HOST RELATIONS OF PARASITIC COWBIRDS 7 



fewer than 42 species have been reported but a single time as hosts. 

 Another sHghtly larger group have been noted more than once but 

 not more than five times apiece. In my extensive compilation of 

 data, I find only 17 species have been recorded as cowbird victims 

 100 times or more; these must be looked upon as the chief fosterers 

 although at times, and in some locahties, still other species may be 

 found to be as important, if not more so, to the parasite. In descend- 

 ing order of frequency of parasitism, these 17 are: yeUow warbler 

 and song sparrow, with about 1,000 instances each; red-eyed vireo, 

 875+; chipping sparrow, 650+ ; eastern phoebe, 375; rufous-sided 

 towhee, 300; ovenbird, 280 + ; yellowthroat, 250+ ; American redstart 

 and indigo bunting, with about 200 records each; yellow-breasted 

 chat and red- winged blackbird, about 180 each; Kentucky warbler, 

 150; Traill's flj^catcher, 150; BeU's and yellow-throated vireo, and field 

 sparrow, with approximately 100 to 125 records each. 



The next group of frequently imposed upon victims comprises some 

 17 species, for each of which there are on record more than 50 but 

 less than 100 instances. In descending order of frequency these are: 

 veery and Kirtland's warbler, with about 80 records each; wood 

 thrush, chestnut-sided warbler, and Louisiana waterthrush, with about 

 75 apiece; cardinal and vesper sparrow, with about 70 each; 

 warbling vireo and myrtle warbler, with over 60 each; eastern 

 wood pewee, prothonotary warbler, scarlet tanager, painted bunting, 

 dickcissel, American goldfinch, clay-colored and swamp sparrow, 

 with over 50 instances each. 



Of lesser frequency, but still important as cowbh'd hosts, are the 

 following birds, for each of which between 25 and 50 records are 

 available. Listed in descending order of frequency they are: rose- 

 breasted grosbeak, with 40 instances; Acadian flycatcher, white-eyed 

 vireo, black-and-white warbler, worm-eating warbler, blue-winged 

 warbler, and white-throated sparrow, with 34 to 40 apiece; eastern 

 bluebird, prairie warbler, blue grosbeak, and lark sparrow, with over 

 30 apiece; and catbird, brown thrasher, hermit thrush, blue-gray 

 gnatcatcher, and savannah sparrow, with 25 to 30 records each. 



These 50 hosts account for approximately 7,800 records out of a 

 total of about 9,000 instances of cowbird parasitism. It would seem 

 that the proportionate role they play in nature is, if anything, even 

 greater than these figures would suggest, since many instances of 

 parasitism upon common hosts are left unrecorded because of their 

 repetitive nature, while most cases involving uncommon victims are 

 pubhshed as records of particular interest. 



In much of the literature one finds statements to the effect that 

 certain species are "commonty," ''frequently," or even "invariably" 

 parasitized. Such statements of course cannot be translated quanti- 



630590—63 2 



