304 INFLUENCE OF WEATHER 



heat production to ofFset the greater heat loss and less remains for maintenance of body weight 

 and other functions. Loss of heat from the body of a bird is also controlled to some extent by 

 its coat of feathers. Thus, with immature grouse, the early brood period, when the chicks are 

 less well protected by feathers, would seem to be the most critical. 



Similarly, precipitation undoubtedly exerts its influence, in the main, through accelerating 

 loss of body heat. The insulating value of a substance is related to its moisture content. Thus 

 a bird's feathers have much less abilit\ to prevent heat loss when they are wet than when they 

 are dry. 



On the other hand, losses which seem to be associated with such conditions, often take place 

 several weeks later. Perhaps cold June weather, especially when accompanied by frequent 

 rainfall, produces a group of chicks which tends to lack the stamina to survive some critical 

 period during the later part of the summer. The moulting of immature feathers and growth 

 of adult plumage, which takes place at this time, represent a severe physiological drain on the 

 bird. It may be that herein lies the answer. 



During the Adult Period 



As with broods, it is not intended that the relationships suggested for adults be considered 

 as conclusions with respect to cause and effect. Nevertheless, the degree of correlation is suffi- 

 ciently marked to strongly indicate a connection*. Temperature appears to be the primary in- 

 fluence. Interestingly enough, it was not the temperature during the coldest months of the 

 year, January and February, but that occurring in March which was most closely associated 

 with differences in overwinter mortality among grouse. Low average temperature levels during 

 this month were regularly followed by higher total losses, while the reverse was true when 

 mild weather occurred at this time. 



Again, it is probably the rate of body heat loss in relation to the energy required to com- 

 pensate for it which is principally responsible. Throughout mid-winter, it is apparently 

 normal for the energy expended in this way to often exceed that derived from the food eaten. 

 Under such circumstances, a bird is forced to draw on its bodily reserve and it loses weight. 

 This is reflected in the average seasonal weight trend'^. But. while birds which enter the winter 

 in good condition can withstand its hardships and still cope with moderate weather during 

 March, a continuation of low temperatures at this time apparently is "the straw that breaks 

 the camel's back". 



The influence of precipitation on adult grouse seems of minor importance. Even the severe 

 cloudbursts of 193.S and 19.37 had no recognizable effect on survival. The bird's feathers 

 apparently form an effective "raincoat". It is possible, however, that snow, under certain 

 conditions, may even aid grouse in maintaining their body temperature. It is well known that 

 dry, fluffv snow possesses a high insulative value. Thus a bird snow-roosting* under condi- 

 tions of tlii> kind will undoubtedly use up less energy in combating heat loss than luu- with- 

 out such protection. This mav explain why grouse are apparentlv capable of remaining in 

 roosts of this sort for considerable |)eriods without food. 



To Mini up this discussion of its indirect influence, weather may be considered primarily as 

 a factor conditioning the susceptibility of grouse to more direct decimating agents through 

 sapping their vitality. Actually, death is usually due to |)rc(Iaticin or disease. 



* Statistical nnAJvait ha« tliuwD ihem to be highly ■ignihcanl. 

 A Sn Chapter U, p. 91. 

 t Sec Chapter \ . p. 270. 



