Plumage polymorphism of red-footed boobies (Sula sula) in the western Indian Ocean: an indicator of biogeographic isolation

The pantropical red-footed booby Sula sula is one of the most polymorphic s eabirds. In the Indian Ocean most extant colonies hold white morph adults e xcept on Europa Island (southern Mozambique Channel), where adults are of t he white-tailed brown morph and on Tromelin Island (western Indian Ocean), where one-third of the birds are of the white-tailed brown morph and two-th irds are of the white morph. On Tromelin, the morph ratio has remained cons tant over the last 40 years. An extinct colony (Glorieuses Islands, norther n Mozambique Channel) once supported a white-tailed brown morph population. These results suggest that the colonies of the western Indian ocean do not constitute one great gene pool where exchanges occur, but are isolated at various degree from each other. Particularly, the population of Europa is t he only current population of the Indian Ocean constituted almost entirely of white-tailed brown morph, suggesting that it is isolated from other near by colonies. Possible causes of isolation are discussed in the light of oce anic features of the Mozambique Channel. Although further studies are neede d to understand the adaptive significance of plumage coloration in red-foot ed boobies, I suggest that white-tailed brown morph at Europa may act as a defensive camouflage against kleptoparasitism by great frigatebirds Fregata minor and brown skuas Catharacta antarctica.