124 U.S. NATIONAL MUSEUM BULLETIN 233 



are three cases from Ontario and one each from New York and Min- 

 nesota. The Canada warbler has been known to hatch and rear 

 cowbirds: Roberts (1932, p. 277) noted a nest with young of the 

 parasite in Minnesota. 



Redstart 



Setophaga ruticilla (Linnaeus) 



The redstart is one of the commonest victims of the brown-headed 

 cowbird, but, probably because it is parasitized so frequently, many 

 observers have felt it mmecessary to record their data. The result is 

 that the total published material is much less than that which exists 

 for a number of species less often imposed upon by the parasite. In 

 my own field work in central New York, I found cowbird eggs or 

 young in 23 out of 34 nests exam.ined; I have not found a similarly 

 high incidence of cowbu'd parasitism reported from any other area. 

 In Ohio, Hicks (1934) discovered 22 nests, of which 7 were parasi- 

 tized; and in southern Quebec, TerriU (1961, p. 8) found 145 nests, 

 of which 23 held cowbu'd eggs. All in all, about 200 records have been 

 noted, ranging from Saskatchewan, Ontario, Quebec, and New 

 Brunswick to Idaho, Minnesota, Iowa, Wisconsin, Michigan, Ohio, 

 Maine, Massachusetts, Connecticut, New York, Pennsylvania, New 

 Jersey, Delaware, Maryland, Virginia, Alabama, Ai'kansas, and 

 Oklahoma. 



The incidence of cowbird parasitism noted in several studies, which 

 have been reported on in some detail, varied from less than 2 percent 

 to 10, 33, 42, and even as much as 70 percent of all the redstart nests 

 found. 



The records include both races of the redstart, ruticilla and tricolora, 

 and two races of the cowbird, ater and artemisiae. Both of these 

 races of the parasite molest tricolora, while only ater has been observed 

 in relation to ruticilla; in fact, only recently have the following two 

 records, involving the northwestern cowbu'd artemisiae, been reported. 

 Street {in Houston and Street, 1959, p. 166) found a nest of tricolora 

 at Nipawin, Saskatchewan, containing only a nestling covv^bird. 

 Burleigh (1952) observed a recently fledged cowbii'd being fed by 

 redstarts at Missoula, Montana. 



Occasionally, the redstart may build a new lining or floor over a 

 cowbird egg, if, as sometimes happens, the cowbu-d deposits her egg 

 before the host female deposits hers. The redstart is usually a very 

 tolerant host, accepting the strange eggs and rearing the young. At 

 times, however, although the redstarts assume charge of the cowbird 

 eggs, they may show an initial hostility toward the female cowbird 

 prior to actual ovulation. Strum (1915, pp. 202-203), noting that 

 both adult redstarts reacted aggressively toward female cowbirds, 

 concluded that this show of hostilitj^ saved them from excessive para- 



