574 FLlCriATlONS IN GROUSE ABUNDANCE 



as noted in Chapter I, the great majority being of an essentially local nature. A few theories 

 have been advanced, however, to explain more extensive fluctuations. 



I'redation 



In one of the first hvpothcses suggested. Burnhani" related unusual invasions of goshawks 

 ( Astur atricapillus) during the winters of 1906-07 and 1916-17 to disappearances of the vary- 

 ing hare and reasoned that, with their staple food gone, goshawks and other predators preyed 

 more heavily on grouse and were responsible for their periodic depletion. Subsequent records, 

 however, have shown that this correlation has not been corroborated. For example Clarke' 

 reported "A goshawk migration is recognized to have occurred in Ontario in the winter 

 of 1934-35. Reference to the map of the areas of dying-ofi for 1934 shows that, for the 

 most part, dying-off has preceded this migration." Similarly, Leopold"' although speaking 

 of Wisconsin, summed up the situation general!) when he wrote "it would appear . . . that 

 invasions are much too local and infrequent to account for the periodic Statewide decima- 

 tion in ruffed grouse, although goshawks doubtless accentuate the shortages where and when 

 they occur." Furthermore such flights take place in winter whereas the data indicate that 

 recurrent periods of general grouse scarcity have resulted mairdy from breeding and rearing 

 losses. Therefore, incursions of migratory predators nia) be eliminated as a primary cause 

 of grouse fluctuations of this kind although the\ undoubtedly are contributory on occasion. 

 .Neither is there evidence that changes in the abundance or food supply of resident predators 

 have been responsible. 



Disease 



Another possible cause is disease. The idea of some scourge periodically sweeping through 

 the ranks of the grouse has been an easily visualized explanation. For many years the com- 

 mon belief was that one disease, the so-called "Grouse Disease," was responsible for the peri- 

 odic diminution of grouse abundance. So far, however, no such ailment has been found to 

 fit the circumstances. 



Although some condition of this kind had been mentioned by a number of writers and 

 Woodruff had included it among the three |)rincipal reasons he listed for the scarcity of 1907, 

 its relationship to grouse abundance was first really studied by Allen' and Gross'' wht)sc in- 

 vestigations were given imjx'lus by the scarcity of 1924-27. Other studies of llii> kind lia\e 

 been carried on by Green and Shillinger* and Clarke"' as well as by this Investigation. 

 Grouse have been found to be subject to a great variety of diseases and parasites. Outstand- 

 ing among them as decimating agents of wild grouse are the stomach worm ( Dispharynx), 

 tularemia and a blood ])arasite ( Leucocytozooii ). The last of these in |)articular was found 

 to be associated primarily with mortality among chicks. Yet none of these maladies has been 

 found widely distributed throughout grouse range, while those parasites which have a wide 

 (lislriliulion appear largely to be innocuous'^. On the other hand, the fact that there is still 

 so little known of the paramount causes of brood mortalily leaves the possibility that some 

 important but as yet unrecognized disease exists. 



But a.'-suniing that sonic coiulilion of ibis kind ucic loiind. a m((lKuii>Mi capable of svn- 

 ilnonizing its irruption o\er large areas and al the same time allowing for e\len>i\e local 

 \ai'ialioiis would remain to be discoM-red. Clarke' has suggested what might lake place if 

 Leucocytozooii were llw principal cause involved. i?ut the presence of Leucocytozoon has 



* See varioui paper* by Green and ShiUinter. 

 A See Chapter X, p. 4-12. 



