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 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 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 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 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, 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 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 I have hunted in vain under the trees for the ejected eggs; and I have never found the eggs of _M. badius_ with holes pecked in the shells, which would have been the case had a _M. bonariensis_ intruded into the nest.