86 



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. bonariensis, though their instincts 

 differ, are both parasitic on a great number of species ; M. rufoaocillaris 

 on M. badius ; and in this last species two or more females frequently 

 lay together. If we suppose that the M. bonariensis , when 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 whole 

 race would degenerate, and would only be saved from final extinction 

 by some individuals occasionally dropping their eggs in the nests of 

 other species, perhaps of a Molothrus, as M. rufoaocillaris still does, 

 rather than of birds of other genera. Certainly in this way the para- 

 sitic instinct may have originated in M. bonariensis without that 

 species ever having acquired the habit of breeding in the covered dark 

 nests of other birds. I have supposed that they once possessed it only 

 to account for the strange attraction such nests have for them, which 

 seems like a recurrence to an ancestral habit. 



95. MOLOTHRUS RUFOAXILLAEIS, Cassin. 



(SCREAMING COW-BIRD.) 



[PLATE VI. FIG. 2.] 



Molothrus rufoaxillaris, Scl. et Salv. Nomend. p. 37 ; Hudson, P. Z. S. 1874, 

 p. 161 (Beunos Ayres) ; Durnford, Ibis, 1877, p. 174 (Buenos Ayres) ; 

 White, P. Z. S. 1882, p. 601 (Catainarca) ; Barrows, Bull. Nutt. Orn. Cl. viii. 

 p. 134 (Entrerios) ; Scl Cat. B. xi. p. 338. 



Description. Silky black, washed with purple ; wings and tail with a slight 

 greenish gloss ; a chestnut spot on the axillaries ; bill and feet black : whole 

 length 8-0 inches, wing 4*5, tail 3*3. Female similar, but somewhat smaller. 



Hab. Argentina and Uruguay. 



This bird has no vulgar name, not being distinguished from the 

 Common Cow-bird by the country people. The English name of 

 Screaming Cow-bird, which I have bestowed on it, will, I think, com- 

 mend itself as appropriate to those who observe this bird, for they will 

 always and at any distance be able to distinguish it from the species it 

 resembles so nearly by listening to its impetuous screaming notes, so 

 unlike anything in the language of the Common Cow-bird. 



