Food intake, digestibility and gut size in red grouse

Abstract

Abstract 1. Fully‐grown captive red grouse were fed 4 increasingly fibrous diets which incorporated ling heather (Calluna vulgaris), their main natural food, at rates of 50, 200, 400 and 600 g/kg. The rest of each diet was a grouse maintenance ration. 2. Gizzard weights showed a close linear relationship with voluntary food intake. Previously published data for wild grouse fell close to the regression line. 3. Birds eating the 600 g heather/kg diet digested it more efficiently than expected by extrapolation from the three lower‐fibre diets. Their voluntary intakes of food varied more and they had longer caeca than birds on the lower‐fibre diets. 4. It was concluded that grouse began to show increased gut lengths and increased abilities to digest fibrous food when the intrinsic digestibility of their diet dropped below a certain, predictable, point. This may have been when bulk began to limit intake.