EASTERN BROWN-HEADED COWBIRD 441 



A lone cowbird, in a nest by himself, grows faster than one that has 

 competition from other nest mates. 



Probably all the altricial species that successfully hatch a cow- 

 bird's egg feed the young imposter, or attempt to do so, while it is in 

 the nest, but the larger species are not always successful in rearing 

 it to a nest-leaving age. The smaller flycatchers, the vireos, 

 the wood warblers, and the smaller sparrows are the most 

 successful in this and therefore make the best foster parents. As 

 Friedmann (1929) remarks: "Obviously only those species that serve 

 as foster-parents of the young Cowbird are important in the economy 

 of the parasite. Of the 195 birds on the list, 91 have been definitely 

 recorded as rearing young Cowbirds. Of the remaining number, a 

 large number doubtless could, and do, act in this capacity but are less 

 commonly victimized and so have been less often recorded." 



In order successfully to rear a young cowbird until it attains its 

 growth and is able to shift for itself, the foster parent must feed it for 

 some time after it leaves the nest. The smaller flycatchers, the vireos, 

 the wood warblers, and the smaller sparrows that have acted as hosts 

 usually do this, and most of them have been definitely recorded as 

 doing so, as have also the house wren and the Carolina wren. 

 Milton B. Trautman (1940) lists, among the larger birds observed 

 feeding fledgling cowbirds out of the nest, the catbird, eastern robin, 

 wood thrush, starling, yellow-breasted chat, eastern cardinal and red- 

 eyed towhee; most of these were observed in the act only once or 

 twice, but the cardinal was seen more than 13 times. Baird, Brewer, 

 and Kidgway (1874) mention that J. A. Allen saw a brown thrasher 

 feeding a nearly full grown cowbird. Probably the above lists could 

 be considerably enlarged. 



In at least three cases, a female cowbird has been seen to feed a 

 young cowbird that was supposed to be its own young, but in no case 

 could the relationship be proven. J. R. Bonwell (1895), of Nebraska 

 City, Nebr., reported seeing a female cowbird feed a young cowbird 

 in a nest with young rose-breasted grosbeaks. "Nearly every evening 

 she would come and feed the young Cowbird, but if the young Gros- 

 beaks would open their mouths for food she would peck them on the 

 head and refuse them food." Forbush (1927) mentions two other 

 cases. He knew Mason A. Walton, of Gloucester, well enough to 

 accept his report of seeing a female cowbird feed a young cowbird in 

 a yellow warbler's nest, but he had no way of knowing that she was 

 the parent. 



The other case cited was based on a careful observation by Laurence 

 B. Fletcher (1925), who trapped a female Cowbird with a young one, 

 saw the female feed it, banded both birds and saw the two together 

 afterward, the same adult still feeding the young. The adult, which 



