Productivity and Populations 309 



the losses of adults, but this figure will vary widely in different 

 localities and years. 



I have already indicated that hunting by man in normally well- 

 hunted areas results in losses approximating fifteen per cent of the 

 birds. This is the equivalent of about thirty per cent of the yearly 

 mortality. It is self-evident that this figure will vary widely in dif- 

 ferent times and places. Adding up the losses from predation, hunt- 

 ing, disease, and physical accidents, we practically account for the 

 whole mortality. An occasional bird may be killed by fire; very rarely 

 it is possible that one succumbs from starvation or over-exposure to 

 bad weather. These factors are negUgible, however, as direct deci- 

 mating agencies. 



Limiting Effects of Environment. There are many environmental 

 influences that condition the decimating agencies in one way or an- 

 other. We have just noted that starvation is of no importance as a 

 killer of adult grouse. But when a grouse wanders out into an apple 

 orchard to get at a desired food, it may thereby expose itself to an 

 easy attack by some predator, or a hunter. Thus the food problem is 

 interrelated with predation and hunting losses. The pursuit of food 

 may likewise be a factor in bringing about disease. Concentration 

 of a food source may mean concenti^ation of grouse, and thereby fa- 

 cilitate the spread of disease organisms. Some items of food carry 

 disease organisms, as for example the sow bug that is the alternate 

 liost for the parasite, Dispharynx spiralis. If the parasitic infection 

 weakens the bird so that it is killed the more easily by a predator, 

 then the food relation, the sow bug, is two steps removed from the 

 actual cause of death. 



Climatic factors are of immense importance in conditioning the 

 decimating agencies. When conditions of snow, temperature, and 

 wind cause grouse to roost in the snow, the birds become more vul- 

 nerable to predator attack. Likewise the quality and distribution of 

 food and shelter play a part in the complex of weather-cover-preda- 

 tion. Then man further complicates these relationships by affecting 

 the area in various ways— by cutting, burning, cultivating, draining, 

 or by adding still another in the chain of factors, livestock, that graze 

 and browse the vegetation. Insects and the other lower forms of 

 animals play many parts in the interrelationships of the grouse en- 

 vironment. Physical factors, as slope and light, add to these complexi- 



