MOLOTHRUS BONARIENS1S. 85 



nest in the angle formed by twigs and the bough of a tree, but prefers, 

 and almost invariably makes choice of, the covered nest of some other 

 species or of a hole in the tree. It is precisely the same with our Wren, 

 Troglodytes furvus. The Yellow House-Sparrow (Sycalis pelzelni] in- 

 variably 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 exposed nests, as 

 M. rixosa still occasionally does. It may be added that those 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 M. rixosa and the 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 large domed 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 Cow-bird 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 M. bonariensis in a struggle for nests, in which it would 

 probably be defeated. In Buenos Ay res the White-rumped Swallow, the 

 Wren, and the Yellow Seed-finch prefer the ovens of the Farnarius to 

 any other breeding-place, but to obtain them are obliged to struggle with 

 Progne tapera,} for this species has acquired the habit of breeding 

 exclusively in the ovens. They cannot, however, compete with the 

 Progne ; and thus the increase of one species has, to a great extent, 

 deprived three other species of their favourite building-place. Again, 

 Machetornis rixosa prefers the greatnest 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 





