18/4.] MOLOTHRI OF BUENOS AYRES. 157 



with red and consequently closely resembling the eggs of the Yellow- 

 breast. 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 from 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 ? 

 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 im- 

 prove 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 (para- 

 sitical) 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 them- 

 selves 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- 



