HOST RELATIONS OF PARASITIC COWBIRDS 19 



and a considerable percentage of these cases failed to come through 

 to the fledging stage. They gave a low success to the parasite; 

 consequently, they were rated as poor hosts from its standpoint. 

 Examples include such frequent victims as Traill's flycatcher, Bell's 

 vireo, the yellow warbler, and the field sparrow. Larger hosts, or 

 hosts with more rapid growth gradients, proved less susceptible to 

 the cowbirds and consequently resulted in a notably higher parasite 

 success rate, even at higher pressures, and these the McGeens rated 

 as good hosts. Examples of these included the song sparrow, the 

 ovenbird, the yellowthroat, and the red-eyed vireo. 



The importance that the relative size of the host species may have 

 is shown in the case of two frequently parasitized species of vireos, 

 the red-eyed and Bell's. The two are similar in their general nesting, 

 egg-laying, incubation, and feeding habits, and yet Bell's vireo is a 

 poor host, with a high frequency of loss to the parasite, while the 

 red-eyed vireo is a good one. The latter is nearly half again as large 

 as the former, which fact appears to be the chief determining factor 

 in the red-eyed vireo 's relations wdtli the parasite. For a further 

 analysis of what may be correlated with mere body size, see my 

 discussion of the hatching potential of the host species (p. 21). 



As is indicated in my account of Kirtland's warbler (pp. 108-111), 

 based on Mayfield's detailed study, this host is an instance of an 

 unbalanced economy, and it suffers perilously because of it. Here 

 again the large size of the warbler — as large as, if not larger than, 

 any of its congeners — makes it a good host to such a degree that it is 

 in danger of depleting its own population in serving its parasite. 



The frequency "with which the brown-headed cowbird parasitizes 

 some of the poor hosts is one of the main checks on its increase and 

 spread, since a poor host is a major factor of loss for the parasite. 

 The egg and nestling mortality are correspondingly higher than they 

 would be if the cowbird restricted its attentions to potential fosterers 

 capable of rearing its young with a higher rate of success. 



In an evolutionary sense, this inefScient host range might con- 

 ceivably be looked upon as an indication of the imperfection and even 

 of the relative recency of the parasitic habit. If the degree of loss 

 it causes the parasite were of critical importance, it might be expected 

 that natural selection would tend to eliminate those individuals that 

 laid their eggs more often in unpropitious nests. However, this 

 would be possible only if there were a marked tendency for each 

 parasitic hen to be more or less host specific. As we have already 

 seen (pp. 14-15), the available evidence for actual or even incipient 

 host specificity is hardly more than a number of isolated instances 

 in a much larger mass of data, either quite devoid of any specificity 



