+Molothrus bonariensis+, _Scl. et Salv. Nomencl._ p. 37; _Hudson, P. Z. S._ 1872, p. 809, 1874, p. 153 (Buenos Ayres); _Durnford, Ibis_, 1877, pp. 33, 174 (Chupat); _White, P. Z. S._ 1882, p. 601 (Buenos Ayres); _Döring, Exp. al Rio Negro, Zool._ p. 41 (Carhué); _Barrows, Bull. Nutt. Orn. Cl._ viii. p. 133 (Entrerios); _Scl. Cat. B._ xi. p. 335. +Molothrus sericeus+, _Burm. La-Plata Reise_, ii. p. 494. _Description._--Uniform shining purplish black; less lustrous on wings and tail; bill and feet black: total length 7·5 inches, wing 4·5, tail 3·0. _Female_ dark ashy brown, beneath paler; slightly smaller in size. _Hab._ Argentina, Paraguay, Bolivia, and Brazil. This species is the _Tordo Comun_ of Azara, and is usually called "_Tordo_" or "_Pajaro Negro_" by the Spanish, and "_Blackbird_" by the English-speaking Argentines. A more suitable name, I think, is the Argentine Cow-bird, which has been given to it by some writers on ornithology, Cow-bird being the name of the closely allied North-American species, _Molothrus pecoris_. This Cow-bird is widely distributed in South America, and is common throughout the Argentine country, including Patagonia, as far south as Chupat. In Buenos Ayres it is very numerous, especially in cultivated districts where there are plantations of trees. The male is clothed in a glossy plumage of deep violaceous purple, the wings and tail being dark metallic green; but seen at a distance or in the shade the bird looks black. The female is inferior in size and has a dull, mouse-coloured plumage, and black beak and legs. The males are much more numerous than the females. Azara says that nine birds in ten are males; but I am not sure that the disparity is so great as that. It seems strange and contrary to Nature's usual rule that the smaller, shyer, inconspicuous individuals should be in such a minority; but the reason is perhaps that the _male eggs_ of the Cow-bird are harder-shelled than the _female eggs_, and escape destruction oftener, when the parent bird exercises its disorderly and destructive habit of pecking holes in all the eggs it finds in the nests into which it intrudes.