104 BIRDS OF LA PLATA 



from me, and I naturally concluded that they were 

 young Common Cow-birds (M, bonariensis), casually 

 associating with the Bay-wings, I was surprised to 

 see them, for the young male M. bonariensis always 

 acquires the purple plumage before March, so that 

 these individuals were changing colour five weeks 

 after the usual time. 



To-day, while out with my gun, I came upon the 

 flock, and noticed four of the birds assuming the 

 purple plumage, two of them being almost entirely 

 that colour ; but I also noticed with astonishment 

 that they had bay- or chestnut-coloured wings, also 

 that those with least purple on them were marvel- 

 lously like the Bay-wings in the mouse-coloured 

 plumage of the body and the dark tail. I had seen 

 these birds before the purple plumage was acquired, 

 and there was then not the slightest difference amongst 

 them, the adults and their supposed offspring being 

 alike ; now some of them appeared to be under- 

 going the process of a transmutation into another 

 species ! I at once shot the four spotted birds, along 

 with two genuine Bay-wings, and was delighted to 

 find that the first were young Screaming Cow-birds, 



I must now believe that the extra eggs twice 

 found in the nest of the Bay-wings were those of 

 the Screaming Cow -bird, that the latter species 

 lays chiefly in the nests of the former, that 

 the eggs of the two species are identical in form, 

 size, and colour, each bird also laying five, and 

 that, stranger still, the similarity is as perfect in 

 the young birds as it is in the eggs. 



