Vulnerability of Black Grouse (Tetrao tetrix) hens to Goshawk (Accipiter gentilis) predation in relation to vole cycles

Abstract

In southern Norway from 1989 to 1996, the annual proportion of hens among Black Grouse killed by breeding Goshawks was not correlated with that of those shot in the previous autumn. The proportion of hens among Black Grouse killed by hawks was usually higher than that among Black Grouse shot, but in three out of four peak vole years, all with an early thaw, there was no such difference. In a stepwise regression model, both vole number (high or low), snow depth in spring and the proportion of hens among Black Grouse shot contributed significantly to explain the proportion of hens among Black Grouse killed by hawks. This was because predation on Black Grouse hens, but not that on cocks, was positively related to snow depth and negatively related to vole number. Vole populations peaked in post-mast years of Bilberry (Vaccinium myrtillus), possibly because the content of chemical defence compounds in Bilberry plants was low after a year with high seed production. Since the Bilberry is an important food plant for both voles and grouse, one possible explanation for the lower number of Black Grouse hens killed by hawks in peak vole years is higher food quality. If the ground is not covered by snow, the higher quality of Bilberry plants may reduce the time needed for foraging during egg formation and incubation, improve the body condition of the hens, and thus reduce their vulnerability to predation.