The perfect similarity of the eggs of _M. rufoaxillaris_ to those of _M. badius_ perhaps was brought about in this way. But, it may be added, if besides the Yellow-breast some one other species laying very different eggs (a _Zonotrichia_ or _Tyrannus_, for instance) should also acquire this distinguishing habit, and eject all eggs unlike its own from its nest, the habit in the two or more species would ultimately cause the extinction of the parasite. It might throw some light on this obscure subject to examine, for several successive summers, a large number of nests, to ascertain whether the nests of the Yellow-breast are often found without any white unspotted eggs, or if the same proportional number of white (parasitical) eggs are found in the nests of the Yellow-breast, Scissor-tail, Song-Sparrow, Pipit, and other species. 103. TRUPIALIS MILITARIS (Linn.). (PATAGONIAN MARSH-STARLING.) +Sturnella militaris+, _Hudson, P. Z. S._ 1872, p. 548 (Rio Negro); _Scl. et Salv. Nomencl._ p. 38; _Durnford, Ibis_, 1877, p. 33, et 1878, p. 394 (Chupat). +Sturnella loica+, _Döring, Exp. al Rio Negro, Zool._ p. 41 (R. Colorado, R. Negro). +Trupialis militaris+, _Scl. Cat. B._ xi. p. 356. +Trupialis loyca+, _Burm. La-Plata Reise_, ii. p. 491 (Mendoza). _Description._--Above brown, especially on the head and back variegated with black; superciliaries in front of the eye red, behind the eye white; beneath black; throat, middle of the neck, breast, and upper belly scarlet; sides of belly and crissum with brownish edgings to the black feathers; under wing-coverts white; bend of the wing red; slight rictal stripe white; bill horn-colour; feet brown: total length 10·0 inches, wing 4·7, tail 3·6. _Female_ similar. _Hab._ Chili, Patagonia, and Falkland Islands. Two species of _Trupialis_ inhabit the southernmost part of the Argentine Republic, the present being confined to Patagonia and South Chili, while its northern representative inhabits the pampas of Buenos Ayres and Uruguay. Probably the Colorado River, which separates two districts differing in soil and vegetation, is the boundary-line dividing their habitats. So nearly alike are these two birds in colour, language, and habits, that they seem rather like races than species; and they were so regarded by naturalists until recently, when the pampas bird was raised to the rank of a distinct species, with the name of _Trupialis defilippii_. Unfortunately the old name _militaris_ fits the Pampas, and not the Patagonian, Starling best; but of this I shall speak when I describe the former species.