CAUSES OF ANIMAL CYCLES 183 



winters (Johanscn, 1929). Blegvad (1929) cites death in cold winters 

 as one of the principal causes of the decline of fish food in Danish 

 waters, and he also frequently found quantities of partially grown and 

 adult invertebrates dead following cold winters. He has shown that 

 this is not due to enemies and is much more important than the de- 

 struction wrought by them. 



Many adult insects are killed by unfavorable winter weather. The 

 survival of the codling moth is poor in cold dry winters. IMost insects 

 unfavorably located in their hibernation quarters succumb to extreme 

 cold lasting over long periods. The box elder bug, Harlequin cabbage 

 bug, brown-tail moth, and many other insects that make their way 

 northward during favorable years are frequently "frozen back" sev- 

 eral hundred miles southward by extremely low temperatures. 



Birds are often, though at irregular intervals, killed by storms 

 while in migratory flight; some are driven to sea or into large lakes. 

 The beaches of some of the Great Lakes haA'e at times been strewn 

 with birds caught in storms (Roberts, 1907; Saunders, 1907; Ken- 

 deigh, 1934). Even the English sparrow succumbs to severe storm 

 (Bumpus, 1896) . The destruction of adult mammals by severe weather 

 is reported for the bison. A large number of bison skeletons are to be 

 found in localities where severe blizzards have occurred, and the last 

 herd of wild bison was practically exterminated by blizzards in 1886 

 (Seton, 1929, 2:677). 



Enemies and Disease. Destruction by enemies is commonly cited 

 as a cause of the decline of game. Examples of decline in the abun- 

 dance of game related to attacks by their predatory enemies, resting 

 upon definite scientific evidence, are still rare. Brooks (1926) has 

 inferred that the general absence of deer in western Canada was due 

 to the work of the puma, which was present in numbers about two 

 years before, when deer had been unusually abundant. The decline of 

 the European lemming is probably correctly ascribed to predators, 

 since hawks and owls are known to congregate at points where lem- 

 mings are abundant. Moreover, the investigations of Blegvad and 

 Petersen have shown that the food taken by the fishes is a real factor 

 in changing the abundance of various species in the Danish seas. 



Disease is commonly cited as the cause of the decline of the num- 

 ber of animals. It may be defined as any abnormality of the form or 

 function of the body or any of its parts, or as any state in which the 

 organism is not in the best condition. It is accordingly necessary to 

 restrict the use of "disease" as a cause of population decline to malig- 

 nant infectious diseases, which commonly occur in epidemic form. 

 The disease question was long ago worked through to its conclusion 



