14 U.S. NATIONAL MUSEUM BULLETIN 23 3 



While it is true that cowbu'd eggs show no tendency toward adap- 

 tive resemblance to those of their frequent hosts, the fact that such 

 similarities have been evolved in some of the Old World cuckoos 

 suggests that there may be, and often probably is, an advantage to 

 the parasite if its eggs are not too readily distinguishable by the 

 hosts from then* own. However, that such similarity is not always 

 essential is amply shown by the success of the cowbirds. 



In some cases, more than one egg is laid in the nest by the same hen 

 cowbird, as Savary (1936) discussed in his account of Bell's vii-eo; 

 in others, more than a single bird is responsible. Byers (1950, p. 

 136-138) judged from the differences in size and coloration of the 

 8 cowbird eggs he found in one nest of a black-and-white warbler, 

 that four female cowbirds had been involved, and Mulliken (1899) 

 concluded that five different birds had laid one egg each in a scarlet 

 tanager's nest that he described. 



At Nickelsville, Virginia, F. M. Jones (1941, pp. 117-119) had the 

 opportunity to study the egg-laying of a female cowbird which laid a 

 very distinctively colored, hence easily recognized, egg. He found a 

 total of 8 eggs from this bird between May 5 and June 15 m eight nests 

 of four different species — two white-eyed vireos, tln-ee praude warblers, 

 two field sparrows, and one summer tanager. In my own field studies 

 (Friedmann, 1929, p. 183) I found that one hen cowbhd had laid 5 eggs 

 in fom" nests of the veery, the chestnut-sided warbler, and the redstart. 

 Another individual had laid 2 eggs in a redstart's nest, and 3 in one 

 nest of a red-eyed vireo; stUl another bu-d had laid 4 eggs in nests 

 of the veery, the red-eyed vireo, and the redstart. These cases 

 indicate that ordinarily the parasite is not individually host-specific. 

 Yet there are some data that suggest that at times and in particular 

 situations some hen cowbirds may show incipient, if not developed, 

 specificity in their choice of victims. 



Such a case is described in our account of the prothonotary warbler 

 (see pp. 91-92). At Burlington, Iowa, J. P. Norris (1890, pp. 177- 

 182) found 35 nests of this warbler within a period of two weeks, 

 and of these, 18 contained 1 or more cowbird eggs. Since this 

 warbler nests in holes in trees and since the cowbird ordinarily shows 

 little interest in nests in such a concealed situation, it appears that 

 some individual hens at Burlington must have had an appreciably 

 consistent tendency to choose the prothonotary warbler as a host. 

 Another, and in some ways even more striking, example is Walkin- 

 shaw's series of observations on a cowbird consistently parasitic on 

 a field sparrow in Michigan (see pp. 164-165). 



Further evidence of host-specific tendencies was found by Dr. 

 and Mrs. D. McGeen, who very kindly sent me a summary of the 

 results of their intensive study of the cowbirds and their hosts near 



