The young _Molothrus_ differs from the true offspring of its foster-parents in its habit of quitting the nest as soon as it is able, trying to follow the old bird, and placing itself in the most conspicuous place it can find, such as the summit of a stalk or weed, and there demanding food with frequent and importunate cries. Thus the little Flycatcher had acquired the habit of perching on the back of its charge to feed it, because parent birds invariably perch above their young to feed them, and the young Cow-bird prevented this by always sitting on the summit of the stalk it perched on. The habit is most fatal on the open and closely cropped pampas inhabited by the Cachila (_Anthus correndera_). In December, when the Cachila Pipit rears its second brood, the _Milvago chimango_ also has young, and feeds them almost exclusively on the young of various species of small birds. At this season the Chimango destroys great numbers of the young of the Cachila and of _Synallaxis hudsoni_. Yet these birds are beautifully adapted in structure, coloration, and habits to their station. It thus happens that in districts where the _Molothrus_ is abundant, their eggs are found in a majority of the Cachilas' nests: and yet to find a young Cow-bird out of the nest is a rare thing here, for as soon as the young birds are able to quit the nest and expose themselves they are all or nearly all carried off by the Chimangos. _Conjectures as to the Origin of the Parasitic Instinct in_ M. bonariensis.