CAUSES 573 



More recently Gordon* stated with reference to poor hunting in 1944 in Pennsylvania, 

 "It is believed that the two bad hatching seasons in succession, 1943 and 1944. are largely 

 responsible". In New Brunswick a good stock of birds at the end of the 1942 season and 

 a good hatch in 1943 were reported but in the fall of the latter year grouse were scarce and 

 few birds of the year were found"". 



That failure of the crop of young in a season can have a pronounced effect on grouse 

 abundance is evident when one considers the proportion of immature birds in an average fall 

 population. As has been shown^ the Investigation's data indicate that the increment of 

 young surviving by September does not greatly exceed the number of breeders even in a 

 year of good productivity. Therefore, if this increment should not materialize, the abundance 

 of grouse in the fall would be only about half what it would have been had productivity 

 been average. Such a loss would also be felt the following year as a result of reduced breed- 

 ing stock. 



If, however, a similar failure of the increment should occur for two years in succession 

 grouse abundance would drop to a very low level. Average adult mortality coupled with little 

 or no replenishment of young would result in the population the first fall being roughly half 

 that of the preceding year and in a further reduction a])proaching the same proportions by 

 the second fall. But about this time another factor would enter in, namely, the average life 

 span of the species in the wild. As has been discussed elsewhere^ it seems likely that few 

 grouse live to he four years old. Following a season of average productivity only about one- 

 quarter of the grouse present in the hunting season are more than two years old. Yet, in ad- 

 dition to greatly reducing overall abundance, failure of the increment for two successive 

 years would also change the age composition of the remaining pojjulation to one in which 

 nearly all the birds were more than two years old and close to half more than three years 

 old. It seems probable that losses among these older birds would be somewhat accelerated 

 and thus still further deplete the ranks. 



Although the evidence points strongly to loss of productivity in terms of birds of the year 

 as the primary manifestation of the forces responsible for the major declines in grouse abun- 

 dance that have been recorded, adult mortality (apart from that associated with longevity) 

 seems to have been a contributory factor in a number of instances. But the data do not 

 support the conclusion that it may have been controlling. As was demonstrated on the Con- 

 necticut Hill study area in 1936-37 above average overwinter losses can result in decreased 

 abundance the following fall. Should such losses occur in conjunction with the season dur- 

 ing which the ensuing increment failed also, they would act to increase the severity of the 

 resultant scarcity. This seems to have been what happened in 1906-07, for example, when a 

 record flight of goshawks invaded the north-central and northeastern states. 



But the question remains as to what have been the principal causes of crop failures, espe- 

 cially brood mortality, and what has effected a sufficient degree of synchronism among them 

 to result in periods of general grouse scarcity over wide areas recurring with the degree 

 of regularity that has been reported. Neither of these problems has been solved. Never- 

 theless certain facts may be cited. 



Contributory Influences 

 Over the years, periods of grouse scarcity have been attributed to a wide variety of causes 



* Gordon, S., personal letter to the authors. February 23, 1945, 

 A See Chapter XII, p. 542. 

 t See Chapter VIII. p. 360. 



