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. 
