The spring and summer diet was determined from 811 blue grouse crops collected from 1957 through 1966 near the town of Campbell River on Vancouver Island. Main foods eaten were conifer needles, broad-leaved vegetation, flowers, fruits, and invertebrates. More than 80% of the plant diet was from nine species. Most foods were selected and selection varied with season, sex, and age. In the spring, females apparently ate more broad-leaved plants than did males, and chicks up to approximately 3 weeks of age fed largely on invertebrates, mainly ants (Formicidae). We attempt to explain selection by relating known nutrients in plants and animals to their use as food. Selection may be partly explained by availability, nutrients, and the nutritional requirements of grouse. Nutrients apparently selected most frequently were ash and nitrogen. The diet of females in May and June seemed richer than that of males in ash, nitrogen, and magnesium. In June, chicks had a diet likely higher in nitrogen, phosphorus, and potassium but lower in carbohydrate, fat, and calcium when compared with adults. The diet of blue grouse compared with that recommended for some other grouse was possibly deficient in ash, phosphorus, and calcium. We postulate food on the summer range might limit populations of blue grouse by the amount of nitrogen and minerals, especially calcium, available to hens in spring and by a range of nutrients available to very young chicks.
