Reversible phenotypic changes, such as those observed in nutritional organs of long-distance migrants, increasingly receive the attention of ornithologists. In this paper we review the cost-benefit studies that have been performed on the flexible gizzard of Red Knots Calidris canutus. By varying the hardness of the diet on offer gizzard mass could experimentally be manipulated, which allowed quantification of the energetic costs and benefits as a function of gizzard size. These functions were used to construct an optimality model of gizzard mass for Red Knots on migration and during winter. Two possible currencies were assumed, one in which Knots aim to balance their energy budget on a daily basis (satisficers), and one in which Knots aim to maximise their daily energy budget (net rate maximisers). The model accurately predicted variation in gizzard mass that we observed (1) between years, (2) within years, and (3) between sites. Knots maintained satisficing gizzards during winter and rate-maximising gizzards when fuelling for migration. The model-exercise revealed the importance of digestive constraints and quality of prey in the life of Knots.