Morphological methods to distinguish between avian groups of research interest (e.g. different sexes, populations or cryptic species) need to be externally validated to ensure reliable performance across situations. In this study, we used stable hydrogen isotope ratios of feathers (δ²H_f) to test the validity of morphological classification functions (MCFs) previously designed to assess the migratory behaviour of European Robins Erithacus rubecula wintering in southern Iberia. Our results show that a great number of migrants (mostly females and juveniles) were erroneously assigned as sedentary, which could compromise the reliability of previous ecological studies that made use of these MCFs. The development of improved MCFs or the use of alternative differentiation methods (δ²H_f) could help us to gain a more realistic insight into the habitat distribution and ecological interactions of sympatric migratory and sedentary robins overwintering in southern Iberia.