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An unlikely survivor: the peculiar natural history of the Raso Lark Paul F. Donald and M. de L. Brooke ABSTRACT The Raso Lark Alauda razae is one of the rarest and most threatened birds in the world. Until recently, almost nothing was known of its population, behaviour or ecology. Recent research has shown that the species exhibits a number of fascinating behavioural and ecological peculiarities.The tiny population, currently fewer than 150 individuals, is dominated by males, which outnumber females by two to one.The males differ from the females in bill structure and feeding behaviour, possibly representing an adaptation to reduce competition between the sexes in a hostile environment.The species’ population is correlated with rainfall, and numbers in the past must have fallen to extremely low levels, possibly even into single figures, during periodic long droughts. Nesting success is very low, owing to heavy predation of eggs by the near-endemic Cape Verde Giant Gecko Tarentola gigas, itself a threatened species. Despite this, and a growing number of other threats, the species still manages to maintain a precarious toehold on its tiny, drought-prone islet home, thanks largely to the continued absence of the introduced mammalian predators that infest neighbouring islands. Recent tourist developments on neighbouring islands, leading to greater visitor numbers, and a predicted increase in drought through climate change pose new and increasing threats. Urgent measures are required to secure the long-term survival of this remarkable and unlikely survivor. n 28th April 1897, the young English southern half, however, he soon realised that the naturalist and explorer Boyd Alexander island was inhabited not only by abundant O(1873–1910) scrambled up the low seabirds, but also by an unfamiliar lark, which cliffs that fringe the southern rim of the tiny, he described as ‘so tame that we could have waterless and uninhabited island of Raso, one knocked many over with sticks’ (Alexander of the smallest of the Cape Verde Islands. Off- 1898 a,b). He compensated for the absence of shore, his ship, ‘a fine old American pilot-boat sticks on this treeless island by shooting a good of nearly eighty tons’, was forced by the absence number of them instead, and 29 of these speci- of a safe anchorage to beat backwards and for- mens still reside in the skin collections of the wards, waiting for him and his companions. British Museum of Natural History in Tring Alexander had previously visited several of the and the American Museum of Natural History other islands in this (at the time) Portuguese in New York. Alexander named his discovery colony, and can hardly have expected to make Spizocorys razae, without explaining why he his greatest discovery on one of the smallest and assigned it to a genus represented at that time least hospitable of the group. After emerging by a single southern African species. After onto the flat, low plain that makes up Raso’s spending taxonomic time in the monotypic 420 © British Birds 99 • August 2006 • 420–430 The peculiar natural history of the Raso Lark (and now defunct) Razocorys, and then in collection. After another long absence from the Calandrella, the Raso Lark was finally recog- attention of ornithologists, the Raso Lark nised by Hall (1963) to be closest to the skylarks briefly reappeared in the writings of Mein- Alauda, a position recently confirmed by DNA ertzhagen (1951) and Bourne (1955), before the analysis (Keith Barnes pers. comm.) and sup- Abbé de Naurois embarked upon a series of ported by detailed behavioural observations visits to the island in the 1960s. David and Mary (Donald et al. 2003). The main structural differ- Bannerman, authors of the first book on Cape ences between Raso Lark and Sky Lark Alauda Verde’s birds (Bannerman & Bannerman 1968), arvensis (smaller size, less pointed wing and preferred to view Raso from the comfort of a longer, heavier bill of the former) may all result passing ship and did not land, instead extolling from adaptations to life on a small, arid island the bravery and hardiness of de Naurois and (Hall 1963; Burton 1971; Hazevoet 1995), and others in landing and staying on the island. the two species are closely related. Further specimens were collected until at least A year after Alexander’s discovery, the Italian 1970 (eight specimens of this date reside in naturalist Leonardo Fea (1852–1903) visited Lisbon Museum), but little was recorded of the Raso and collected a further 30 specimens, now species’ numbers, ecology or behaviour until in the Museo Civico di Storia Naturale, Genova. the late 1980s, and the standard account given (Fea was also, incidentally, perhaps the last nat- in BWP is based largely on early and sketchy uralist to observe alive the remarkable Cape accounts. Verde Giant Skink Macroscincus coctei,once endemic to Raso and nearby Branco and prob- Distribution, population and sex ratio ably hunted to extinction around a century Cape Verde is the southernmost of a number of ago.) The lark was then left in peace until 1922, northeastern Atlantic island groups known col- when José Correia (1881–1954), an Azorean lectively as Macaronesia, which also contains the naturalist, collected specimens for the American rather better known (at least ornithologically) Museum of Natural History in New York. Two Canary Islands, Azores and Madeira. It is the years later, 16 further specimens were collected only politically independent member of this during an expedition in the sailing ship Blossom group, having ceded from Portugal in 1975. The by the Cleveland Museum of Natural History, archipelago lies in the northeast trade-wind belt, Ohio, some of which are also in the New York at about 15°N and approximately 500 km west of Paul Donald Paul 207. Raso in October 2001. Recent rainfall resulted in a growth of green cover and most birds were nesting. The sandy areas in the foreground contained large numbers of holes, excavated mainly by male Raso Larks Alauda razae to extract small bulbs from below the surface. British Birds 99 • August 2006 • 420–430 421 The peculiar natural history of the Raso Lark Senegal, and consists of nine inhabited islands equally isolated Warsangli Linnet Carduelis and a number of smaller islets, the island group johannis may owe its existence on the opposite having a total land area of 4,033 km2, scattered side of the African continent to a similar over approximately 60,000 km2 of ocean. Raso process of postglacial stranding in a habitat lies in the northern group of islands, the so- with few competitors. It is perhaps no wonder called ‘Windward Islands’ (Ilheus do Barlavento), that, despite noting several similarities to the and is one of three uninhabited islands (the Sky Lark (including its call), Boyd Alexander other two are Branco and Santa Luzía) that lie did not consider the possibility of his new bird between São Nicolau and São Vicente. being closely related to that familiar but geo- Where did the Raso Lark, so closely related graphically distant species. to the Sky Lark yet separated from the nearest Given these alternative scenarios, the time of population of that species by more than 2,000 arrival of the Raso Lark’s ancestors in Cape km of sea and desert, come from? There are at Verde is unclear. In conformity with the idea of least two possible explanations. The first is that late Pleistocene speciation, Hazevoet (1995) and this population represents the last remnant of a others have suggested that the species’ arrival is species that was formerly far more extensive likely to have been recent, within the last across Africa, perhaps a western counterpart of 100,000 years. Ongoing genetic studies, the Oriental Lark A. gulgula, which replaces the however, indicate a difference between Sky Lark Sky Lark in southern Asia today. This may have and Raso Lark in the mitochondrial become extinct on the mainland as climatic cytochrome-b gene of around 7.2%. A conven- conditions changed, but persisted in Cape tional estimate is that avian cytochrome-b DNA Verde. Alternatively, the species may have diverges by approximately 2% per million years. evolved in situ on Cape Verde following coloni- Although the application of this standard has sation by Sky Larks (or their ancestors) during been severely criticised, a 7.2% divergence one of the Ice Ages, when the Palearctic fauna between Sky Lark and Raso Lark strongly sug- was pushed southwards into what is now the gests a divergence at least 2–3 million years ago, Sahara. As the ice retreated and the Sahara rather than during the glacial and interglacial reverted to desert, the Sky Larks gradually cycles of the late Pleistocene in the last few returned northwards, leaving some stragglers hundred thousand years (Keith Barnes pers. stranded behind on Cape Verde to evolve into comm.). This new evidence suggests that either the species that survives there today. The the Raso Lark and its ancestors must have sur- vived on Cape Verde for far longer than originally thought, or the Raso Lark began its separation from the Sky Lark elsewhere before colonising the Cape Verde Islands. There is no written or physical evidence of the species from any of the other Cape Verde islands, but unless it is an extremely recent arrival (within the last 20,000 years, which now seems highly unlikely), its ancestors would have been present when Raso was intermittently connected through low sea levels to the neighbouring islands of Branco (3 km2), Santa Luzía (35 km2) and São Vicente Michael Brooke 2 208. Raso Lark Alauda razae, Raso, Cape Verde, December 2002. (227 km ). It is therefore 422 British Birds 99 • August 2006 • 420–430 The peculiar natural history of the Raso Lark likely to have had a far larger distribution than it few birds are found here during the breeding has today, as the resulting island (which would season, but up to half the population moves of course have included all the submerged areas here when the birds are not breeding.