94 BIRDS OF LA PLATA 



in its turn this instinct was also lost, A diminution 

 in the number of birds that build domed nests or an 

 increase in the number of species and individuals 

 that breed in such nests, would involve M. bonari- 

 ensis in a struggle for nests, in which it would probably 

 be defeated. In Buenos Ayres the Common Swallow, 

 the Wren, and the Yellow Seed-Finch prefer the 

 ovens of the Furnarius to any other breeding place, 

 but to obtain them are obliged to struggle with the 

 Tree-Swallow, 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 great nest of the Anwnbius ; 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 



