Recent changes in climate have advanced the timing of long-distance migration in birds, although there are still no studies showing a change from migration to residence in previously migratory populations. Barn swallows Hirundo rustica are long-distance migrant songbirds with an almost global breeding distribution and with resident populations in Egypt and possibly in south China. In Europe, they have been exclusively migratory with wintering populations in Africa where they also undergo an annual moult. In December 2015, barn swallows started wintering in large numbers in Portugal resulting in these individuals also moulting there. Here, we tested the origin of these novel resident European birds by using measurements of feather deuterium (δ2Hf) of juveniles that had grown their feathers at the breeding grounds in Europe and in adults that had grown their feathers in the winter quarters to test for a difference in moult distribution of these two individual categories of barn swallows. Second, we tested the prediction that barn swallows have changed their migratory habits from wintering in Africa to wintering in Portugal as revealed by differences in δ2Hf compared to known Palearctic-Afrotropical migrants. Third, we tested whether moult phenology had changed following recent residency in Portugal. Our results confirm the existence of an entirely resident population of barn swallows in Europe with a shorter and earlier moult period.