The eggs of the Bay-wing are five in number, nearly round, and densely marked with dusky reddish brown. Once I observed two young Bay-wings following a Yellow-breast (_Pseudoleistes virescens_) with their usual peculiar hunger-cry, and while I watched them they were fed several times by their foster-parents. Naturally I concluded that the Bay-winged Cow-bird is sometimes parasitical on other species, but I never saw anything afterwards to confirm me in that belief, and I believe now that I was mistaken, and that the young Bay-wings were not _real_ Bay-wings, but the young of _Molothrus rufoaxillaris_. 97. AGELÆUS THILIUS (Mol.). (YELLOW-SHOULDERED MARSH-BIRD.) +Agelæus thilius+, _Burm. La-Plata Reise_, ii. p. 492 (Mendoza, S. Juan, Catamarca); _Scl. et Salv. Nomencl._ p. 37; _Durnford, Ibis_, 1876, p. 159 (Buenos Ayres), 1877, p. 33 (Chupat), p. 174 (Buenos Ayres), et 1878, p. 394 (Chupat); _White, P. Z. S._ 1882, p. 602 (Buenos Ayres); _Döring, Exp. al Rio Negro, Zool._ p. 40 (Colorado); _Barrows, Bull. Nutt. Orn. Cl._ viii. p. 134 (Entrerios); _Scl. Cat. B._ xi. p. 343. _Description._--Black; lesser upper and under wing-coverts yellow; bill and feet black: whole length 5·5 inches, wing 3·6, tail 2·7. _Female_: above pale brown striated with black; distinct superciliaries white; beneath paler, cineraceous white with black striations; smaller, and bill shorter. _Hab._ S. Peru, Chili, Paraguay, and Argentina. This bird is abundant everywhere on the pampas, and does not migrate, but inhabits marshy situations in summer, building its nest amongst the rushes, and in winter ranges over the country. The male is entirely of an intense black, except the shoulders, which are pure yellow; the female is dull grey with fuscous markings, and, as was long ago remarked by Azara, the grey-plumaged are very much more numerous than the black individuals. The young birds are like the females, and possibly do not acquire the full black plumage until the second year, which would account for the great number of grey birds. These birds are extremely sociable, being seen in flocks all the year round, even during the breeding-season; in winter a great many males separate themselves from the females, and are found associating together in flocks of from thirty to forty individuals.