ROLE OF PREDATION 



321 



Apparently the relationship between predators and adult grouse is largely an incidental 

 one. No species depends on this game bird for any large proportion of its food. But the 

 greater the number of birds in a covert the more frequently they are encountered and killed. 



It is significant that the basic effects of predation should have been so similar on the two 

 study areas in spite of substantial differences in the composition of the predator population. 

 For example, the great horned owl was not observed on the Adirondack area while the 

 goshawk was a regular visitant. Specific data from the latter area are insufficient to draw 

 conclusions as to the comparative relationships of the various species. There is some indi- 

 cation, however, that the fox preys to a greater extent on adult grouse on this area than on 

 Connecticut Hill. In any case the principle of compensatory predation as discussed by 

 Errington"' seems to be corroborated. 



At the same time the comparative level of the grouse population is not the only factor 

 affecting the degree of adult mortality. In figure 28 the data have been statistically adjusted 

 by analysis of covariance to eliminate the effect of the relative abundance of the birds. 



lOOa ■ lOO 



> O 



iij cc loo 

 Q lij 2^00 

 a. 

 300 



4.00 





40 



30 j 

 20 I 

 10 



o 



Carouse R.emciins in Owl Pellets 



4-0 

 30 

 2,0 

 10 

 O 



1930-31 3\-32 3233 3334- 3435 SbiC 3t37 37-3& 36-33 3940 40-4I 41-42 



YEAR 



FIGURE 28. RELATIONSHIP OF INCIDENCE OF GROUSE REMAINS IN GREAT HORNED OWL PELLETS 

 TO GROUSE ADULT MORTALITY AND TO BUFFER ABUNDANCE — CONNECTICUT HILL AREA — 



1930-31 TO 1941-42 



Other influences, such as variations in weather and predator pressure have been chiefly 

 responsible for the differences remaining. But just what they are and how they exert them- 

 selves is still largely obscure. Attention may be called, however, to certain interesting cor- 

 relations. 



