The numbers of bird species on islands

ST. PATRICK EXPLICITLY banned snakes from Ireland, but the present state of the island suggests that his prejudices extended to woodpeckers, nightingales, and many other animals and plants.In all, only three-fifths of the land and freshwater birds that breed regularly in Britain breed regularly in Ireland (derived from Parslow 1967-68).Some other saints were more exclusive.St. Michael, for instance, allowed on his island in the Azores only about 21 native species, a tenth of the number in Iberia (derived from Bannerman 1966, Voous 1960), and St. Thomas permitted 33 on his island in the Gulf of Guinea, compared with 287 on Mount Cameroon (Amadon 1953, Eisentraut 1968).These are just three examples of the fact that far fewer species of land birds reside on islands than the adjoining mainland.Most earlier workers assumed that this was due solely to difficulties of dispersal.However, Stresemann (1927-34) and Mayr (1940) showed that the number present is correlated with island-area, as demonstrated later in detail for the islands west of Sumatra by Ripley (1944), and for other types of animals on islands by Darlington (1957) and Preston (1962).But the degree of isolation is also important, and the clear need to combine the influence of the two factors led to the equilibrium theory of MacArthur and Wilson (1963, 1967): the distance of an island from other lands greatly influences the rate of colonisation of new species, while the size of the island greatly influences the rate of extinction of those present.Later, Hamilton and his co-workers (1964-67) showed for various other archipelagoes that the number of bird species on each island is closely linked with its size, and that its isolation is usually of minor importance, though the major factor in Darwin's finches in the Galapagos.They followed Ripley in considering that islandsize is important because it reflects habitat-diversity, as well shown for the avifaunas of the Aegean islands by Watson (1964).In this lecture, I will discuss these problems in relation to the land (including freshwater) birds of various islands in the Atlantic, seabirds