92 ICTERIDvE. 



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. 



Formerly I believed that though M. badius is constantly seen rear- 

 ing 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 Yellowbreast 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 ex- 

 clusively 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 rarely, pale blue. It is not 

 possible to confound the eggs of the two species. Now, ever since I 

 saw, many years ago, the Yellowbreast feeding the supposed young 

 Bay-wings, I have looked out for the eggs of the latter in other birds' 

 nests. I have found hundreds of nests containing eggs of M. bonariensis, 

 but never one with an egg of M. badius, and, I may now add, never one 

 with an egg of M. rufoaxillaris. It is wonderful that M. rufoaxillaris 

 should lay only in the nests of M. badius but the most mysterious thing 

 is that M. bonariensis, indiscriminately parasitical on a host of species, 

 never, to my knowledge, drops an egg in the nest of M. badius, unless 

 it be in a forsaken nest ! Perhaps it will be difficult for naturalists to 

 believe this ; for i the M. badius is so excessively vigilant and jealous 

 of other birds approaching its nest as to succeed in keeping out the 

 subtle, silent, grey-plumaged, omnipresent female M . bonariensis, why 

 does it not also keep off the far rarer, noisy, bustling, conspicuously 

 coloured M. rufoaxillaris ? I cannot say. The only explanation that 



