84 U.S. NATIONAL MUSEUM BULLETIN 233 



Bell's Vireo 



Vireo bellii Audubon 



Bell's vireo is a frequent victim of the cowbird in an area that 

 extends from Illinois, Wisconsin, Iowa, Nebraska, Kansas, Oklahoma, 

 and Texas to Ai-izona and southern California. All four races of this 

 bird recognized within the United States are loiown to be affected — 

 typical bellii from Ilhnois to north-central Texas, medius in Brewster 

 County, Texas, arizonae in southern Arizona, and pusillus in southern 

 CaHfornia. The first of these is parasitized by the eastern form of 

 the cowbird, M.a. ater; the other three, by the dwarf race, M.a. 

 obscurus. In the course of many years I have learned of 82 actual 

 cases of cowbird parasitism, but these constitute only a fraction of 

 the number that lie back of the nimierous estimates put forward by 

 various authors. Attwater (1892, p. 237) considered it a rare occur- 

 rence to find an unparasitized nest in Bexar County, Texas. Bendire 

 (1895, p. 442) found it "almost impossible to obtain a full set of eggs 

 of the Least Vireo, nearly every nest containing one or two eggs of 

 this parasite, and usually only one or two of its own, and the latter 

 were frequently punctured." R. W. QuiUin wrote me that, in San 

 Antonio, Bell's vireo is a very frequent victim; he added that it some- 

 times covers over the parasitic eggs with a new lining to the nest 

 and that it occasionally may "push the foreign eggs from the nest, as 

 I have seen many, many eggs of the Cowbird on the ground under a 

 nest of this species. Yet the Bell's Vireo will hatch the eggs in the 

 majority of cases." Other observers have had different experiences 

 with this bird. Lantz (1883, p. 95) concluded that parasitized nests 

 were usually deserted. Moore (1928) came to a similar conclusion, 

 suggesting, as a result of his observations, that the vireo leaves its 

 old nest when molested by the cowbird and builds a new one near the 

 original site. He foimd 10 such nests within about a hundred yards 

 and only one pair of vireos in the vicinity. Since only two of the 

 nests showed evidence of parasitism, the explanation he offered, 

 however, does not fit the case too well. Pitelka and Koestner (1942) 

 described an instance wherein the evidence indicated that cowbird 

 parasitism was the probable cause of desertion of the first two, and 

 possibly three, nests of a pair of these vireos. At each of the nests 

 the desertion took place after the removal of an egg of the host. 

 Dawson's statement (1921, p. 31) that "one irate vireo I saw who 

 seized a cowbird three times her size and dragged her off the nest by 

 main force" may be discounted as highly inaccurate. The most that 

 a vireo could do would be to hover about excitedly and possibly peck 

 at a visiting cowbird which was on its nest. 



Barlow's recent work (1962, pp. 291-292) in Kansas has revealed 

 that the incidence of cowbird parasitism is greater in the case of 



