Choices facing foraging red grouse

Abstract

The feeding ecology of grouse was studied for 3 years on a moor in N.E. Scotland where the heather had been burned regularly in small patches to maintain high grouse stocks. The birds ate mainly heather and thus had a range of ages and heights of heather on which to feed. They ate steadily throughout the day, but more intensively in the evening, when amounts stored in their crops depended on the length of night. Judging from the distribution of their droppings, they spent the most time on heather patches aged 2-5 years (10-25 cm) in spring, 4-7 years (20-32 cm) in summer, 3-6 years (15-28 cm) in autumn, 4-8 years (20-37 cm) in winter, and more time near the edges of patches on some age classes. They could not be watched in summer, when they were secretive, but counts of their pecking rates at other times showed that they ate mainly 2-5-yearold heather in spring, 3-5-year-old in autumn and 3-8-year°old in winter. These preferences were related to the heather's height (a standing grouse is 30 cm high, a squatting one 15 cm), percent ground cover and food value (young heather being more nutritious than old). Judging from analyses of crop contents, they ate particles 5-15 mm long, 74% (mean) of which were shoot tips and the rest bitten at both ends, and selected for (i.e., more in crop heather than in samples picked at feeding sites) nitrogen and phosphorus (the two nutrients in shortest supply relative to their requirements), but not for energy or other nutrients. Selection for N and P was correlated negatively with both feeding rate (as indicated by the weights of crop contents) and mean weight of particles eaten. They preferred to feed on 3-year-old heather in the evening and ate more 2and 3-year-old heather in spring than at other times. It is concluded that all seasonal and diurnal variation in the utilisation of heather by grouse can be accounted for by variation in their need for: (a) feeding at a certain rate; (b) selecting particular