620 REPORT OF NATIONAL MUSEUM, 1893. 



seems impossible for mimicry to go further than tliis. A slight differ- 

 ence m size is quite iniperc'e])tible when the birds are flying about, 

 while in language and i)luniage the keenest ornithologist would uot be 

 able to detect a difference. But it may be questioned whether this is 

 really a case of an external reseniblan(;e of one species to another 

 acquired by natural selection for its better preservation. Possibly the 

 young .1/. rufoaxillaru in the first stage of its plumage exhibits the 

 ancestral tyi)e — that of the ])rogeuitor of both species. If M. badius 

 belonged to someother group — Sturnclla or rseudolchtes, for iustance — 

 it would scarcely be possible to doubt that the resemblance of the young 

 M. riifoaxiUaris 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 dro])ped their eggs in the uests 

 of other birds. 1 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, lor those two birds were probably 

 the young of M. ritfoa.villaris. There are, however, good reasons for 

 believing that- ^1/. rufoaxillaris is parasitical almost exclusively on M. 

 badius. 1 have spoken of the many varieties of eggs M. bonarieH,sis 

 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 color, varying to dusky brown ; the ground (;olor 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 

 Yellow breast feeding the supposed young Bay-wings, I have looked 

 out for the eggs of the latter in other birds' nests. I have found hun- 

 dreds of uests containing eggs of ^[. bonariensis, but never one with 

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

 31. rufoaxillaris. It is wonderful that M. rufoaxillaris should lay only 

 in the uests of M. badius, but the most mysterious thing is that M. 

 bonariensis, indiscriminately i)arasitical on a host of species, never, to 

 my knowledge, drops an egg in the iiest of M. badius, unless it be in a 

 forsaken nest. rerhai)S it wdl be difficult for naturalists to believe 

 this, for if the M. badius is so excessively vigilant and jealous of other 

 birds approaching its nest as to succeed in keeping out the subtle, 

 silent, gray-i)lumaged, omnipresent female 21. bonariensis, why does it 

 not also keep ott' the far rarer, noisy, bustling, conspicuously colored 

 il/. rufoaxillaris f I cannot say. The only explanation that has occurred 

 to me is that M. badius is sagacious enough to distinguish the eggs of 

 the common parasite, and throws them out of its nest. But this is 

 scarcely probable, for 1 have hunted in vain under the trees for the 

 ejected eggs, and I have never found the eggs of il/". badius with holes 

 pecked in the shells, which would have been the case had ilf. bonariensis 

 intruded into the nest. 



