Three experiments with captive ruffed grouse (Bonasa umbellus) demonstrated that dietary metabolizable energy (ME) can be predicted from neutral detergent solubles, total phenols, and percent acorns of grouse diets. The weight of the fat attached to the gizzard was highly correlated with percent carcass fat in 82 grouse and was judged a useful index of body condition. Crop contents of 1005 grouse collected during fall and winter 1981-84 in Maine, New York, Wisconsin, Washington, Virginia, West Virginia, Indiana, Ohio, North Carolina, and Georgia were used to make regional comparisons of food habits and diet quality. The ME of crop contents was predicted from chemical composition. Evergreen leaves of woody plants were the most common late winter forages of grouse in southeastern states, whereas buds, twigs, and catkins were the most common late fall and winter forages in diets of grouse in northern states. Winter diets in the Southeast tended to have higher levels of predicted ME than diets in the North; however, southeastern diets tended to have higher levels of total phenols and lower levels of protein than typical northern diets. Evergreen leaf forages had higher levels of tannin phenols than buds, twigs, and catkins. Dietary ME appeared adequate in both the North and the Southeast, but low levels of protein and high levels of tannins may result in poorer quality winter diets along the southeastern edge of the range of the ruffed grouse. Acorns comprised 63% of the crop contents of 22 grouse collected in Virginia in March and April 1982, the spring following a year of high acorn production. Leaves and flowers of herbaceous forbs were the primary forages of 41 grouse collected in spring 1983 and 1984. Body fat levels were greater for females than males and declined from March to April. Fat declines appeared to be related to breeding activities. Evergreen leaves were the most abundant forages available to grouse in late winter on a study site in southwestern Virginia. Biomass of high quality herbaceous leaves-was insufficient to meet estimated energy requirements of grouse in late winter, indicating a need for a dietary shift to low quality evergreen leaves. / Ph. D.