104 ICTERID.E. 



species, which enables it to distinguish, and cast out of its nest, eggs 

 very different from its own an instinct, in fact, the object of which 

 would be to counteract the parasitical habit of Molothrus. What would 

 be the effect of such an instinct should the species acquire it ? Doubt- 

 less it would be highly prejudicial to the parasitical birds laying white 

 eggs, but favourable to those laying mottled eggs. This would be 

 natural selection operating in a very unusual manner ; for the Yellow- 

 breast, or other species, would improve another to its own detriment, 

 since the more the parasitical eggs assimilated to its own, the greater 

 would be the likelihood of their being preserved. The perfect similarity 

 of the eggs of M. rufoaxillaris to those of M. badius perhaps was 

 brought about in this way. But, it may be added, if besides the 

 Yellow-breast some one other species laying very different eggs (a 

 Zonotrichia or Tyrannus, for instance) should also acquire this dis- 

 tinguishing habit, and eject all eggs unlike its own from its nest, the 

 habit in the two or more species would ultimately cause the extinction 

 of the parasite. 



It might throw some light on this obscure subject to examine, for 

 several successive summers, a large number of nests, to ascertain 

 whether the nests of the Yellow- breast are often found without any 

 white unspotted eggs, or if the same proportional number of white 

 (parasitical; eggs are found in the nests of the Yellow-breast, Scissor- 

 tail, Song-Sparrow, Pipit, and other species. 



103. TRUPIALIS MILITARIS (Linn.). 

 (PATAGONIAN MARSH-STARLING.) 



Sturnella militaris, Hudson, P. Z. S. 1872, p. 548 (Rio Negro) ; Scl. et Salv. 

 Nomencl p. 38; Durnford, Ibis, 1877, p. 33, et 1878, p. 394 (Chupat). 

 Sturnella loica, Doring, Exp. al Rio Negro, Zool. p. 41 (R. Colorado, R. 

 Negro). Trupialis militaris, Scl. Cat. B. xi. p. 356. Trupialis loyca, 

 Burm. La-Plata Reise, ii. p. 491 (Mendoza). 



Description. Above brown, especially on the head and back variegated with 

 black ; superciliaries in front of the eye red, behind the eye white ; beneath 

 black ; throat, middle of the neck, breast, and upper belly scarlet ; sides of belly 

 and crissum with brownish edgings to the black feathers ; under wing-coverts 

 white ; bend of the wing red ; slight rictal stripe white ; bill horn-colour ; 

 feet brown: total length 1OO inches, wing 4-7, tail 3'6. Female similar. 



Hub. Chili, Patagonia, and Falkland Islands. 



Two species of Trupialis inhabit the southernmost part of the Argen- 

 tine Republic, the present being confined to Patagonia and South 



