The Band-tailed Gull (also called Belcher's Gull and Sime6n Gull) has long been known to breed on the arid Pacific coast and adjacent islands of Perui and northern Chile (Murphy, 1936:1053-1056; Goodall, Johnson, and Philippi, 1951: 283). Earlier authors (Saunders, 1896:226; Dwight, 1925:158; Alexander, 1928: 129) included in the range the coast of Argentina, the Magellanic region, and the Falkland Islands, but Hellmayr (1932:409), Peters (1934:314), and Murphy (1936) rejected such reports, regarding Larus belcheri as endemic species of the Humboldt Current littoral. The collection of specimens on the Atlantic coast of Argentina (Daguerre, 1933:214; MacDonagh, 1934:312; Steullet and Deautier, 1938:2; 1946:654), with the publication of morphological data, satisfied Hellmayr and Conover (1948:257) that this species did in fact occur on the Atlantic. The question still remained whether it bred on that coast. Daguerre (1933:215) had reported finding on an island in Bahia San Blas, in the southern part of the province of Buenos Aires, deserted gull nests with some dead chicks, which he attributed to this species. During the probable breeding season there had been sight observations of Larus belcheri at Puerto Ingeniero White and Puerto Belgrano, a little to the north of the Bahia San Blas (Casares, 1939:286; Olrog, 1958a:31). Sight reports and specimen records indicated that the species ranged in Argentina at least from Puerto Deseado on the Patagonian coast to Mar del Plata in Buenos Aires, and even to Uruguay during the Southern Hemisphere winter (Olrog, 1948:492 and 1958b:8; Cuello and Gerzenstein, 1962:84). Field experience and examination of specimens convinced me that there was a distinct Atlantic population, which I described as Larus belcheri atlanticus (Olrog, 1958b:8), because it differed from the nominate Peruvian population in longer culmen and wing. With assistance from the Mae P. Smith Gull Fund of the American Museum of Natural