18 U.S. NATIONAL MUSEUM BULLETIN 2 33 



of nests, each containing a single cowbird egg, was under less 

 "pressure" than a similar group with 2 or 3 cowbird eggs apiece. 

 On this basis the McGeens concluded that these multiple eggs 

 were a readily usable key to the pressure rate: the higher the pro- 

 portion of multiple eggs to single ones in the total parasitized nests, 

 the greater the cowbhd pressure. By averaging these two parts 

 of the picture, it was possible to arrive at a single "pressure index" 

 for the whole host population in a study area, and this index could 

 then be used in correlating parasite successes and host losses. 



It seems to me, however, that multiple cowbird eggs, beyond a total 

 of three, are almost always lost or wasted eggs. From tliis it follows 

 that any cowbird "pressure" of more than 3 eggs in a nest may not 

 have more effect on the host than does the "pressure" of just 3 eggs, 

 a situation which, in itself, usually speUs disaster to the entire 

 nest contents. If the presence of 3 cowbird eggs in a nest is fatal 

 to the outcome of the nest, 4 or 5 eggs cannot be more so. The 

 only effect these extra eggs can have is that the parasite is Hterally 

 expending its reproductive resources in the one way that cannot 

 further harm its victims. 



It may be explained, at this point, that whereas the McGeens 

 appeared to be thinking of the cowbird "pressure" as a descriptive 

 element in the demographic situation, the concept of "pressure," as 

 I am using it here, applies directly to the degree to which, or the 

 intensity with which, the host is caused to feel the effect of the popu- 

 lation density of the parasite. This would account for the difference 

 in the significance attached by them or by me to the matter of multiple 

 eggs above three in a nest. The difference is important, but actually 

 it applies only to a smaU percentage of the instances of parasitism 

 because, in the great majority of cases, less than 3 cowbird eggs 

 are involved. 



One other thought may be introduced here. As Lack (1954b, 

 p. 155) has expressed it in his study of the evolution of reproductive 

 rates, the clutch size in nidicolous birds is limited by the number of 

 young that the parents can feed. "With broods above normal size, 

 fewer, not more, young are raised per brood. . . . The decline in 

 fecundity with rising population density, found in both birds and 

 invertebrates, is primarily a response to a dwindling food supply for 

 the young, and not to population density as such. . . ." 



Within the total range of resultant host-parasite relationships which 

 emerge from the McGeens' study, at least two balanced situations, 

 or balanced economies, were found to exist. Host species of small 

 size or of relatively slow incubation and rate of nesthng development 

 were very susceptible to the presence of eggs or young of the parasite. 



