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The biodiversity of the BIOT is very rich, particularly in the marine environment which contains some of the world's healthiest coral reefs, the world's largest coral atoll (the Great Chagos Bank) and an exceptional diversity of deep water habitats. On land, BIOT is recognised as globally important due to the large numbers of congregating and nesting seabirds. The birds of the BIOT fall into three groups: the resident landbirds, the migrants and vagrants, and the seabirds. Only three resident landbirds are believed to have arrived as natural colonists: Common moorhen (Gallinula chloropus), White-breasted waterhen (Amaurornis phoenicurus), and Green-backed Heron (Butorides striata).
