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Show 162 MR. W. H. HUDSON ON T H E [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 complement 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 m y 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 m y 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 m e ; 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 m y 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 astonishment 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- Ayrean 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 |
