1674.] MOLOTHRI OF BUENOS AYRES. 173 



nests of other birds of the same genus*. In some species the nesting 

 habit is in a transitional state. Machetomis 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 it- 

 self. 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 diffi- 

 cult 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 in- 

 volve 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 Machetomis rixosa prefers the great nest of the Anumbius ; and 

 when other species compete with it for the nest, they are inva- 

 riably defeated. I have seen a pair of Machetomis 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 

 Machetomis compelled them to raise the siege. Thus some events 

 in the history of our common Molothrus have perhaps been ac- 

 counted for, if not the most essential one — the loss of the nest-mak- 

 ing 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 un- 

 marked 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. 



