242 FOOD HABITS AND REQUIREMENTS 



perature, and the intensity, duration and composition of sunlight. 



Weather records of local stations, however, fail to exhibit such a cyclic trend in most of 

 these factors. In New York, wide differences in the amount of precipitation as between 

 individual sections of the State over the same year normally occur. No substantial rela- 

 tionship was found when an attempt was made to link precipitation with subsequent grouse 

 abundance from State records covering the past 40 years. Several records of high populations 

 of grouse produced during abnormally wet as well as in dry years were found. But no 

 studies of food composition on wet vs. dry years were made. 



Temperature also affects plant composition, but the extremes are more likely to be felt 

 than the average. Many plants are capable of growing under wide variations, therefore a 

 trend differing only a few degrees from year to year in the average temperature would be 

 unlikely to have an appreciable influence. The most noticeable effect of temperature, to date, 

 is connected directly with grouse mortality rather than indirectly through food. 



The third factor to consider is the relation of the intensity, duration and composition of 

 sunlight to food. The first of these, changes in the intensity of sunlight, is closely tied up 

 with sunspot activity and is, therefore, cyclic in nature. The possibility that light intensity 

 affects plant composition, however, is largely offset by the phenomenon known as solarization. 

 This arrests the action of chlorophyll as the intensity approaches a certain degree that 

 is reached on all but the cloudiest of days. 



Duration of sunlight, on the other hand, although extremely important to plant growth, 

 bears no relation to sunspot activity, for the length of daylight is normally much the same 

 year by year. 



Sunspots control or affect still another physical condition, that of the quality of sunlight. 

 Little is known of their effect on plant composition, though changes in the amounts of ultra- 

 violet and infra-red light have been observed. The presence of vitamin D is dependent upon 

 the former, but it is doubtful if quantitative changes in plant foods, thus brought about, are 

 great enough to seriously affect the health of grouse. Experiments with poultry" have shown 

 that a relatively small quantity of sunlight will offset the absence of this vitamin from the 

 diet. It is noteworthy that the Investigation has never found a grouse, either in the wild or 

 in captivity, suffering from rickets. 



In considering the broad picture it should not be forgotten that 994 separate food items 

 have been identified from New York grouse alone. Then, too, their food is the product of 

 at least two growing seasons. Buds and leaves are formed from the energy of past years. 

 whereas fruits are more largely the result of the present year's activity. Such considerations 

 tend to discount the probable effects of cyclic changes in ])lant composition on grouse abun- 

 dance, even if such do occur. 



The diet, too, can influence population by afft'cling directly their resistance to the various 

 decimating influences in their environment. A grouse in pot)r coiKlilioii is more susceptible 

 to disease or ])r('dators than one in good health. It is possible that such a condition could 

 be brought ai)out by a generally inadequate diet. However, in view of the foregoing discus- 

 sion, no cyclic tendency in grouse abundance, due to diet, is indicated. 



Likewise dietary deficiencies could disrupt reproduction. Here again, there is no evidence 

 to verifv a cvclic relationship. It is noteworthy that the number of eggs laid by New York 

 grouse remained almost constant over the 13 years of the Investigation*. 



• See Chaplrr VUI. p. 360. 



