CAUSES 577 



^^Tiile no severe scarcity was experienced during the Investigation, this, nevertheless, suggests 

 that the immediate causes of such shortages may be effective primarily with respect to high 

 breeding population densities. It also suggests that they will result in general scarcity mainly 

 when they occur in conjunction with a preponderantly high level of abundance among the 

 component local populations of an area. If this should prove to be the case it would help 

 to explain why abnormal weather, such as that of 1912 for example*, was not associated 

 with a pronounced low in grouse abundance"^. In other words it may be that density is a 

 necessary catalyst. 



Other observers have reported similar experiences. After stating that, although apparently 

 scarce early in the season, grouse in most localities turned up later, a Connecticut report™ 

 continued "However, in places where birds had been very numerous in the previous year, 

 they appeared to be very scarce in 1924." A news release of the Wisconsin Conservation 

 Department in October 1933 stated regarding mortality in certain northern counties in the 

 fall of that year "Up to the present time it has affected ruffed grouse principally, and on 

 areas where the birds were most plentiful." Commenting on a general slump of more than 

 50 per cent throt|ghout most counties of both peninsulas of Michigan in the fall of 1934 as 

 compared with 1933 Ruhl* stated "In other counties .... characterized by scattered coverts 

 which are heavily hunted, grouse were almost unanimously reported to be more abundant 

 than in the autumn of 1933." Similarh. in speaking of the decline in Minnesota in 1942 and 

 1943 Blair' re]5orted that it "a|)parentlv was not as effective in the areas where birds were 

 most hunted as it was in the places off tlie beaten trails where less hunting was done." It 

 may be, therefore, that reduction of high population densities through hunting for sport, 

 regulated in accordance with actual ])roductivitv from year to year, can serve to lessen the 

 severity of recurrent scarcity. 



In this connection it is possible that the lack of niaikcil fluctualioM in numbers reported 

 by Leopold""' for the remaining grouse habitats of the Middle West during recent years, and 

 which he suggested might indicate that the species did not originallv exhibit such behavior 

 in that part of its range, is not an inherent characteristic but rather a result of the fact that 

 the birds no longer attain high densities in that region. 



It is evident from the foregoing that, while the record concerning major fluctuations in 

 grouse abundance suggests a considerable degree of regularity in their recurrence, there 

 is still a great deal to be explained regarding the mechanism involved. If anything is certain 

 it is that the interaction of several factors is usually responsible. Nevertheless, the data pre- 

 sented seem to narrow the problem to some extent. They indicate that the key lies among 

 the forces which cause failure of the annual increment of young, but that a preponderantly 

 high level of abundance among the component unit populations involved may be a neces- 

 sary prerequisite to the synchronization of the effect of these forces. They also indicate that 

 the rate and degree of decline are closely related to the relative size of the increment com- 

 pared with the breeding population and to the average life span of the species. Further they 

 suggest some of the factors which may implement these relationships and point to weather 

 as of primary importance among them. The remainder of the solution must await further 

 research. 



* See figure 23. p. 305. 



A Assuming ihat the temperature faotor eiled also proves signiBrant. 



t Ruhl, H. D,, jiersonal letter la the authors, January 2J. 1935, 



t Blair. F. D„ personal letter to the authors, February 28, 1945. 



