20 U.S. NATIONAL MUSEUM BULLETIN 23 3 



or positively opposed to it. It appears conceivable, however, that 

 additional detailed (as opposed to relatively "loose") field studies 

 may increase the evidence for host specificity. 



One might speculate that the low survival rate between the egg 

 stage of the cowbird and the fledging stage might be due to a selective 

 mechanism which residts in something partaking of the nature of 

 differential reproduction. If this were the case, it could easily lead 

 to a relatively rapid adaptive evolution of the parasite with respect 

 to its frequent host relations. Something of this sort appears to 

 have transpired in the case of the European cuckoo, Cuculus canorus, 

 the Indian koel, Eudynamis honorata, and some other cuckoos, in 

 which the degree of adaptive similarity in egg coloration achieved 

 and the concomitant development of individual host specificity 

 can hardly be explained on any other basis than the selective pressure 

 brought to bear by the hosts. In the cowbirds, however, evolutionary 

 changes in the parasitic habit, after its original appearance, seem to 

 have been influenced surprisingly little by the hosts. The changes 

 in the habit from the screaming cowbird through the shiny cowbird 

 to the brown-headed species are chiefly a widening of the choice of 

 hosts used, a trend towards egg removal, and, after a relatively high 

 percentage of waste of eggs (in AI. honariensis) , a more economical 

 disposition of the eggs (in M. ater). 



We may note at this point some of the better quantitative data 

 available on a few of the frequently used host species. Since all of 

 these are discussed in greater detail under each of the species in our 

 catalog, only a brief statement is given here. 



In the case of the song sparrow (see pp. 169-170), Nice (1937, p. 200) 

 asked whether each cowbird raised was reared at the expense of a 

 brood of sparrow young. The data showed that, while 66 successful 

 unparasitized nests raised an average of 3.4 song sparrows each, 28 

 successful parasitized ones averaged 2.4 song sparrows. In other 

 words, each young cowbird was reared at the expense of one young 

 sparrow, not of a whole brood. 



The red-eyed vireo (see pp. 87-89) was carefully studied by Southern 

 in Michigan. He found 104 nests, of which 75 were parasitized. The 

 nesting success of the vireo was 87.49 percent in unparasitized nests, 

 66.66 percent in parasitized ones. The last figure is not as different 

 as one might have expected on the basis of Lawrence's study (1953) 

 of this vireo in an area where there were no cowbirds and where 

 the nesting success in 35 nests was 63 percent. Southern concluded 

 that the local vireo population he studied did not suffer serious deple- 

 tion even though it was heavily victimized. He considered the cow- 

 bird a necessary check on the undue increase of the vireo although 

 he admitted that, if the parasite were equally successful with most 



