216 The Ruffed Grouse 



themselves, they tend to act as buffers in reducing predator pressure 

 on other prey. If, because of a scarcity of buffers, the predatory 

 population pursues the game birds more intensively, more game 

 birds will succumb. Thus there is a close theoretical relationship be- 

 tween the rodents and grouse. How does it work out actually? 



Determination of populations of rodent and predatory species on 

 extensive areas is a trick still to be mastered. However, methods for 

 determining population trends are available and, we believe, are 

 reasonably accurate. These include state game kill records; "sign" 

 recorded on field censuses, reduced to a number-per-man-day basis; 

 sample-track counts taken over marked trails of standard length un- 

 der as nearly uniform conditions as possible. All of these methods 

 have their shortcomings, but using all of them some trends appear. 



Several interesting trends come to light. Beginning with the early 

 1930's, all of these rodents increased to an irruptive peak in numbers 

 in 1935. Then from 1935 to 1936 they all suffered a catastrophic de- 

 cline. Three out of four rabbits died, four of five field mice, and nine 

 out of every ten red and gray squirrels. This fact is supported by 

 general census data, by the game kill reports ( see Fig. 9 ) for cotton- 

 tails and gray squirrels (although the general decline in cottontails 

 apparently began two years before), and by the well-defined and 

 much-publicized migration and die-off of gray squirrels in the fall 

 of 1935. 



The decline in cottontails was not so complete from 1935 to 1936 

 as that of the other animals, but it continued for two more years 

 while recovery of all the others began. This three-year period of cot- 

 tontail decline is substantiated by the state game kill records. 



The uniformity of the rise and fall of these species from 1930 to 

 1936 is notable. But very significant variations in the recovery rate 

 followed. As already pointed out, the cottontails continued to de- 

 cline for another two years; the mice began a rapid recovery the 

 next year, and the red squirrels a gradual one. The recovery in gray 

 squirrels began the second year, as the mice and red squirrels com- 

 pleted their recovery to normal abundance. Then from 1938 to 1939, 

 as the cottontails began their increase, the others again declined, 

 the squirrels only moderately and for a single year, the mice signifi- 

 cantly and for at least two years. The cottontails also showed some 

 loss in the second year of the latter period. 



From the standpoint of the effect of these fluctuations on the 



