HOST RELATIONS OF PARASITIC COWBIRDS 139 



New York, New Jersey, Ohio, Indiana, Illinois, Michigan, Wisconsin, 

 Minnesota, Iowa, and North Dakota. The northwestern race of the 

 parasite, M.a. artemisiae, is involved in the Saskatchewan and North 

 Dakota records ; the eastern race, M.a. ater, in all the others. Although 

 most of the records merely refer to eggs in nests, the rose-breasted 

 grosbeak has been known to rear young cowbirds to the fledgling stage. 

 At Ithaca, New York, on July 6, 1922, I observed a pair of grosbeaks 

 feeding an almost fully grown young cowbird. In southern Quebec, 

 Terrill (1961, p. 9) noted three instances of parasitism in 42 nests 

 examined. 



Black-headed Grosbeak 



Pheucticus melanocephalus (Swainson) 



The black-headed grosbeak has been reported as a cowbird victim 

 in only a few places in Kansas, Nebraska, Colorado, Utah, and Mon- 

 tana. In Lincoln County, Nebraska, Tout (1947) recorded the fact 

 without giving any detailed cases. Sclater (1912, p. 401) wrote 

 that this grosbeak is imposed upon not infrequently bj^ the cowbird 

 in Colorado, but he also gave no specific instances. Silloway (1917, 

 p. 159) and A. A. Saunders (1914, p. 136, p. 143) reported parasitized 

 nests from Montana, while Bee and Hutchings (1942, p. 82) found 

 an instance at Utah Lake, Utah, on June 5. Mr. Guy Love 

 informed me years ago that he found two parasitized nests in Decatur 

 County, Kansas — one on June 11, 1909, and the other on May 25, 

 1912. The Kansas records refer to the eastern race of the cowbird, 

 M.a. ater, whereas the others relate to the northwestern race, Al.a. 

 artemisiae. All involve the nominate race of the host. 



Recent studies of hybridization between the rose-breasted and the 

 black-headed grosbeaks suggest that the two are related very closely. 

 Although outside the interest of the present study, it should be 

 mentioned that there is no apparent difference between the two 

 birds in their relation to cowbu'd parasitism. As far as our present 

 knowledge goes, the cowbird treats them alike. 



Blue Grosbeak 



Guiraca caerulea (Linnaeus) 



The blue grosbeak is a fairly frequent victim of the brown-headed 

 cowbird, but, at times and in certain localities, it may be imposed 

 upon very much. Whereas in most areas it has been recorded as a host 

 on the basis of one or a very few instances, the opposite is the case 

 in some areas, as seen in the experience of Bleitz (1956) in southern 

 California. In a single season, within the limits of a very restricted 

 locality, he found seven nests of the blue grosbeak, every one of which 

 had been parasitized. If this were a typical or even fairly usual 

 condition, it could be expected that many more instances would have 



