This review looks at diet selection in grouse and waterfowl in the context of their herbivorous diets and discusses adaptations to herbivory in these groups. Grouse eat plant foods containing lower concentrations of protein than foods of waterfowl, especially during the breeding season. Several species of grouse and small waterfowl (ducks) eat invertebrates during periods of high protein requirement, such as growth or egg production. Grouse and waterfowl differ little from each other in the morphology of digestive structures. The principal exception is the ceca, which are 5 times as long in grouse as in comparably sized waterfowl. Enlarged ceca are associated with improved nitrogen economy in grouse; evolution of these structures in geese may have been precluded by energetic costs of carrying enlarged ceca during migration.
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