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

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The pantropical red-footed booby Sula sula is one of the most polymorphic seabirds. In the Indian Ocean most extant colonies hold white morph adults except on Europa Island (southern Mozambique Channel), where adults are of the 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-thirds are of the white morph. On Tromelin, the morph ratio has remained constant over the last 40 years. An extinct colony (Glorieuses Islands, northern 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 the only current population of the Indian Ocean constituted almost entirely of white-tailed brown morph, suggesting that it is isolated from other nearby colonies. Possible causes of isolation are discussed in the light of oceanic features of the Mozambique Channel. Although further studies are needed to understand the adaptive significance of plumage coloration in red-footed 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.