging from a kestrel Falco newtoni to a fody Foudiaem inentissima (family Ploceidae). Two of these, a drongo Dicrurus aldabranus and a warbler Nesillas aldabranus , are regarded as endemic full species. The latter was only discovered during phase I of the expedition, in December 1967. Seven of the others are endemic subspecies. Another one, a turtledove Streptopelia picturata coppingeri , probably occurred formerly throughout the Aldabra archipelago, and on Gloriosa, but may only now survive on Aldabra. Yet another, Centropus toulou insularis , was formerly on Assumption as well as Aldabra, but has probably been extirpated on the former. (c) Twenty migratory species (mostly palaearctic breeders), but exclusive of shore-frequenting species in the families Charadriidae, Scolopacidae and Dromadidae, are dealt with in a separate paper. The species in these categories (a) and (b) are considered under the same headings, the main points from which can be summarized:— Status : A barn owl Tyto alba has not been recorded since 1906, and is believed to be extinct, though the cause of this is uncertain. Colonization was probably unaided by man. It had not developed into an endemic subspecies. Neither have any of the others been introduced by man directly. His impact on the avifauna has indeed been relatively slight in comparison to that on many other islands in the western Indian Ocean. Nonetheless the numbers of Threskiornis aethiopica and Streptopelia picturata are thought to be less than formerly, because of killing for food around Settlement at the western end of the atoll. But they still thrive further east, as does a blue pigeon Alectroenas sganzini , which may have suffered likewise. A rail Dryolimnas cuvieri has been extirpated on South and West Islands, probably because of the introduction of cats and rats. Cats may eat appreciable numbers of a nightjar Caprimulgus madagascariensis . Care should be taken to avoid the introduction of a fody Foudia madagascariensis , brought by man from Malagasy to some other islands in the western Indian Ocean, and which might compete with the endemic subspecies of F. eminentissima . At Settlement the latter fills to some extent the role of the house sparrow Passer domesticus in some other parts of the world, being unusually tame and picking up scraps of food around houses. Threskiornis aethiopica readily deserts its nesting colonies, which should be left undisturbed. Moreover the population probably does not exceed 200. Of the other Ciconiiformes, the numbers of the Malagasy Squacco Heron Ardeola idae may be only about 100. But it is believed to be a recent colonizer, probably resulting from its migrations from breeding quarters in Malagasy to winter in eastern Africa. On Aldabra it was found breeding in November 1967 in a mixed colony with other Ardeidae. This is the only known breeding area outisde Malagasy. The Cattle Egret Bubulcus ibis may also have been quite recently established, although a specimen in breeding dress was collected as long ago as about 1903. The numbers in South Africa, and probably Malagasy, have increased because of new methods of land usage by man, which may have caused an overflow of this species into neighbouring areas. It apparently associates with the goats introduced by man, and its increase would therefore be favoured by any increase in the numbers of goats. The status of Phoenicopterus ruber is uncertain. Aldabra may be a dead-end for wanderers from elsewhere. An egg was found on 25 September 1967. But it was not on a nest-mound, and this is not accepted as evidence of breeding. Most of the land birds proper, in category (b), are still numerous. An exception is the kestrel Falco newtoni , whose numbers may never have exceeded 100. Another is the recently discovered Nesillas aldabranus , only so far known from the western end of Middle Island. It may be a relic from a former wetter regime. Although the numbers of a crow Corvus albus may not exceed 100, it thrives, especially around Settlement. As elsewhere, it is a commensal of man, and but for this it is unlikely that it would have colonized Aldabra. In turn Falco newtoni , perhaps dependent on man or the crow for nesting sites, may have followed them to Aldabra. A warbler Cisticola cherina has recently colonized Cosmoledo and Astove, from Malagasy, and may eventually reach Aldabra. Another possible colonizer is the Palm-Swift Cypsiurus parvus , which could utilize coconut palms, introduced by man, for breeding. The most plentiful species, in descending order as follows, are probably four passerines:-A sunbird Nectarinia sovimanga , a white-eye Zosterops maderaspatana , a fody Foudia eminentissima , and a bulbul Hypsipetes madagascariensis . The drongo Dicrurus aldabranus is much less numerous, and can probably be numbered by hundreds, rather than by thousands as for the sunbird. But at present is is not possible to make more than an informed guess as to the numbers of any species, even the less numerous ones. It can however be added that a count at East Channel resulted in a total of 3168 Streptopelia picturata crossing the channel in 3 h. Ecology and food : Some of the Ciconiiformes are shore-feeders, this applying in particular to three of the Ardeidae, Ardea cinerea , Egretta garzetta and Butorides striatus , which feed alongside each other, E. garzetta being much the most numerous. Butorides striatus also feeds inland. The two other Ardeidae, Ardeola idae and Bubulcus ibis , feed almost entirely inland, the latter largely on insects. Threskiornis aethiopica feeds principally at inland pools on the platin in the southeast of the atoll. The category (b) species feed inland, showing in general no particular ecological preference. Most of them occur to some extent in mangroves, especially Alectroenas sganzini and Dicrurus aldabranus , possibly a reflection of an ancestral evergreen forest habitat. Nectarinia sovimanga and Foudia eminentissima and their nests are much in evidence in the scattered groves of Casuarina trees on the north side of the atoll. The spread of this tree on Aldabra may have been assisted by man. But it is not thought that there is any particular association of these birds with this relatively open habitat, in which observations are especially easy. Ardea cinerea is suspected of taking some chicks of the noddy tern Anous stolidus . The crow Corvus albus certainly takes some noddy eggs, and probably of some land birds too. Behaviour and voice : No detailed ethological studies were possible in the limited time available, though many random observations were. A detailed study is available from the Seychelles for Foudia sechellarum , and F. eminentissima aldabrana might be a suitable subject for a similar study on a comparative basis. There is also much information available on sunbirds Nectariniidae and white-eyes Zosteropidae from southern Africa, which could be utilized in studies of the Aldabra representative of each of these families. Tameness was noted in a number of species, though in some (but not all) this also applies to related species in Africa. Examples of species whose tameness is probably peculiar to Aldabra are Threskiornis aethiopica and Streptopelia picturata , but both may be rather less so than formerly. Dicrurus aldabranus , which has been long enough on Aldabra to have developed into a full species, is not particularly tame, yet is not known to have any enemies. The song-call of Streptopelia picturata on Aldabra and the Comoros appears to differ markedly. This should be verified by tape-recordings. Breeding : Although much further information is needed, it would appear that breeding seasons are generally similar to those in Malagasy and south-central Africa, where the incidence of the seasons is also similar. A possible exception is Threskiornis aethiopica , egg-laying in Aldabra being apparently confined to November and December, whereas egg-dates for Zambia are much more extensive. Clutch-sizes are less in several of the Ciconiiformes than in South Africa. In the land birds proper, there is some evidence of a progressive reduction in clutch-size northwards from Malagasy to the Seychelles, this apparently being much accentuated in Falco newtoni (Malagasy and Aldabra)/ F. araea (Seychelles). The 1967/68 season may have been a poor one for breeding by Hypsipetes madagascariensis , the ultimate cause of this being perhaps a shortage of rain. There is a mixed breeding colony of species of Ardeidae on lie aux Aigrettes, in the southeast of the lagoon. The identity of a group of 79 nests therein was undetermined, though it may have been a case of colonial breeding by Butorides striatus , which is normally a solitary breeder. Systematics, including origins and variation : Excepting Butorides striatus crawfordi , perhaps of direct Asiatic origin, the Aldabra avifauna is predominantly of Malagasy origin, though in at least one species ( Alectroenas sganzini ) an immediate one in the Comoros is much more likely. Colonization from Malagasy could have been via Gloriosa and the other islands of the Aldabra archipelago. Compared to Aldabra, these islands are so small that they can maintain viable populations of only a fraction of the number of species that Aldabra can. A case in point is Caprimulgus madagascariensis , only known outside Malagasy from Aldabra. Poor fliers such as Dryolimnas cuvieri and Centropus toulou could have been brought to Aldabra on rafts of floating vegetation, of which there were examples off the north coast of the atoll in February 1968. Certain trends of variation are apparent. The most general one would appear to be the result of a fairly dry climate, leading to reduction in melanin, usually manifest as pallor. A case in point is Nesillas aldabranus , which is most similar in colour to a form in dry southwestern Malagasy. There are several cases of a reduction in size (inferred from a relatively short wing-length), perhaps the result of unusually high temperatures. But the reduction in wing-length, which has resulted in almost complete flightlessness, in Dryolimnas cuvieri , is a special adaptation also known in Rallidae on various other islands. There are some instances of an increase in length or size of bill, which might be an adaptation to deal with a greater range of food-sizes. There are three examples of an increase in tail-length, but the significance of this is not understood. Migrants: The 20 migratory species which have been recorded are for the most part mere strays. They are mostly palaearctic breeders which winter in southern Africa. But a wagtail and a wheatear Oenanthe oenanthe may also do so on Aldabra in very small numbers. A proper assessment of their status is difficult, owing to the nature of the terrain, over which quick movement is not possible. Other species, especially a flycatcher Muscicapa striata and a swallow Hirundo rustica , may pass through regularly on northward passage, and it is possible that they winter in very small numbers in Malagasy as well as in much larger numbers in southern Africa. There is one probable record of Falco eleonorae , already known to winter in Malagasy, and another such species likely to occur is F. concolor . There are a few species known to breed in Malagasy and winter in eastern Africa, notably a roller Eurystomus g. glaucurus . But it was only once recorded on Aldabra in 1967/68. There are many species which breed in southern Africa and winter further north, some even crossing the Equator. There is one record each from Aldabra of Threskiornis a. aethiopica and a rail Porzana marginalis , which could have been blown there accidentally during the course of such movements. Many more records of this kind are likely to be forthcoming. Further investigations on migrants generally should be carried out on an island nearer to the coast of Africa, such as Grand Comoro, and could prove still more fruitful than those already made on Aldabra.