118 U.S. NATIONAL MUSEUM BULLETIN 23 3 



in some areas than to a real differential in the incidence of cowbird 

 parasitism. In the case of the salt marsh race of San Francisco Bay, 

 sinuosa, reported so far as a cowbird victim only by Grinnell and 

 Wyeth (1927, p. 104), it is probable that the nature of this bird's 

 habitat is not especially attractive to the cowbird; this is not the case 

 with other sparsely reported victims such as occidentalis, campicola, 

 and chryseola. 



In my own field work at Ithaca, New York, I discovered five 

 instances, and I was told of many others which had been observed 

 in earlier years. In southern Quebec, Terrill found 113 nests in 

 about 60 years of observations; of these, eight were parasitized — a 

 much lower percentage of parasitism than occurs farther south. In 

 Ohio, Hicks (1934) found that 19 out of 41 occupied nests were 

 parasitized. 



In spite of the fact that the yellowthroat long has been known as 

 a frequent victim of the brown-headed cowbird, little in the way of 

 an analysis of the host-parasite relationship has been available until 

 recent studies by Stewart (1953) and especially Hofslund (1957), 

 both conducted in Michigan. Stewart found that the average 

 number of yellowthroat fledglings which were produced from a 

 nonparasitized nest was 1.9, whereas in nests parasitized by the 

 cowbird this average was only 0.1 (the average number of fledgling 

 cowbirds produced per parasitized nest was 0.4). During a period 

 of four years, Hofslund found 52 nests, 20 of which were parasitized. 

 In these 52 nests, 152 yellowthroat eggs were Imown to be laid; 

 of these 152 eggs, 52 were lost for various reasons. The loss of 30 

 of these 52 could be attributed to the cowbird: 10 eggs were removed 

 by the parasite, 6 were punctured by it, 4 were lost because of nest 

 desertion induced by cowbird activity, and there were 10 abortive 

 eggs whose loss was due to lack of heat during incubation — the 

 presence of larger cowbird eggs prevented close contact between 

 the smaller eggs and the body of the incubating warbler. The 

 percentage of egg loss attributed to the cowbird varied in the four 

 years of the study from 16.6 to 88.8 percent of the total egg loss. 

 The hatching success of the warbler was related to the number of 

 cowbird eggs in a nest: "No more than two Cowbird eggs, or one 

 Cowbird egg and two Yellow-throat eggs hatched in any nest; and 

 if more than one Cowbird egg was present, no Yellow-throat eggs 

 hatched." 



Hofslund has attempted a quantitative analysis of hatching success 

 in the yellowthroat. He started with Hann's estimate (1947, p. 174) 

 that the probable limit of egg volume which a related warbler species, 

 the ovenbird, can successfully incubate and hatch was between 1.3 

 and 1.8 times the volume of its normal clutch of 5 eggs and that 



