Red Knots (Calidris canutus rufa) are long-distance migratory shorebirds that fly from the Arctic to the southern tip of South America. They stop over in a few places with high concentrations of trophic resources that act like population bottlenecks. At a population level, unavailable resources during a stopover can have catastrophic consequences. Because of their importance, these sites must be managed and conserved. On their northward migration, Red Knots stop over on Península Valdés (Patagonian Argentina) between March and May, during which we surveyed them every year between 2006 and 2013. This peninsula is surrounded by 2 gulfs where the Blancas, Fracasso, Conos, and Sarmiento Beaches were monitored (in the San José Gulf), and the Colombo Beach (in the Nuevo Gulf). We recorded encounter–reencounter data of individually tagged Red Knots through telescopes. To evaluate Red Knot site fidelity, we used the Standardized Site Fidelity Index, which was calculated for individual beaches and grouping the beaches by gulf. In order of importance, Red Knots were more faithful to Blancas, Fracasso, and Colombo Beaches. When we grouped beaches, site fidelity was greater in the San José Gulf, which can be linked to reports of its greater food supply. The use of this index is a useful and powerful tool to make decisions or propose management strategies.