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Show 1874.] MOLOTHRI O F B U E N O S A Y R E S . 157 with red and consequently closely resembling the eggs of the Yellowbreast. I was surprised to find five more eggs of M. bonariensis on the ground, near together and about three feet from the bush ; these five eggs were all pure white and spotless. Naturally I asked, How came these eggs on the ground ? They had not fallen frum the nest, which was very deep ; this one contained few eggs, and was scarcely 30 inches from the ground. Then they were all white, while those in the nest were mottled. That the eggs had been laid in the nest I was quite sure; and the only way 1 can account for their being in the place I found them, is that the Yellowbreast itself removed them, taking them up in the bill and flying to the ground. If I am right we must believe that this individual Yellowbreast had strongly developed an instinct unusual to the species, by which it is able to distinguish, and cast out of its nest, eggs very different from its own-an instinct, in fact, the object of which would be to counteract the parasitical instinct of the Molothrus. What would be the effect of such an instinct should the species acquire it 1 Doubtless it would be very prejudicial to all the parasitical birds that laid white eggs, but those that laid mottled eggs would be preserved. This would be natural selection operating in a very unusual manner; for the Yellowbreast, or other species, would improve another to its own detriment, because the more the parasitical eggs resembled its own the better chance would they have of being preserved. But, it may be added, if besides the Yellowbreast some one other species laying very different eggs (a Zonotrichia or Tyrannus for instance) should also acquire this distinguishing-habit and eject all eggs differing greatly from its own from its nest, the instinct in two species would ultimately cause the extermination of the parasite. Some light might be thrown on this obscure subject by examining for two or three summers a large number of nests, to ascertain if the nests of the Yellowbreast are often found without any white eggs, or if the same proportional number of white (parasitical) eggs are found in the nests of the Yellowbreast, Scissor-tail, Sturnella, and other species. IV. Habits of young M . bonariensis.-Little birds of all species when just hatched closely resemble each other ; after they are fledged the resemblance is less, but still comparatively great; grey interspersed with brown is the colour of most of them, or at least of the upper exposed plumage. There is also a great similarity in their cries of hunger and fear-shrill, querulous, prolonged, and usually tremulous notes. It is not to be wondered at, then, that the foster-parents of the young M. bonariensis so readily respond to its cries, understanding the various expressions denoting hunger, fear, or pain, as well as when uttered by their own offspring. But the young Molothrus never understands the language of its foster-parents as other young birds understand the language of their real parents, springing up to receive food when summoned, and concealing themselves or striving to escape when the warning note is given. Again the young Molothrus does not learn to distinguish, even by sight, its foster-parents from any other bird approaching the nest. It gene- |