PRODUCTIVITY OF POPULATIONS 



547 



Whether this net productivity is high or low has been primarily a consequence of the de- 

 gree of mortality to nests, broods, or adults during the overwinter period. Considering first 

 circumstances under which a net gain in the number of breeders would result figure 52 is 

 representative. Survival during both nest and brood periods is good while overwinter mor- 



FIGURE 52. AVERAGE LIKE EQUATION OF INCREASI-NG GROISE POPULATION 



tality is below average. The latter is the difference between this situation and the one in 

 figure 53 depicting a stable condition and is usually associated with populations well below 

 the carrying capacity of their habitat. By the same token when adult losses increase apprecia- 

 bly a net reduction may follow as in figure 54. That either nest or brood failure can also be 

 the cause of occasional set-backs is shown in figures 55 and 56*. 



Other losses, with one exception, have not been observed to vary sufficiently to exert any 

 appreciable influence. The exception is non-breeding among the females, which apparently 

 approached 25 per cent on Connecticut Hill in 1933, 1935 and 1936. Since, however, the 

 evidence of this is indirect, more specific appraisal of its role must await further study. 



With respect to the effect of hunting, studies'^ have shown that in the better grouse coverts 

 of New York about 17 per cent of the birds present in the fall are usually taken. Of this pro- 



* The reader should bear in mind that there is no evidence as yet to correlate those situations depicting net losses with any 



cyclic tendency which may exist. 

 A See Chapter IX. p. 378. 



