Dietary flexibility is key to adjusting to environmental change. In Mauritania, the seemingly obligatory molluscivorous red knots Calidris canutus were observed to eat seagrass rhizomes. To study the ability of knots to live on plant material, we performed a diet-change experiment on captive individuals. Two groups of five were fed protein-rich pellets for 13 weeks, then plant-based pellets for 6 weeks, then reversed back to protein-rich pellets for 4.5 weeks. During the first days following the shift to the plant diet, body mass declined by 14% before increasing and stabilizing to lower levels. Faecal colour changed from green (i.e. gall, suggesting starvation) to brown and was produced in larger quantities when the birds ate plant pellets. These experimental data prove that knots can indeed live on a plant-based diet, with the observed changes suggesting that adjustments of the digestive system, i.e. gut morphology and microbiome, take only a few days.