HOST RELATIONS OF PARASITIC COWBIRDS 21 



of its other hosts, it might become too numerous. He suspected, 

 however, that the parasite has few other hosts with which it is equally 

 successful. 



Another frequently imposed upon victim is the ovenbird (see pp. 

 112-113). Hann (1937) estimated that 52 percent of all the active nests 

 he watched were parasitized. The loss due to the cowbirds was 18 

 percent of the eggs and young of the ovenbird. The chief loss was 

 due to the removal of eggs by the cowbird. Out of a total of 40 

 cowbird eggs, only 22 hatched and only 10 of these lived to the 

 fledging stage, and not more than 5 survived to independence of their 

 foster parents. 



In the case of the yellowthroats (see pp. 118-1 19), studied by Hofslund 

 over a period of four years, 152 eggs were laid m 52 nests, 20 of which 

 were parasitized. Of the 152 eggs, 52 were lost; and of these, the 

 loss of 30 seemed to be due to the cowbird. The 20 parasitized nests 

 produced only 0.6 cowbird each; 9 of the 20 produced young cowbirds, 

 or an average of 1.3 each. 



As noted in my discussion of the field sparrow (p. 165), cowbird 

 parasitism at times may reduce the percentage of nest-success for the 

 host without producing any parasitic young. 



With each of these host species, the survival rate of the cowbirds 

 in their nests is relatively low. Even in species that seem to be 

 overly burdened, the results are not especially disastrous as a rule. 

 In the case of the Kirtland warbler (see pp. 108-111), however, the 

 situation is other\vise. The survival rate of the cowbird with this 

 host is high, about 41 percent, and approximately 55 percent of all 

 Kirtland v\^arbler nests are parasitized. It is estimated that the 

 whole Kirtland warbler population would produce annually about 

 60 percent more fledglings of its own species if the cowbh-d menace 

 were not present. 



Hatching Potential of Host Species 



Another factor that limits the degree to which a species can be a 

 successfid host to the parasite is what may be termed its hatching 

 potential. A word of explanation may be in order, as the concept 

 involved is seldom used in studies of wald birds. Incubation nnplies 

 maintaining a fairly constant temperature in a clutch of eggs, the 

 body of the incubating bird being the source of the heat. It implies 

 the warming of the whole volume of each egg, not merely the portion 

 that happens to be on top at any given moment of incubation. The 

 amount of heat that the Vjird is able to give off is limited by its metabo- 

 hsm, which is, or more properly, is thought to be, fairly constant for 

 each species. If a species usually lays 4 eggs of an average volume 

 of, for example, 2 cc. each, incubation cannot succeed unless the bird 



