128 U.S. NATIONAL MUSEUM BULLETIN 233 



distributed from Illinois, Minnesota, Montana, and "northwestern 

 Canada" to Wyoming, Colorado, and California. T. S. Roberts 

 (1909) reported three parasitized nests in one breeding colony of 

 yellow-heads. Lincoln (1920, p. 69) wrote that, in the Clear Creek 

 district of Colorado, this blackbird was one of the most imposed 

 upon local hosts. These observations suggest that the j^ellow- 

 headed blackbird locally may be a not uncommon victim; but over 

 the extensive range of its sympatry with the cowbird it is not Imown 

 to be affected to any degree. 



As many as 6 eggs of the parasite along with 4 of the owner have 

 been reported in a single nest at Laramie, Wj^oming, by Henninger 

 (1915, p. 232). 



All three races of the parasite are involved in the known records: 

 ater in those from Ilhnois, Michigan, and Minnesota; artemisiae in 

 those from Montana, Wyoming, Colorado, and northwestern Canada; 

 and obscurus in one case from Los Banos, California. H. R. Eschen- 

 berg informed me that there he found a nest on May 6, 1936, containing 

 a dwarf cowbu*d's egg in addition to 4 eggs of the blackbird. 



Redwinged Blackbird 



Agelaius phoeniceus (Linnaeus) 



The redwinged blackbu'd is a fairly common victim of the brown- 

 headed cowbu'd in some areas, but it is almost unmolested by the 

 parasite in other localities. In regions where the redwings are largely 

 colonial nesters in cattail swamps, they are rarely bothered by the 

 cowbird; where their nests are more scattered and in bushes, they 

 often are parasitized. 



In Michigan, Nickell (1955, p. 91) examined 1300 active nests of 

 this blackbird over a period of 15 years and found the frequency of 

 known parasitism to be about 1 in every 185 nests. All the para- 

 sitized nests were either solitary or at the perimeters of colonies — 

 a hundred feet or more from their nearest conspecific neighbors. 

 He concluded that, because of the combined aggressiveness and 

 vigilance of the blackbirds, few cowbird eggs are deposited in red- 

 wing nests that are in definite colonies. NickeU also concluded 

 that any cowbird which was hatched in a nest with two or more red- 

 wings would have slight survival chances because of the larger size 

 and aggressiveness of the host's own nesthngs. NickeU's observa- 

 tions are in agreement with those first made by Sutton (1928) in the 

 Pymatuning Swamp in western Pennsylvania; these data demon- 

 strated the tendency of the redwings to drive away any intruding 

 cowbirds, thereby protecting not only their own nests, but also — 

 though not intentionally — those of vireos and warblers breeding close 

 by. Trautman's experience at Buckeye Lake, Ohio, as summarized 



