Evolutionary history of Anas oustaleti.—In the past, most of the studies have pointed to a northern ancestry for A. oustaleti. Bryan (1941:187) has noted a relationship between A. oustaleti and the Laysan Duck (A. laysanensis Rothschild) and the Hawaiian Duck (A. wyvilliana Sclater). Amadon (1943:1) suggests that these three species of ducks are rather recent derivatives of the Common Mallard (A. platyrhynchos) and postulates the evolution of A. wyvilliana from migrants from North America. He further states that A. laysanensis and A. oustaleti may have been derived from A. wyvilliana or may represent independent colonizations. Delacour and Mayr (1945:21) go a step further and make these forms subspecies of A. platyrhynchos, saying that they are "dull-colored editions" of the Common Mallard, that because of isolation they have become reduced in size and have lost many of the characteristics of their ancestors. Recently, however, Yamashina (1948) has concluded that the Marianas Mallard has evolved as the result of hybridization between the two species, A. platyrhynchos and A. poecilorhyncha. His conclusions are based on a study of a large number of specimens, both museum skins and captive birds, in which he has been able to detect plumages of the A. platyrhynchos type and of the A. poecilorhyncha type (see Characters). He has noted specimens which have ninety percent of the characteristics of A. platyrhynchos and ten percent of the A. poecilorhyncha type. These percentages are reversed in specimens favoring the A. poecilorhyncha type. In his series of skins he finds the A. poecilorhyncha type of plumage most frequently, in forty-four specimens out of fifty examined, while only six specimens have the A. platyrhynchos type of plumage. Yamashina cites also as evidence favoring his conclusion that hybridization has taken place the results obtained from the crossing of captive A. platyrhynchos and A. poecilorhyncha. It is his assumption that there has been a resident form of A. poecilorhyncha in the Marianas, apparently resembling closely that which occurs in the Palaus and at Truk (A. p. pelewensis), and that stragglers of A. platyrhynchos from the north occasionally reach the Marianas where hybridization between the two species occurs. Yamashina remarks (1948:123): "The opportunity for hybridization should occur more rarely in the south, and thus more frequent back-crossing of the hybrid with the indigenous Anas poecilorhyncha on Tinian and Guam explains the superabundance there of the poecilorhyncha type. As the hybridization should have taken place more frequently to the north in Saipan, the ratio of the occurrence of the platyrhynchos type is logically higher there." The Common Mallard (A. p. platyrhynchos) has not been recorded in Micronesia, but according to Yamashina (1948:123) "winters frequently just north of the Marianas in the Bonin and Volcano Islands."
This remarkable explanation for the development of the Marianas Mallard is not questioned by this author, who feels that hybridization may be found to be the cause for other unusual forms of life in island habitats whose ancestry has not been explained. As Yamashina comments, the special environments of islands together with small and restricted populations of animals are factors which could favor such development.
Anas poecilorhyncha pelewensis Hartlaub and Finsch
Australian Gray Duck
Anas superciliosa var. pelewensis Hartlaub and Finsch, Proc. Zool. Soc. London, 1872, p. 108. (Type locality, Pelew Islands.)
Anas superciliosa Hartlaub and Finsch, Proc. Zool. London, 1868, pp. 8, 118 (Pelew); Sclater, Proc. Zool. Soc. London, 1869, p. 659 (Pelew); Gray, Hand-list Birds, 3, 1871, p. 82 (Pelew); Salvadori, Ornith. Papuasia, 3, 1882, p. 395 (Pelew); Wiglesworth, Abhandl. und Ber. Zool. Mus. Dresden, no. 6, 1890-1891 (1891), p. 70 (Pelew); Salvadori, Cat. Birds British Mus., 27, 1895, p. 206 (Pelew); Oustalet, Nouv. Arch. Mus. Hist. Nat. Paris, (3), 8, 1896, p. 50 (Palaos).
Anas superciliosa pelewensis Dubois, Syn. Avium, 2, 1904, p. 990 (Pelew); Mathews, Birds Australia, 4, 1915, p. 90 (Pelew); Phillips, Nat. Hist. Ducks, 2, 1923, p. 113 (Pelew); Mathews, Syst. Avium Australasianarum, 1, 1927, p. 215 (Pelew); Hartert, Novit. Zool., 36, 1930, p. 112 (Pelew); Peters, Check-list Birds World, 1, 1931, p. 160 (Pelew); Hand-list Japanese Birds, rev., 1932, p. 184 (Palaus, Truk); Hand-list Japanese Birds, 3d ed., 1942, p. 205 (Babelthuap, Peliliu); Amadon, Amer. Mus. Novit., no. 1237, 1943, p. 3 (Palau); Mayr, Birds Southwest Pacific, 1945, p. 286 (Palaus, Truk); Baker, Smithson. Misc. Coll., vol. 107, no. 15, 1948, p. 45 (Peleliu, Truk).
Anas pelewensis Finsch, Journ. Mus. Godeffroy, 8, 1875, pp. 5, 40 (Palau); Schmeltz and Krause, Ethnogr. Abth. Mus. Godeffroy, 1881, p. 407 (Palau); Bolau, Mitteil. Naturhist. Mus. Hamburg, 1898, p. 71 (Palau).
Polionetta superciliosa pelewensis Kuroda, in Momiyama, Birds Micronesia, 1922, p. 38 (Pelew).
Anas superciliosa rukensis Kuroda, "Gan to Kamo" (Geese and Ducks), 1939, page not numbered, description between pls. 52 and 53 (Type locality, Ruk); Hand-list Japanese Birds, 3d ed., 1942, p. 206 (Truk).