FOOD AND ITS RELATION TO HEALTH 241 



composition should be determined. To answer some of the problems a far more complete 

 analyses than the '"crude" determination of basic nutrients is required. The present studies 

 must be continued for a long time to come before many relationships can be even reasonably 

 well understood. Finally, the limitations of the data provided by chemical anlyses alone must 

 not be overlooked. Knowledge of the nutritive constituents is largely academic until the abil- 

 ity of grouse to use them is determined by biological assays. 



Relation of Food to Weight 



One of the less exact yardsticks bv which the relation of the diet to the health of grouse 

 may be measured is the ability of a food to sustain weight and condition. That the birds are 

 able to survive from year to year on their normal diet would seem in itself indirect proof of 

 its adequacy. But. as has been indicated, such an assumption does not present the entire 

 picture. The physiological experiments reported elsewhere* indicate that grouse have a re- 

 markably high rate of metabolism. The amount of i-iicrgN they expend, even in a simple 

 flight, is tremendous. Even the varied diet enjoxed l)\ grouse in .New \ork is not always 

 sufficient to meet completely the normal demands of the bod\ when the energy output is 

 high. Weight records of collected specimens bear this out. I he females show an almost con- 

 stant weight loss from January until the end of the l)rccding season in June, and the males 

 until after the moult. When these periods are completed, the liirds then gain steadil\. build- 

 ing up reserves against the advent of cold weatlict. until ;i peak is reached at the beginning 

 of the following winter. 



With the coming of snow and low tem|jeratures greater energy is required to move about 

 and to combat the loss of heat from the body. Then, too, the birds are forced oflF the ground 

 to feed in the trees. It is indeed fortunate that they arc then capable of existing upon browse, 

 for an enforced migration in search of food might well prove disastrous. 



Yet there is no indication that the diet is wholly inadequate, for in no case was the weight 

 recorded in a healthy bird dangerousK low. nor has the Investigation ever found a grouse 

 in its natural habitat that died from star\atioii. Ihe normal weight-health relationship of 

 New York grouse, adults and birds of the \ear. is indicated in tigure II. p. 90. 



It is interesting to find that the weights of captive birds at the Research Center likewise 

 drop during the winter, despite the fad that the\ rccei\c an artificial ration calculated to best 

 suit their needs. Noteworthy, too. is the fact that in winter and spring the quantities of 

 food eaten by wild grouse is considerably greater than is taken in sunnner. 



Relation of Food to Cycles 



The possibility has been suggested that food, through cyclic qualitatixe changes in its nu- 

 tritive components, may have a bearing upon the periodic fluctuations in grouse populations. 

 It has been clearlv established that the composition of plants, on which the grouse must de- 

 pend for food, may vary somewhat with weather conditions. Criddle' has reported on fluctu- 

 ations in the abundance of grouse and other species of wildlife in Canada. DeLury"" has 

 connected these observed variations with a close correlation between sunspot cvdes and some 

 weather phenomena. 



All this is interesting but not even circumstantialh conclusive for. as indicated in Chapter 

 VI, weather may have a host of other direct as well as indirect effects on grouse abundance. 

 The weather changes most likelv to influence grouse food composition are precipitation, teni- 



* See AjipeiHlix. p. 719. 



