HOST RELATIONS OF PARASITIC COWBIRDS 85 



Bell's vireo than in that of any other local host. Out of 35 occupied 

 nests of this bird, 24, or 68.6 percent, were parasitized, whereas, out 

 of 43 nests of other parasitized passerine species, 14, or 32.6 percent, 

 were so affected. Barlow found that, with this species as host, the 

 percentage of cowbird eggs which hatched as compared to the number 

 that were laid was relatively low. He considers that Bell's vireo is 

 less tolerant of cowbird parasitism than are many of the other fre- 

 quently chosen victims. 



The study by Nice (1929, pp. 13-20) suggests that the nest mor- 

 tality of this vireo is often very high and, since the cowbird is often 

 a factor in the mortality, the parasite's importance in the population 

 dynamics of the host is heightened thereby — although difficult to 

 estimate. Of 17 nests studied by Nice, the outcome of two was un- 

 known, but the other 15 came to untimely ends. In tliree cases the 

 cowbirds seemed to have caused desertion, in three others there were 

 cowbu'd eggs, but only in one case was a young cowbird raised success- 

 fully. Nice pointed out that Bennett (1917) reported on 13 nests, 

 of which nine were failures, three were successes, and one remained 

 incomplete for study purposes. Of the nine failures, seven ostensibly 

 were due to cowbirds. 



Savary (1936, p. 64) reported one parasitized nest of a Bell's vheo 

 containing 4 cowbhd eggs and none of the vireo. He considered the 

 eggs to be so similar that they were almost certainly the product of 

 one cowbird. 



Gray Vireo 



Vireo vicinior Coues 



This is a frequently imposed upon victim, for which I have noted 

 dwarf race of the brown-headed cowbird: at Cajon Pass, northwest 

 of San Bernardino, California, on June 4, 1944, Hanna (1944, p. 244) 

 found a nest containing 2 eggs of the vheo and 1 of the parasite. In 

 the collection of the San Bernardino County Museum, however, there 

 are two additional records — one from near Hesperia, San Bernardino 

 County, and the other from Sheep Creek Canyon, San Gabriel Moun- 

 tains. 



Yellow-throated Vireo 



Vireo flavifrons Vieillot 



This is a frequentl}^ imposed upon victim, for which I have noted 

 about 100 records. They are distributed am.ong the following areas: 

 Ontario, Quebec, Massachusetts, Rhode Island, Connecticut, New 

 York, New Jersey, Pennsylvania, Maryland, and Ohio to Indiana, 

 Michigan, Minnesota, Iowa, Illinois, Kentucky, Tennessee, Alabama, 

 Arkansas, Oklahoma, and Kansas. In some places this vheo is re- 

 ported as a common host while in others, even where both it and the 



