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



VII. The M. rufoaxillaris is parasitical on the M. badius. — 

 April 12, 1873. — To-day I have made a discovery, and am as pleased 

 with it as if I had found a new planet in the sky. The mystery of 

 the Bay-wings' nest twice found containing over the usual comple- 

 ment of eggs is cleared up, and I have now suddenly become 

 acquainted with the procreant instinct of M . rufoaxillaris. I esteem 

 it a great piece of good fortune ; for I had thought that the season 

 for making any such discovery was already over, so near are we now 

 to winter. The Bay-wings are so social in their habits, that they 

 appear reluctantly to break up their companies in the breeding- 

 season ; no sooner is this over, and when the young birds are still 

 fed by their parents, than all the families about a plantation unite 

 into one flock. About a month ago all the birds about my trees had 

 associated in this way together, and wandered about in a scattered 

 party, frequenting one favourite spot very much, about fifteen 

 minutes' walk from the house. The flock was composed, I think, of 

 three families, about fifteen or sixteen birds in all : the young birds 

 are indistinguishable from the adults ; but I know that most of these 

 birds were young hatched late in the season, from their incessant 

 strident hunger-notes. From the time of my first seeing them 

 together before the middle of March, I never observed the flock 

 closely till to-day. A week ago I rode past the flock and noticed 

 among them three birds with purple spots on their plumage. They 

 were at a distance from me ; and I of course concluded that they 

 were young of M. bonariensis casually associating with the Bay- 

 wings. It surprised me very much at the time ; for the young male 

 M. bonariensis always acquires the purple plumage before March. 

 To-day while out with my gun 1 came upon the flock and observed 

 four of the birds assuming the deep-purple plumage, two of them 

 being almost entirely that colour ; but I also noticed with astonish- 

 ment that they had bay wings like the birds they followed, also that 

 those that had least purple on them were marvellously like the Bay- 

 wings in the mouse-coloured plumage and blackish-brown tail. I 

 had seen these very birds a few weeks ago and before the purple 

 plumage was acquired, and there was not the slightest difference 

 amongst them ; now they appeared to be undergoing the process of 

 a transmutation into another species ! I immediately shot four of 

 them along with two genuine Bay-wings, and was delighted to find 

 the purple-spotted birds to be the young of M. rufoaxillaris. 



I must now believe that the extra eggs twice found in the nest of 

 the Bay- wings were those of M . rufoaxillaris, that the latter species 

 has a particular predilection for laying in the nests of the former, 

 that the eggs of the two species are identical in form, size, and 

 coloration, and that, stranger still, the mimicry is as perfect in the 

 young birds as in the eggs. 



The M. rufoaxillaris is the fourth Molothrus with the procreant 

 habits of which we are now acquainted ; for besides the three Buenos- 

 Ay rean and the single North-American species, I know not that the 

 habits of any others have been ascertained. There is a network of 

 affinities in the nesting-habits, colour of plumage and the changes it 



