April 15.--This morning I started in quest of the Bay-wings, and observed one individual, that had somehow escaped detection the day before, assuming the purple dress. This bird I shot; and after the flock had resettled a short distance off, I crept close up to them, under the shelter of a hedge, to observe them more narrowly. One of the adults was closely attended by three young birds; and these all, while I watched them, fluttered their wings and clamoured for food every time the old bird stirred on its perch. The three young birds seemed precisely alike; but presently I noticed that one of them had a few minute purple spots, and on shooting this one I found it to be a young _M. rufoaxillaris_, while the other two were true young Bay-wings. The hunger-cry of the young _M. badius_ (Bay-wing) is quite different from that of the young _M. bonariensis_: the cry of the latter is a long, shrill, two-syllabled note, the last syllable being prolonged into a continuous squeal when the foster-parent approaches with food; the cry of the young _M. badius_ is short, reedy, tremulous, and uninflected. The resemblance of the young _M. rufoaxillaris_ to its foster-brothers in language and plumage is the more remarkable when we reflect that the adult bird in its habits, gestures, guttural notes, also in its deep purple plumage, comes much nearer to _M. bonariensis_ than to _M. badius_. It seems impossible for mimicry to go further than this. A slight difference in size is quite imperceptible when the birds are flying about; while in language and plumage the keenest ornithologist would not be able to detect a difference. But it may be questioned whether this is really a case of an external resemblance of one species to another acquired by natural selection for its better preservation. Possibly the young _M. rufoaxillaris_, in the first stage of its plumage, exhibits the ancestral type--that of the progenitor of both species. If _M. badius_ belonged to some other group--_Sturnella_ or _Pseudoleistes_, for instance--it would scarcely be possible to doubt that the resemblance of the young _M. rufoaxillaris_ to its foster-brothers resulted from mimicry; but as both species belong to the limited, well-defined group _Molothrus_, the resemblance may be ascribed to community of descent.