The Christmas shearwater or aoū (Puffinus nativitatis) is a medium-sized shearwater of the tropical Central Pacific. It is a poorly known species due to its remote nesting habits, and it has not been extensively studied at sea either. It is one member of a very ancient lineage of the small Puffinus species. Its only close living relative is the Galápagos shearwater (P. subalaris). Taxonomy The species was described and given its current binomial name by the American naturalist Thomas Hale Streets in 1877. A genetic analysis using mitochondrial DNA has shown that the closest living relative is the Galápagos shearwater (Puffinus subalaris). Description The Christmas shearwater is a slender-bodied shearwater, about 36 cm long, with a wingspan of around 75 cm, and weighs around 350 g. It has dark plumage all over, generally blackish-grey with a rusty-brown tinge, slightly paler on the underside of the bird, and with some small edging of white under the chin and pale fringes to the upperwing coverts. It has brown-grey feet and a dark bill and eyes. Both sexes are alike, as are the young after fledging. Nestlings are covered in dark grey down feathers. The species closely resembles the sooty (Puffinus griseus) and short-tailed shearwaters (P. tenuirostris), but has dark brown underwings and is smaller. The short tail of the Christmas shearwater does not appear blunt except when spread, but in flight usually tapers to a point, enhanced by the feet which protrude beyond the tail-tip. It often flies in a leisurely way like the related Procellariidae, and thus can be mistaken for a petrel. In particular, it can appear similar to the extremely rare Fiji petrel (Pseudobulweria macgillivrayi), a gadfly petrel-like relative of the shearwaters. The two species share a similar morphology and colouration, but the bulbous head and rather thin long bill – typical for shearwaters – distinguish P. nativitatis from the slim-headed thick-billed Fiji petrel. Distribution and habitat The Christmas shearwater nests on remote islands of the Central Pacific: the Northwestern Hawaiian Islands, Tuamotu, the Marshall Islands, Kiritimati (for which the species is named) and Sala-y-Gómez. It has become locally extinct on a number of islands, including Wake Island. Outside of the breeding season it ranges across the Pacific, having been recorded off the coast of Mexico and Guatemala in the east, and Bonin Islands in the west. Further south it is rare, having been recorded off Fiji only twice