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.
They feed on the ground, keeping to the moist borders of marshes during summer; they avoid woods, but occasionally alight on trees, where they all sing in concert. The song, when an individual is heard singing alone, is, though limited in its range, very sweet, some of the notes being remarkable for their purity and expression. The bird sits on a rush or stalk while singing, and makes a long pause after every note or two, as if to make the most of its limited repertory. There is in the song one rich full note, which, to my mind, is unequalled for plaintive sweetness, and I am therefore surprised that Azara says only of this species that it sings passably well—‘canta razonablemente.’
The nest is neatly made of dry grasses, and attached to the rushes growing in the water. The eggs are four, pointed, and spotted at the larger end with dull brown and black on a white ground.