PRODUCTIVITY OF POPULATIONS 



535 



TABLE 86. RELATIONSHIP OF DENSITY OF FALL GROUSE POPULATIONS TO 

 SUBSEQUENT SURVIVAL ON CONNECTICUT HILL 

 AND ADIRONDACK STUDY AREAS— 1930-1942 



t Group I^years having a fall population deoaily, on Connecticut Hill of under 12 per 100 acres, on the Adirondack area of 

 under 7.5. 



Group II — years having dcnaities between 12 and 15, and over 7.5 respectively. 



Group III — years on Connecticut Hill with densities over 15. 



In figure 48 the composition of the fall population has been graphed in relation to the 

 subsequent mortality. It is evident from this that the trend in number of adult birds lost 

 each year has paralleled to a remarkable degree that of the number of young birds present 

 in the fail. Probably it is the l)irds of the year which absorb the bulic of the overwinter loss. 

 It may be, however, that sometimes these young birds usurp the territories of older individ- 

 uals, forcing them into poorer situations where they become more vulnerable. 



On the other hand, it does not follow that the size of the annual increment is more than 

 a general index to the proportion of the total ])opulation destined to survive. In 1935-36 on 

 Connecticut Hill, for example, the latter was practically identical with that of the previous 

 year (table 84) in spite of a drastic change in the ratio of young to old birds. A similar 

 situation occurred in 1940-41. On the Adirondack area there is more indication of such a 

 relationship. This suggests that some fundamental principle of the sort may exist but that 

 its effect is often obscured by other influences. 



In order to determine the extent to which these variations in survival have been due to 

 causes other than the varying levels and composition of the fall population, the data were 

 correlated statistically to remove the effect of those factors. The adjusted relationships* are 

 presented in figure 49. It is interesting that these other forces affecting adult survival exerted 

 less pressure in 1931-32 on the Connecticut Hill area than during the ])receding year when 

 the population level was lower. It is also noteworthy that on the Adirondack area in 1935-36, 

 in spite of the apparently high degree of security of the very low fall population (table 84) 

 this analysis indicates that the relative weight of the limiting forces was next to the highest 

 recorded on this tract. 



Since predation has been shown to be the primary decimating agent responsible during 

 this period, the graph reflects mainly differences in predator pressure as well as in the vul- 

 nerability of the birds. The reasons for these variations are many. Some may be traced to 

 fluctuations in the abundance of the predators themselves. In general, however, a number of 

 environmental influences acting as cafalists are involved. The comparative quality of the 

 particular coverts which hold the bulk of the birds in a given year is highly important. 

 Units which produce well during the breeding and rearing seasons may have comparatively 

 low wintering value. The weather is another factor capable of affecting vulnerability. Buf- 

 fers, too. may exert a substantial control through their effect on predator activity as dis- 



'■• Data correlated by regression technique. 



