106 BIRDS OF LA PLATA 
ornithologist would not be able to detect a difference. 
It may, however, 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 pos- 
sible 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. 
Formerly I believed that though M. badius is 
constantly seen rearing its own young, they also 
occasionally dropped their eggs in the nests of other 
birds. I could not doubt that this was the case after 
having witnessed a couple of their young following 
a Yellow-breast, Pseudoleistes virescens, and being fed 
by it. I must now alter my opinion, for what then 
appeared to be proof positive is now no proof at all, 
for those two birds were probably the young of M. 
rufoaxillaris. There are, however, good reasons for 
believing that M. rufoaxillaris is parasitical almost 
exclusively on M. badius. I have spoken of the many 
varieties of eggs M. bonariensis lays. Those of M. 
badius are a trifle less in size, in form elliptical, densely 
and uniformly marked with small spots and blotches 
of dark reddish colour, varying to dusky brown; the 
ground-colour is white, but sometimes, though 
