io6 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 



