The Mariana mallard or Oustalet's duck (Anas oustaleti) is an extinct species of duck of the genus Anas that was endemic to the Mariana Islands. Its taxonomic status is debated, and it has variously been treated as a full species, a subspecies of the mallard or of the Pacific black duck, or sometimes as a subspecies of the Indian spot-billed duck.
Taxonomy The taxonomic status of the Mariana mallard is disputed, since it resembles an intermediate of the mallard and the Pacific black duck, two closely related allopatric species which frequently hybridise. Its males had two intergrading color morphs, called the "platyrhynchos" and the "superciliosa" types after the species they resembled more. It was first scientifically described by Tommaso Salvadori as a full species in the genus Anas, named after its collector, the French zoologist Emile Oustalet. Salvadori suggested it was related to the Pacific black duck. It was previously known to the Chamorro people, who called it ngånga' (palao) in Chamorro, and to the Carolinian people, who called it ghereel'bwel in Carolinian. After Salvadori, most taxonomists, such as Dean Amadon and Ernst Mayr, considered it a subspecies of the mallard. Yoshimaro Yamashina examined those specimens in Japanese museums in 1948, and decided that the Mariana mallard was an example of hybrid speciation, and was descended from the mallard and the Pacific black duck's Palau subspecies (Anas superciliosa pelewensis). However, no molecular genetic evidence is available to support this hypothesis. Some scientists, such as Jean Delacour, have considered the Mariana mallard a simple hybrid, so it was absent from Delacour's four-volume monograph on the ducks and from the IUCN Red List. If Yamashina's hypothesis is correct, the Mariana mallard would have presumably evolved into near species status in only about ten thousand years. Neither Mariana mallards nor their progenitor species are known from fossils on the Marianas, casting into doubt the assumption that a resident black duck population had been long established on the islands. However, most rock shelters and caves on the Marianas were obliterated in the 1944 Battle of Guam. A species of flightless duck is known from a prehistoric bone found on Rota in 1994; it was apparently not closely related to the Mariana mallard.
Description Mariana mallards were 51–56 cm (20–22 in) long and weighed approximately 1 kilogram (2.2 lb), making them marginally smaller than mallards. Two intergrading color morphs were found in males, called the "platyrhynchos" and the "superciliosa" type after the species they resembled more. Only the former had a distinct nuptial (breeding) plumage: the head was green as in mallard drakes, but less glossy, with some buff feathers on the sides, a dark brown eyestripe and a faint whitish ring at the base of the neck. The upper breast was dark ruddish chestnut brown with blackish-brown spots. The wing patch (speculum) and the tail was also like in mallard drakes' nuptial plumage, including curled-up central tail feathers, but the tips of the speculum feathers were buff. The underside was a mix between the vermiculated grey feathers of the mallard and the brown ones of the Pacific black duck. The remainder of the bird looked like a male Pacific black duck with lighter underwings. The bill was black at the base and olive at the tip, the feet reddish orange with darker webs and the iris brown. The eclipse plumage looked similar to a dark eclipse mallard drake. Males of the "superciliosa" type resembled a Pacific black duck with a less distinctly marked head, the supercilium and cheeks being buffy and the cheek (malar) stripe hardly visible. The upper breast, flank and scapular feathers had broader buff edges, and the underwings were lighter. The speculum was usually as in the "platyrhynchos" type, i.e. mallard-like, but at least two specimens have the green speculum of the Pacific black duck. The bill was like that of A. superciliosa, and the iris and legs similar to the "platyrhynchos" type. Females looked essentially like a dark mallard female with the orange of the feet and near the bill tip usually a bit more pure.
Distribution It occurred, in recent times at least, on the islands of Guam, Saipan and Tinian. Two unidentified ducks were seen on Rota in 1945, but as no movement of A. oustaleti between Saipan and Tinian, which are just 8 kilometres (5.0 mi) apart, was recorded, these were probably vagrant migrating ducks, although Marshall (1949) suspected from circumstantial evidence that such movement did indeed take place. However, the distance between Guam and Rota is nearly 80 kilometres (50 mi), making intentional migration between these islands unlikely.
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