HOST RELATIONS OF PARASITIC COWBIRDS H 



can bird seldom uses wrens as fosterers, but it does not explain the 

 marked diminution of interest M. ater shows in the open and readily 

 available nests of mockingbirds. The latter difference between the 

 two species of cowbirds — ^and it is a marked difference — still remains 

 a puzzling change in host selection. 



Throughout its enormous range, the brown-headed cowbird has 

 shown very slight alterations in its major host relations. Expanding 

 into additional breeding areas, it has necessarily come into contact 

 with, and made use of, host species not previously available to it, 

 such as Kirtland's warbler in the jack-pine areas of northern Alichigan, 

 and the redpoll in northern British Columbia. In other areas of 

 recent occupancy there is some evidence that suggests AI. ater has 

 made more frequent use of certain fosterers than it regularly does in 

 areas of older cowbird equilibrium. An example is the blue grosbeak 

 in southern California (p. 139). In all these instances, however, no ob- 

 vious change in the general pattern of habits is involved. In aU cases 

 the hosts are fairly similar to other, previous hosts in their nest con- 

 struction and in their main ecological choice of nesting sites. When a 

 wide-ranging host shows a marked ecological difference in various 

 parts of its range, its status as a cowbird victim may change accord- 

 ingly. Perhaps the best example of this is the redwinged blackbird. 

 In the eastern portions of its range it nests almost entirely in the 

 cattail swamps, where it is rarely molested by the parasite. Else- 

 where it builds its nests frequently in bushes, and there it is commonly 

 parasitized. 



The broad spectrum of host choice and the general lack of speciah- 

 zation within this wide latitude seem to have made it unnecessary 

 for the parasite to develop any strildngly differential host relation- 

 ships. That some such may be formed eventually is suggested by 

 the data presented in our discussion of the prothonotary warbler, a 

 hole-nesting bird for which a surprisingly high frequency of parasitism 

 has been recorded in the vicinity of Burlington, Iowa (see p. 92). 



While our present knowledge of the wide-ranging shiny cowbird 

 is much less complete than is that of the brown-headed species, the 

 former does offer some evidence which is suggestive of geographical 

 changes in host selection. For example, the wren, Troglodytes 

 musculus, is parasitized in Argentina, but not nearly as frequently 

 as are some sparrows, mockingbirds, and flycatchers, whereas in 

 Surinam and adjacent areas it appears to be the chief fosterer. In 

 Chile, where the shiny cowbird may be a fairly recent intruder, the 

 primary host is the diuca finch, Diuca diuca, although the chingolo, 

 Zonotrichia capensis, by far the most frequent victim in Argentina, 

 Uruguay, and Brazil, occurs in Chile as well and is not disregarded 

 completely by the Chilean cowbirds. 



