Assuming that animals respond optimally to environmental changes, both behavior and physiology should be useful indicators of the way that animals perceive the quality of their environment. For verification, we examined foraging time and gizzard size of the red knot (Calidris canutus), a long-distance migrant shorebird that ingests hard-shelled mollusk prey whole and therefore readily faces digestive constraints. Nevertheless, even when digestively bottlenecked, knots can still enhance their daily energy intake by flexibly increasing the size of their digestive system, notably the gizzard, and/or by feeding longer per day. Whether such adjustments are actually necessary depends on the quality (condition) of their food, i.e., the ratio of flesh to shell mass. Hence, gizzard mass and daily foraging time in knots may be reliable indicators of the quality of their invertebrate prey. This idea is explored by using field data on radio-marked knots during late summer in the Dutch Wadden Sea. Their gizzards were measured using ultrasonography and the knots' movements and working hours were monitored using handheld receivers and automated radio-tracking systems. Indeed, gizzard mass alone predicted annual variation in food quality satisfactorily; however, adding estimates for daily foraging time significantly improved the predictions. We discuss and conceptualize how knots may trade off digestive organ size against foraging time upon arrival in the Wadden Sea, given the quality and quantity of the food on offer, and why these findings are relevant in the context of conservation of migration systems.