Selection of Heather Age and Chemical Composition by Red Grouse in Relation to Physiological State, Season and Time of Day

Abstract

When given a choice of heather of the same age but different chemical composition, captive Red Grouse in poor body condition preferred to eat plants containing more nitrogen and phosphorus, but not calcium, whereas birds in good condition showed no such preference. When given a choice of different ages of heather at the same height at different times of year, grouse in poor condition ate more of the young, more nutritious, heather than did those in good condition, at all seasons; all birds preferred young heather in spring. This confirms the importance of physiological state of the bird, and chemical composition of the heather, in determining choice. Selection for N and P (not Ca) from heather by wild Red Grouse were correlated negatively with mean weights of heather particles eaten and with total weights of crop contents (which were assumed to reflect feeding rates). Grouse thus tended to be more selective for N and P when they ate smaller particles and fed slowly. In autumn and winter, wild grouse selected more for N and P, and less for Ca, in the morning and afternoon than in the evening, but there was no difference with time of day in spring. In spring, selection for N and P were lower in the morning and afternoon, and greater in the evening, than in autumn and winter. Seasonal and diurnal variation in selection for nutrients was associated with variation in feeding rates (as indicated by weights of crop contents) and in weights of heather particles eaten.