8 U.S. NATIONAL MUSEUM BULLETIN 233 



tatively into the figures here presented, but they do establish that the 

 actual hsted instances form only a portion of the total experience of 

 all the observers. In my first compilation (1929) I did attempt to esti- 

 mate them and added the resultant figures to the records, but they are 

 not included in our present totals as there is now enough data to be 

 able to afford to disregard the earlier estimates. 



Aside from the frequency with which they are parasitized, some 

 host species are important to the brown-headed cowbird in terms of 

 their unusually extensive geographic ranges, as contrasted to some 

 others that are much imposed upon in relatively limited areas. 

 The comm.on hosts of great geographic availabihty to the parasite 

 are, in descending order of importance : song sparrow, yellow warbler, 

 red-ayed virco, chipping sparrow, rufous-sided towhee, yellowthroat, 

 yellow-throated vireo, yellow-breasted chat, redwinged blackbird, 

 warbhng vireo, American goldfinch, and lark sparrow. Even among 

 these, there are geographical and ecological differences; for example, 

 in the eastern areas, where it nests chiefly in cattail swamps, the 

 redwinged blackbird is seldom molested, whereas in other areas in 

 which it nests in bushes, it is a frequent host, and, in an over-all 

 picture, by virtue of its abundance over a vast range, it has been 

 reported as a host a great many times. Similarly, the American 

 goldfinch, while aval] able geographically, is often unavailable season- 

 ally since it is a late nester. The prime example of an extremely 

 localized, but locally very important, host, is Kirtland's warbler; 

 the golden-cheeked warbler is another case, but it appears to be less 

 intensively parasitized. 



Inasmuch as data are still relatively sparse and incomplete south of 

 the Mexican border, the following breakdown of the cowbird's hosts 

 is restricted to the area covered by the official checklist of North 

 American bu'ds, comprising the United States and Canada and includ- 

 ing Baja California but not the Mexican mainland. In North America 

 the bulk of the cowbird's victims are found in eight families — the 

 tyrant flycatchers, the thrashers and their allies, the thrushes, the 

 vireos, the wood warblers, the orioles and blackbirds, the tanagers, 

 and the finches. 



Of the 32 species of flycatchers in North America, 17 are known to 

 be parasitized. Of the remaining 15, about half do not nest within 

 the breeding range of the parasite, or they are birds whose nests 

 seldom have been observed, or they are hole-nesters and are thereby 

 relatively immune to the attentions of the cowbird. Two flycatchers, 

 the eastern phoebe, and Traill's flycatcher, are important, frequent 

 hosts. 



The thrashers are somewhat infrequently imposed upon, but 7 of 

 the 10 North American species have been found to be so affected; 



