614 REPORT OF NATIONAL MUSEUM, 1893. 



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 lighting it will not build one for itself. Let us 

 suppose that the Oowbird also once acquired the habit of breeding in 

 domed nests, and that through this habit its original nest-making 

 iustiuct was completely eradicated. It is not difrtcult to imagine how 

 in its turn this instinct was also lost. A diminution in the number of 

 birds that built domed nests would involve 3/. bonariensis in a struggle 

 for nests, in which it would probably be defeated. In Buenos Ayres 

 theWhite-rumped Swallow, the Wren, and the Yellow Seed-tinch prefer 

 the ovens of the Furnarius to any other breeding i)lace, but to obtain 

 them are obliged to struggle with Progue trtprra. for this species has 

 acquired the habit of breeding exclusively in the ovens. They can 

 not, however, compete with the Prague, and thus the increase of one 

 species has, to a great extent, deprived three other species of their 

 favorite building place. Again, Maclietornis rixosa prefers the great 

 nest of the Annmhius, 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 Hock of or 8 

 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 Mohthrus have per- 

 haps 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. honariensis 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 together; the progenitors of all 

 the species of Molothrus may have been early infected with this habit, 

 and inherited with it a facility for acquiring their present one. M. 

 pecoris and M. honariensis, though their instincts differ, are both para- 

 sitic on a great number of species; i\[. rufoaxiUaris on M. badim; and 

 in this last species two or more females frequently lay together. If we 

 suppose that the 71/. honariensis, wheu it was a nest builder or reared 

 its own young in the nests it seized, possessed this habit of two or 

 more females frequently laying together, the young of those birds that 

 oftenest abandoned their eggs to the care of another would probably 

 inherit a weakened maternal instinct. The continual intercrossing of 

 individuals with weaker and stronger instincts would prevent the 

 formation of two races differing in habit; but the Avhole race wouhl 

 degenerate, and would only be saved from final extinction by some 

 individuals occasionally (Iropi)ing their eggs in the nests of other 

 species, perhaps of a Molothrus, as M. rufoaxiUaris still does, rather than 



