Seasonal mortality of black grouse Tetra tetrix during a year with little snow

The seasonal mortality of black grouse Tetrao tetrix was studied in southeastern Norway (60 degrees 26'N, 10 degrees 54'E), in a study area dominated by up to 80% Norway spruce Picea abies. Modern forestry with clear-cuttings has been practised, and 50% of the forest was younger than 30 years. Fourteen males and seven females were captured on a lek in spring 1991 and equipped with radio transmitters. During the following 12 months, 72% of the birds were killed, mostly by predators. No birds died from capture through July. In autumn, predation was higher, with four black grouse being killed. Mortality was highest during winter, with 58% of the birds dying. The goshawk Accipiter gentilis was the most important predator during winter. High predation by goshawks in winter 1991-92 may have been the result of black grouse being more vulnerable to predation because limited snowfall precluded snow roosting, or of an invasion by goshawks. Therefore, snow roosting may be an important antipredator behaviour in black grouse. The effect of increased adult mortality is discussed in relation to chick production.