192 U.S. NATIONAL MUSEUM BULLETIN 233 



since the chimango hawk, Milvago chimango, was common there and 

 picked off the noisy fledglings to feed its young. 



Hudson's comments (1920, pp. 82-86) are worth repeating. "The 

 young of Synallaxis spixi, though in a deep domed nest, will throw 

 itself to the ground, attempting thus to make its escape. The young 

 Mimus patagonicus sits close and motionless, with closed eyes, mim- 

 icking death. The young of our common Zenaida, even before it is 

 fledged, will swell itself up and strike angrily at the intruder with 

 beak and wings; and by making so brave a show of its inefficient 

 weapons it probably often saves itself from destruction. But any- 

 thing approaching the young Molothrus is welcomed with fluttering 

 wings and clamorous cries, as if all creatures were expected to minister 

 to its necessities. 



"I found a young Molothrus in the nest of a Screaming Finch, 

 Sporophila caerulescens ; he cried for food on seeing my hand approach 

 the nest; I took him out and dropped him down; when finding him- 

 self on the ground he immediately made off half flying. I succeeded 

 in recapturing him, and began to twirl him about, making him scream 

 so as to inform his foster-parents of his situation, for they were not 

 by at the moment. I then put him back in the nest, and plucked 

 half a dozen large measure worms from an adjacent twig. The cater- 

 pillars were handed to the bird . . . and with great greediness he 

 devoured them all notwithstanding the ill treatment he had just 

 received and utterly disregarding the wild excited cries of his foster- 

 parents, just arrived and hovering within three or four feet of the 

 nest . . . ." 



Some lack of attunement between the reaction of the parasitic 

 nestling or fledghng and those of its foster-parents exists in the 

 brown-headed cowbird as well as in the shiny one, but it appears 

 to be more noticeable and more disastrous to the species in the shiny 

 cowbird than in its North American relative. 



Frequency of Host Selection 



Although it is entirely parasitic in its breeding, the shiny cowbird 

 generally evinces more interest in nests than does the brown-headed 

 cowbird. This interest is shown by males as well as females before 

 and after, as well as dui-ing, their breeding season. In Argentina I 

 often noticed both males and females of this species examining nests 

 of ovenbirds, Furnarius, and of woodhewers, Anumhius, Synallaxis, 

 etc., without actually entering them. My observations were antici- 

 pated by many years by Hudson, who wrote (1874, pp. 171-172) that 

 this interest did not seem like idle cm-iosity but "precisely like that of 

 birds that habituaUy make choice of such breeding places. . . . 

 Whenever I set boxes up in my trees the female Cowbirds were the 



