Nutrition and reproduction of the Willow Grouse Lagopus lagopus in Central Sweden

In a 6 yr study of Willow Grouse diet, Vaccinium species dominated crop contents in autumn, together with Eriophorum species in spring. During winter, a diet dominated by shoots and catkins from mountain birch Betula pubescens var. tortuosa was eaten. In vitro digestibility, protein and ash levels of crop contents were 24-34% lower, while fibre content was 20% higher in winter than in the other seasons. Marked annual variation in the yield of mountain birch shoots and catkins were registered. The yield of shoots showed a gradual 4-yr increase followed by a marked decline in the following year. The percentage of different birch parts in late winter crop contents, and their chemical composition, varied significantly between years. Annual variation in winter diet composition as well as the fat level of grouse were to a large extent related to food abundance. Multiple regression equations showed that 92-96% of the observed variability in grouse fat levels was accounted for by the yield of mountain birch shoots and catkins. In spring, large annual variation in species composition of the food was related to variation in the in vitro digestibility. This food quality parameter was positively correlated with the size of the fat reserves of breeding females, which in turn was related to breeding success, indicating that maternal nutrition affects Willow Grouse reproduction. However, the two worst grouse breeding years were associated with declines in the populations of small mammals, so that predator "switching" may have contributed to the low grouse reproductive success. On the other hand, nutrition alone may have been responsible for the synchrony.