172 mr. w. h. Hudson on the [Mar. 3, 



little space, she will return again and again, as if fascinated with 

 the comfort and security of such an abode. It is amusing to see 

 how pertinaciously they hang about the ovens of the Oven-bird3, 

 apparently determined to take possession of them, flying back to 

 them after a hundred repulses, aud yet not entering them, even 

 when they have the opportunity. Sometimes one is seen follow- 

 ing a Wren or a Swallow to its nest beneath the eaves, and then 

 clinging to the wall beneath the hole into which it disappeared. 

 I could fill many pages with instances of this habit of the M. bo- 

 nariensis, which, useless though it be, is as strong an affection as the 

 bird possesses. That it is a recurrence to a long disused habit, I 

 can scarcely doubt ; at least, to no other cause, that I can imagine, 

 can it be attributed ; and, besides, it seems to me that if the M. bo- 

 nariensis, when once a nest-builder, had acquired the semiparasi- 

 tical habit of breeding in domed nests of other birds, such a habit 

 might conduce to the formation of the instinct which it now possesses. 

 Iu my former letter on the M. bonariensis I mentioned that twice I 

 had seen birds of this species attempting to build nests, and that on 

 both occasions they failed to complete the work. So universalis the 

 nest-making instinct that one might safely say the M. bonariensis had 

 once possessed it, and that in the cases I have mentioned it was a re- 

 currence, too weak to be efficient, to the ancestral habit. Another in- 

 teresting circumstance may be adduced as strong presumptive evidence 

 that the M. bonariensis once made itself an open exposed nest as 

 M. badius occasionally does — viz. the difference in colour of the male 

 and female ; for whilst the former is rich purple, the latter has what 

 naturalists consider an adaptive resemblance in colour to the nest 

 and to the shaded interior twigs and branches on which nests are 

 usually built. How could such an instinct have been lost ? To say 

 that the M. bonariensis occasionally dropped an egg in another bird's 

 nest, and that the young hatched from these accidental eggs pos- 

 sessed some (hypothetical) advantage over those hatched in the 

 usual way, and that so the parasitical habit became hereditary, sup- 

 planting the original one, is an assertion without any thing to sup- 

 port it, and seems to exclude the agency of external conditions. 

 Again the want of correspondence in the habits of the young parasite 

 and its foster-parents would in reality be a disadvantage to the 

 former ; the unfitness would be as great in the eggs and other cir- 

 cumstances. For all the advantages the parasite actually pos- 

 sesses in the comparative hardness of the egg-shell, rapid evolution 

 of the young, &c, already mentioned, must have been acquired 

 little by little through the slowly accumulating process of natural 

 selection, but subsequently to the formation of the original parasitical 

 inclination and habit. I am inclined to believe that M. bonariensis 

 lost the nest-making instinct by acquiring that semiparasitical habit, 

 common to so many South-American birds, of breeding in the large 

 covered nests of the Dendrocolaptidse. We have evidence that this 

 semiparasitical habit does tend to eradicate the nest-making one. The 

 Synallaxes build great elaborate domed nests ; yet we have one 

 species (S. cegithaloides) that never builds for itself, but breeds in the 



