Productivity and Populations 315 



has been that grouse follow the rodents by about two years in their 

 own die-ojff. This did not occur on this area in anything like a cyclic 

 decline although the grouse population did show a marked decline 

 in 1937. 



The cause and effect synchronism in one group of animals is set 

 up by the loss of lemmings, white hares, and ptarmigan in the far 

 north. This dislocates the food supply of such predators as the snowy 

 owl, lynx, Arctic fox, and goshawk. A shortage in their food supply 

 causes them to migrate southward in search of better hunting, or 

 die where they are. Those that migrate come into the grouse range 

 and exert abnormal pressure on that species, which results in their 

 marked decline. There is no doubt that this chain of events occurs 

 to some degree. Southward winter incursions of snowy owls and 

 goshawks have often been recorded. A few of each species were 

 seen on Connecticut Hill in some years. And there is no doubt that 

 the goshawk is a very efficient grouse predator (see page 206). 

 Griddle ( 1930 ) suggests that a single goshawk may take as many as 

 fifty grouse in a winter. An "invasion" of these birds into a grouse 

 range already adequately populated with indigenous predators may 

 therefore have a significant effect on grouse numbers. Criddle rec- 

 ords that such an invasion occurred in Manitoba in the winter of 

 1907-08. This period was immediately following the drop in grouse 

 and could not have caused it, although, as Criddle says, "we . . . 

 need to know more of the influence of goshawks on a grouse popula- 

 tion already depleted by other agencies." 



The evidence seems clear that the recorded declines in grouse 

 are not consistently synchronous with die-off periods in other 

 species. This adds weight to the evidence against the grouse being 

 typically cyclic in this respect. 



4. Uniformity of the Die-off. 



When a cyclic decline occurs it strikes aU coverts within the 

 broad area of effect, possibly continent-wide for some species some 

 times. King (1937), referring to grouse says: "Cyclic declines are 

 not the result of overpopulation . . . when the crash comes it in- 

 cludes all areas, those on which there are peak populations and those 

 on which there are very sparse populations." There is considerable 

 evidence to the contrary. 



When grouse disappeared in Wisconsin in September 1933, it was 

 noted as "spotty" (Wisconsin Conservation Department, News Re- 



