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Show 1874.] MOLOTHRI OF BUENOS AYRES. 173 nests of other birds of the same genus*. In some species the nesting habit is in a transitional state. Machetornis rixosa sometimes makes a shallow elaborate nest in the angle formed by twigs and the bough of a tree, but prefers, and almost invariably makes choice of, the the covered nest of some other species or of a hole in the tree. It is precisely the same with our Wren, Troglodytes fuscus. The Sycalis pelzelni invariably breeds in a dark hole or covered nest. The fact that these three species lay coloured eggs, and the first and last very darkly coloured eggs, inclines one to believe that they once invariably built shallow exposed nests, as the M. rixosa still occasionally does. It may be added that these species that lay coloured eggs in dark places construct and line their nests far more neatly than do the species that breed in such places, but lay white eggs. As with the M. rixosa and Wren, so it is with the Bay-winged Molothrus ; it lays mottled eggs, and occasionally builds a neat exposed nest; yet so great is the partiality it has acquired for the domed large nests, that whenever it can possess itself of one by dint of fighting, it will not build one for itself. Let us suppose that the M. bonariensis also once acquired the habit of breeding in domed nests, and that through this habit its original nest-making instinct was completely eradicated, it is not difficult to imagine how in its turn this instinct was also lost. A diminution in the number of birds that built domed nests, or an increase in the number of species and individuals that breed in such nests, would involve the M. bonariensis in a struggle for nests, in which it would probably be defeated. In Buenos Ayres the Common Swallow, the Wren, and the Sycalis chloropsis prefer the ovens of the Furnarius to any other breeding-place, but to obtain them are obliged to struggle with the Progne tapera; for this species has acquired the habit of breeding exclusively in the ovens. They cannot, however, compete with the Martin ; and the increase of one species has thus deprived three other species of their favourite building-place. Again, the Machetornis rixosa prefers the great nest of the Anumbius; and when other species compete with it for the nest, they are invariably defeated. I have seen a pair of Machetornis after they had seized a nest attacked in their turn by a flock of six or eight Bay-wings; but, in spite of the superior numbers, the fury of the Machetornis compelled them to raise the siege. Thus some events in the history of our common Molothrus have perhaps been accounted for, if not the most essential one-the loss of the nest-making instinct from the acquisition of the habit of breeding in the covered nests of other birds, a habit that has left a strong trace in the manners of the species, and perhaps in the pure white unmarked eggs of so many individuals ; finally we have seen how this habit may also have been lost. But the parasitical habit of the M. bonariensis may have originated when the bird was still a nest-builder. The origin of the instinct may have been in the occasional habit, common to so many species, of two or more females laying * The nest in which Darwin (Voy. of Beag. iii. p. 79) found this Synallaxis breeding, and which he naturally supposed to have been built by the bird, was probably a nest of S. modesta. |