Plant growth and nitrogen metabolism of Red Grouse Lagopus lagopus scoticus in spring

Annual variations in the total nitrogen content of Red Grouse faeces in spring reflected variations in the growth and N content of their main food, ling heather Calluna vulgaris. The N metabolism of birds eating low-protein, high-polyphenol diets such as heather is poorly understood. This was described by analysing the N in faeces into six chemical fractions. Variation in these fractions was partitioned statistically into principal components on the working assumption that each component reflected a distinct metabolic process. Annual variations in the first principal component were related to the amount of overwinter dieback in the heather but not to the heather's total N content or growth. Variations in the second were closely related to the total N content and growth of heather but not to overwinter dieback. Protein appeared to be in short supply in the birds' diet. However, annual variations in the first principal component of faecal N occurred independently of the total N contents of food and faeces. Therefore the total N content of the birds' diet probably did not reflect its nutritive value and total faecal N was probably not a reliable index of dietary quality. No measured aspect of N metabolism was related to the birds' breeding success.