Forester T. Grasaas' data on numbers of capercaillie Tetrao urogallus cocks and hens observed at leks and clutch sizes in Vegårshei, southern Norway, during 1953–1962 (high population level) and 1969–1978 (low population level) were analysed with regard to bilberry Vaccinium myrtillus production, autumn population indices and snow conditions in spring. From the mast depression hypothesis, it was predicted that the number of capercaillies counted at leks and the mean clutch size should be high after high seed crops (masts) of bilberry, usually produced at intervals of 3–5 years. In stepwise regression models, both the bilberry index of the preceding year and the autumn population index one (hens) or two (cocks) years earlier contributed to explain the mean number of capercaillies counted at leks during 1970–1978. Capercaillie clutch sizes were highest in years with early thaw, but the effect was significant only for the period 1969–1978. For this period, there also was a positive effect of the bilberry index and a negative effect of the autumn population index of the previous year. It is concluded that the synchronous population fluctuation of grouse and voles in Norway and Sweden cannot be explained by the alternative prey hypothesis alone, and that food quality should also be considered as a possible contributing factor when analysing population fluctuations of grouse and other herbivorous species.