4 U.S. NATIONAL MUSEUM BULLETIN 23 3 



gruities in the eggs themselves. Sunilarly, many hosts which regularly 

 accept and incubate the parasitic eggs lay eggs readily distinguishable 

 in size and coloration from those of the cowbird. Examples, to 

 note but a few, are the eastern phoebe, red-eyed vireo, and chipping 

 sparrow. 



While the brown-headed and the shiny cowbird parasitize birds 

 of a considerable range in size (from Polioptila and Empidonax to 

 Toxostoma in the case of the former species; from Myiophobus and 

 Sporophila to Mimus in the latter), the parasites do not lay notice- 

 ably small eggs for birds then* size, as do some species of Cuculus. 

 There has been no evolution in size or in coloration tending to make 

 cowbird eggs more readily acceptable to their hosts, as we find in 

 some cuckoos. 



The data presented in this report on the several hundred species 

 of hosts of the various cowbirds are not only of interest from the 

 standpoint of the habits of the parasites, but also serve as a digest 

 of all that is laiown of this aspect of the natural economy of each 

 of the victimized species. In not a few instances, the parasitism 

 of the cowbirds forms an important element in the breeding success 

 rate, and hence, in the population dynamics of the host. 



