302 INFLUENCE OF WEATHER 



and 12 weeks, they become able to cope with such conditions. 



With respect to the adults, also, losses resulting directly from exposure to the elements ap- 

 pear to be of little significance. Even the above mentioned flood had no recognizable effect 

 on this age class. It has been suggested many times that an important cause of winter mortality 

 is the imprisonment and subsequent starvation of snow-roosting birds*. The Investigation did 

 not encounter this condition and has been unable to find a single positive record of this kind. 

 This is understandable when one considers that experience during this study has indicated 

 grouse seldom indulge in this habit except when below freezing temperatures are accompanied 

 by light, fluffy snow. Yet a wet snow is required for crusting. Only when a bird remained in 

 such a roost long enough to allow the surface to become soggy and then freeze, might it be en- 

 dangered. 



On the other hand, there is some evidence that snow-roosting birds are somewhat more vul- 

 nerable to predation. 



The possibility that sleet storms result in starvation seems quite remote. The most severe 

 conditions of this kind experienced during the 12 years of the study began on March 17, 

 1936. Heavy ice-coatings remained on the trees for three days, but no evidence was found to 

 indicate that the grouse had been affected. 



WEATHER AS AN INDIRECT INFLUENCE 



The principal relationships of weather to fluctuations in grouse abundance are of an in- 

 direct nature. Physiological studies'^ have shown that elements, such as temperature and 

 precipitation, definitely affect a bird's vital processes. Often these conditions are adverse but 

 their effect is seldom severe enough to result in immediate death. Rather, by lowering the 

 bird's vitality, they tend to make it more vulnerable to other forces such as predators, disease 

 or the strain of moulting. 



Because of the complexity of these interacting influences, only data from the most inten- 

 sively worked study area, Coimeclicut Hill, have proven suitable for analysis. The weather 

 records used here have been largely those of the nearby Ithaca (N. Y.) Station. In the fol- 

 lowing discussion the correlations recognized are grouped according to llic three life periods 

 of a grouse. It must be emphasized ajiain that the degree to which thev represent cause and 

 effect has not been completely established. 



During the Nest Period 



It is well recognized that the progressive lengthening of daylight during the spring is directly 

 associated in many birds with the enlargement of the reproductixe organs and the onset of the 

 breeding season. With grouse, this relationship has been demonstrated by subjecting experi- 

 mental birds during the winter at the Research Center to increased duration of light each day 

 by means of artificial illuniination". These birds produced fertile eggs many weeks before 

 those living under natural conditions. There seems little doubt lliat this is the primary influ- 

 ence governing the nesting jieriod of grouse. 



With only slight differences due to cloudiness, however, the progressive increase in daylight 

 hours is the same one spring as another. If light were the sole influence this would .seem to 

 indicate a constant date of nesting each spring, 'let in ;ii lualit\. the average date on the Con- 



* Sec Chaptfr \. ['. 1*70. 

 A See Chapter U, p. 60. 



