164 CONSERVATION OF CANADIAN WILD LIFE 
under the snow, under which they are accustomed to shel- 
ter, but which proves to be a death-trap when alternate 
thawing and freezing causes the formation of an ice-crust. 
The most important and at the same time the most in- 
teresting cause of the scarcity of grouse during recent 
years has been the migration southward of certain large 
species of northern predatory birds such as the goshawk 
and great horned owl. This southern migration of these 
predatory birds was evidently due to the scarcity through- 
out the northern regions of the rabbit or varying hare, 
which experiences, as is well known, periodic cycles of abun- 
dance and scarcity, as I have described in another chapter 
(see p. 216). This relation between the scarcity of rabbits 
and the decrease in the numbers of grouse was indepen- 
dently studied by Mr. P. A. Taverner and myself in Canada, 
and Mr. John B. Burnham, president of the American Game 
Protective Association, who has published an interesting 
report on the subject.* Owing to the scarcity of rabbits, 
which normally constitute the chief food of such predatory 
birds as the goshawk and great horned owls, these birds mi- 
grated southward in large numbers and sought other prey. 
This migration brought these enemies of our game-birds 
in abnormal numbers into the natural haunts of the various 
grouse, and these birds suffered from the consequent ex- 
cessive slaughter. In cases of this kind the natural balance: 
usually becomes restored in a few seasons, and, if close sea- 
sons are wisely fixed to counteract the baneful influences of 
natural factors over which we have no control, it is possible 
to prevent excessive reduction in numbers provided steps 
are taken at the same time to prevent overshooting. These 
species of grouse afford such excellent sport and recreative 
exercise that the greatest care should be taken in the fixing 
of open seasons, bag limits, and in regulating the methods 
* Bulletin of the American Game Protective Association, vol. VII, no. 1, 
pp. 4-8, January, 1918, 
