HOST RELATIONS OF PARASITIC COWBIRDS 17 



young cowbirds reared. But this, again, might give the fosterers the 

 opportunity to increase — and so on, in endless waves of depletion and 

 increase of the population, both of the parasite and of its usual victims. 



The natural fecundity of almost all the frequent hosts is sufficient 

 to stand the losses due to cowbird parasitism. The idea that the 

 two factors are necessarily mutually counteracting forces involved 

 in maintaining or upsetting the so-called ''balance of nature" is sup- 

 positious. What we may have here is another, if somewhat special, 

 type of predator-prey relationship. In a study, of which only a 

 summary has been pubHshed, Darling (1959, pp. 62-63) minimized 

 the supposed importance of predation in maintaining the "balance of 

 nature." He pointed out that recent, critical, analytical studies 

 have indicated that predation is, in itself, quite unimportant as a 

 factor in regulating the size of the populations of the prey species 

 and that the latter are, to a large degree, self-regulating. He sugges- 

 ted that the effects of predation — and it seems that brood parasitism 

 may be looked upon as a form of predation on the next generation 

 (the eggs), if not the present one (the adults), of the prey species — 

 will be found to vary with the degree to which the population of the 

 prey species is experiencing "optimal conditions in its ecological niche. 

 The complex of predation may be important in conservation of habitat 

 and consequently of the prey, by softening zenith and nadir of popula- 

 tion oscillations and so lessening the percussive effects on habitat." 



At a meeting of the American Ornithologists' Union at Ann Arbor, 

 Michigan, in August, 1960, the McGeens reported on a study of the 

 effect of parasitism by the brown-headed cowbird upon several of the 

 more frequently chosen species of hosts. The McGeens correlated the 

 incidence of parasitic successes and of host losses due to the parasite 

 with the population density of the cowbird. In areas where the cow- 

 bird was more numerous, it was observed that not only were more nests 

 victimized, but also that a larger proportion of these nests contained 

 mor-3 than a single parasitic egg. To express the "cowbird pressure" 

 as a factor in these correlations, the McGeens used the average of the 

 percent of host nests parasitized and the percent of multiple cowbird 

 eggs as compared with single ones in these nests. It was assumed, and 

 correctly so, that even though it was not possible to find all the nests 

 of a given host species in a study area, the sample observed, if not too 

 small, should give a proper picture of the degree of parasitism suffered 

 by that host. 



The "pressure" which cowbirds exert was divided by the McGeens 

 into two phases: the size of the affected segment of the entire nesting 

 population of a given host (in other words, the percentage of its 

 nests parasitized), and the degree or intensity of parasitism inflicted 

 upon this segment. The reasoning was that a parasitized group 



