This Molothrus is common in large flocks on the grassy plains of La Plata, and is often mingled with the _Leistes anticus_, and other birds. In the same flock with the usual black kind, there were generally a few dull brown coloured ones, (_Icterus sericeus_ of _Licht._) which I presume are the young. Azara states that the brown coloured birds are smaller than the black glossy ones, and that they sometimes form one-tenth of the whole number in a flock. In the single specimen which I brought home, the size, with the exception of the length of the wing, is only a very little less. Sonnini, in his notes to Azara, considers the brown birds as the females; I can, however, scarcely believe that so obvious a solution of the difficulty could have escaped so accurate an observer as Azara. These birds in La Plata often may be seen standing on the back of a cow or horse. While perched on a hedge, and pluming themselves in the sun, they sometimes attempt to sing or rather to hiss: the noise is very peculiar; it resembles that of bubbles of air passing rapidly from a small orifice under water, so as to produce an acute sound. Azara states that this bird, like the cuckoo, deposits its eggs in other birds’ nests. I was several times told by the country people, that there was some bird which had this habit; and my assistant in collecting, who is a very accurate person, found in the nest of the _Zonotrichia ruficollis_ (a bird which occupies in the ornithology of S. America the place of the common sparrow of Europe), one egg larger than the others, and of a different colour and shape. This egg is rather less than that of the missel-thrush, being ·93 of an inch in length, and ·78 in breadth; it is of a bulky form, thick in the middle. The ground colour is a pale pinkish white, with irregular spots and blotches of a bright reddish brown, and others less distinct of a greyish hue. This species is evidently a very close analogue of the _M. pecoris_ of North America, from which, however it may at once be distinguished by the absence of the glossy brown on the head, neck, and upper breast,—by the metallic blueness of its plumage in the place of a green tinge, and by its somewhat greater size in all its proportions. The young or brown coloured specimens of these Molothri resemble each other more closely; that of the _M. pecoris_ is of a lighter brown, especially under the throat, and the small feathers on its breast and abdomen have each an obscure dark central streak. The eggs of the Molothri, although having the same general character, differ considerably; that of the _M. pecoris_ being smaller and less swollen in the middle; it is ·85 of an inch in length, and ·78 in breadth. Its colour cannot be better described than in the words of Dr. Richardson[15]—it is “of a greenish white, with rather small crowded and confluent irregular spots of pale liver brown, intermixed with others of subdued purplish grey.” From this description it is obvious that the egg of _M.