370 IN THE WILDS OF SOUTH AMERICA 



birds. So persecuted were the oven-birds that it is difficult 

 to understand how any of them survived in this immediate 

 locaUty. The nests were common enough, it being not 

 unusual to find several of them in a single tree, but the birds 

 themselves were not abundant. It is possible that some of 

 the pairs may have built several nests each in their vain 

 attempts to escape the attentions of the cowbirds. 



In no instance had the walls or top of the oven-birds' 

 nests been broken or perforated in any manner, in order 

 that light could penetrate to the interior; they were not 

 tampered with in any way, and the cowbirds seemed con- 

 tent to use them just as the oven-birds had constructed 

 them. 



I believe that the greater number of M. b. bonariensis 

 that reach maturity are reared by the smaller birds, such as 

 finches, warblers, and vireos, in whose nests only a few eggs 

 are laid, which increases the favorable chances of their in- 

 cubation. Also, the larger and heavier eggs of the cowbird 

 frequently crush at least a part of the smaller eggs which 

 naturally have a more fragile shell, thus forestalling to a 

 marked degree the competition that might arise between 

 the young birds in the nest. 



We collected about two hundred eggs of this species, 

 nearly all of them at Rosario de Lerma, and a great varia^^ 

 tion in marking exists; there is also some difference in color. 

 As a general rule the eggs are greenish or bluish, rather 

 heavily spotted with reddish-brown; in a very few speci- 

 mens the background is of a pale flesh-color, and in a small 

 number of others it approaches white, having, however, a 

 dull grayish tinge; of the entire lot, four only are so lightly 

 marked as to appear unspotted. Not a single egg is pure 

 white or has a pure white background (my standard of com- 

 parison is an egg of the oven-bird) "like the eggs of birds 

 that breed in dark holes"; the majority of these eggs, were 

 taken from the darkened interiors of oven-birds' nests. 



A type of egg not uncommon is heavily and evenly 

 marked all over with fine dots and larger spots of reddish- 



