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Show 1874.] MOLOTHRI OF BUENOS AYRES. 159 although the bird itself cannot understand its foster-parents, and welcomes all things that, whether with good or evil design, come near it, the unfitness is not so apparent as when the nest is in open fields and plains. The young M. bonariensis differs from the true offspring of its foster-parents in its habit of quitting the nest as soon as it is able, trying to follow the old bird, and placing itself in the most conspicuous place it can find, such as the summit of a stalk or weed, and there demanding food with frequent and importunate cries. Thus the little Polioptila had acquired the habit of perching on the back of its charge to feed it, because parent birds invariably perch above their young to feed them ; and the young M. bonariensis prevented this by always sitting on the summit of the stalk it perched on. The habit is most fatal on the open and closely cropped pampas inhabited by the Cachila (Anthus correndera). In December, when the Cachila rears its second brood, the Milvago chimango also has young, and feeds them almost exclusively on the young of other, chiefly small, birds. At this season the Chimango destroys great numbers of the young of the Cachila and of Anumbius rufigularis. Yet these birds are beautifully adapted in structure, coloration, and habits to their station. It thus happens that in districts where the Molothrus is abundant, their eggs are found in a majority of the Cachilas' nests : and yet to find here a young M. bonariensis out of the nest is a rare thing; for as soon as they are able to quit the nest and expose themselves they are all or nearly all carried away by the Chimango. • V. Nidification of Molothrus badius.-A pair of Leiiateros (Anumbius acuticaudatus) have been nearly all the winter building a great nest on a locust-tree within sixty yards of the house. This nest is about 27 inches deep and 16 or 18 in circumference, and appears now nearly completed. To-day (September 28, 1872) I saw a bay-winged Molothrus on the nest ; it climbed about it, deliberately inspecting every part, taking up and rearranging some sticks and throwing others down. Whilst thus engaged, two Blackbirds (M. bonariensis), male and female, came to the tree; the female dropped into the nest, and began also to examine it, peering curiously into the entrance and quarrelling with the first bird. After a few minutes she flew off followed by her consort. The Bay-wing continued its strange futile work until the owners of the nest appeared, whereupon it hopped leisurely to one side, sung for a few moments, and then flew away. The similarity in the behaviour of the two birds struck me very forcibly ; in the great interest they take in the nests of other birds, especially in large covered nests, the two species are identical. But when the breeding-season has come their habits begin to diverge : then the M. bonariensis lays in nests of other species, abandoning its eggs to their care ; whilst the Bay-wings usually seize on the nests of other birds, and rear their own young. Yet, as they do occasionally build a neat elaborate nest for themselves, the habit of taking possession of nests of other birds is probably recently acquired; probably also its tendency is to eradicate the primitive building-instinct. |