Food Consumption of Red Grouse in Relation to the Age and Productivity of Heather

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Abstract
In 1969-1970, the consumption of heather [Calluna vulgaris] by red grouse [Lagopus agopus scoticus] at Spyhill moor in northeastern Scotland [United Kingdom] was measured by counting pecking rates and weighing the particles eaten. The annual heather intake of a grouse was at most 25 kg (dry wt), if it ate only heather, and at least 18 kg if it ate other plants. By applying this figure to the density of birds present and to an estimate of heather productivity, it was calculated that grouse ate only 1.4-2.5% of the total annual production of heather shoots and flowers. Grouse at Spyhill preferred to feed on heather aged 3-5 yr in autumn, 3-8 in winter and 2-5 in spring, and always avoided the youngest and oldest material. Preferences for particular age classes were related to the heather''s height, cover and food value (young heather being more nutritious than old). The preferences in autumn, when territories are established, help to explain the timing of the increase of grouse numbers following heather burning. The age classes most heavily grazed by grouse at Spyhill were 2,3 and 4 yr old, but less than 10% of their annual production was consumed. It is unlikely that territory size could be limited directly by the total amount of preferred heather available to birds in autumn.