POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 



375 



in an adept way, making use of several me- 

 thods appropriate for each case. For exam- 

 ple, the sparrow population was estimated 

 by sampling the number of males per house 

 during the spring, after which this mean 

 figure was multiplied by the number of 

 houses and then converted to total popula- 

 tion through knowledge of the winter sex 

 ratio derived from field studies. 



An approach based on male singing fre- 

 quency was used for the chaflBnches and 

 titmice. The mean monthly mortahty fac- 

 tor, s, for the latter (Parus) was calculated 

 as follows: 



(1-fi) (l-s)«=l 



in which / is the number of young reared 

 per adult and the exponent 12 represents 

 months of the year. 



For the sparrows and chafiinches, s was 

 obtained by the notation 



(1-t-e) (l-s)«=l 



in which e is the ratio; number of first 

 year birds -=- number of adults. 



One specific illustration of sparrow hawk 

 predation can be given as it affects the two 

 titmice species in a particular location for 

 two intervals of the year. Tinbergen's 

 method allows him to compute the total 

 expected mortality against which the num- 

 ber of birds actuallv killed by the hawks 

 can be contrasted. This illustration is as 

 follows: 



June 16- 

 May Sept. 15 

 Coal Tit 



Expected (total mortality) 357 5,569 

 Sparrow hawk mortality . 132 138 



Great Tit 

 Expected (total mortality) . 210 2,906 

 Sparrow hawk mortality . . 93 529 



Similar data representative of the entire 

 study led Tinbergen to the generalizations 

 that sparrow hawk predation caused ap- 

 proximately 50 per cent of the summer 

 mortality for the house sparrow, 25 per 

 cent for both the chaffinches and great tits, 

 and only a negligible percentage for the 

 coal tits. 



For our present purposes the major 

 ecological principle emerging from this in- 

 vestigation is that, within certain limits, 

 the intensity of predation is proportional 

 to the abundance of the prey, or, in other 



words, predation is demonstrated to have 

 density-dependent aspects. Hartley (1947) 

 says: "The upper limit of predation inten- 

 sity is reached when the prey species is in- 

 creasing so much more rapidly than the 

 predator species that the most intense pre- 

 dation contributes a smaller and smaller 

 part of the total mortahty." 



Errington (1937a) discussed certain 

 of the more complicated aspects of 

 predation as it affects a prey population. 

 His general theme is stated thus: "Life to 

 wild animals unquestionably is often harsh, 

 but the demands of predators in temperate 

 regions are not apt to be so drastic as to 

 make existence a neck and neck race be- 

 tween the great appetite of predation and 

 the breeding rates of the prey animals" (p. 

 243). Errington goes on to quote McAtee 

 (1936), who concluded that animal popu- 

 lations only rarely approach the limits of 

 food supply. Errington is of the opinion 

 that predators, at least those that are 

 higher vertebrates, are no exception to this 

 rule. "Predators may occasionally starve, 

 and predator pressure may at times be 

 about all that a prey species can stand, or 

 conceivably more than it can stand; but, for 

 all that, predation still seems to be essen- 

 tially a byproduct of population rather 

 than a broadly dominant influence on 

 population" (pp. 243-245). 



Errington elaborates this concept in re- 

 porting observations on the effect of pre- 

 dation by the great horned owl (Bubo vir- 

 oinianus) upon populations of the bob- 

 white quail (Colintis vir^iniamis) . Winter- 

 ing bobwhites were studied for six years 

 at Prairie du Sac, Wisconsin, where Er- 

 rington determined the quails' density and 

 the intensity of owl predation. 



These findings are partially reported in 

 Table 28. It should be noted that this 

 study area had a "carrying capacity" that 

 remained constant so long as hunting, star- 

 vation, or unusual weather conditions did 

 not obtain. The major point brought out 

 in the table is that when the quail popula- 

 tions did not exceed this carrying capacity, 

 the predation was low; conversely, when 

 the area was supersaturated, the predation 

 was high. For this case it thus appears that 

 Errington views predation as an incidental 

 rather than a controlling factor affecting 

 population growth form. The owls remove 

 the excess quail, and, as the latter return 



