10 U.S. NATIONAL MUSEUM BULLETIN 233 



ences. The latter bird parasitizes species of the following famihes, 

 arranged in descending order of number of included host species: 

 finches and their allies, 41 species; tyrant flycatchers, 27; blackbirds 

 and orioles, 24; tanagers, 10; wrens, 7; and mockingbirds, 6. It also 

 uses wrens and mockingbirds much more frequently than does M. ater. 

 A number of purely neotropical families, absent from North America, 

 such as ant-thrushes, spinetails, woodhewers, cotingas, and honey- 

 creepers, are affected to some extent, but the real difference between 

 the two cowbirds is found in their relation to the vireos and the wood 

 warblers. 



The South American M. bonariensis seldom uses nests of vireos and 

 wood warblers, two families that are heavily parasitized by M. ater. 

 It is not altogether valid to compare data on the two since the picture 

 is about as incomplete in South America as it is complete in North 

 America, but it may be noted that only 2 species of vireos and 2 of 

 wood warblers are known to act as hosts for the shiny cowbird despite 

 the fact that there are resident in South America some 18 species of 

 the former family and 30 of the latter. (Figures compiled from 

 Hellmayi''s Catalogue of the Birds of the Americas.) To emphasize 

 the difference involved here, one should recall that M. ater has been 

 found to affect every North American species of vireo (11 in all) that 

 breeds within its range. Moreover, the brown-headed cowbird is 

 recorded as parasitizing 45 of the 50 North American species of wood 

 warblers that are sympatric with it. 



The shiny and the brown-headed cowbirds are similar in their 

 frequent use of fringillids as hosts. In this family M. ater is known 

 to impose upon 56 of the 63 species that breed sympatricaUy with it 

 in North America. M. bonariensis is recorded as victimizing 41 of 

 the 181 fringUlid species listed by Hellmayr as breeding in South 

 America. The figure 181 probably includes some species allopatric 

 to the parasite, but the present state of knowledge makes it difficult, 

 if not impossible, to determine this in many individual cases. This 

 statement possibly applies also, and to a lesser degree, to the vireos 

 and warblers discussed above. 



Since the brown-headed cowbird appears to be a more recent 

 evolutionary entity than the shiny cowbird, the differences between 

 the two in host selection should be examined more closely. As 

 pointed out in the discussion (pp. 192 ff .), the shiny cowbird's interest in 

 domed nests has some of the attributes of atavism, a lingering of past 

 stages in the history of that species; conversely, it is in keeping with 

 the newer, if not necessarily more advanced, status of the brown- 

 headed cowbird that this bird evinces much less tendency to enter 

 and parasitize nests with dark interiors, such as domed structures or 

 holes in trees. This could account for the fact that the North Ameri- 



