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 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 
