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Background Most of long-distance migratory raptors suspend moult during migration but detailed information is patchy for most of the Palearctic species. The aim of this research is to verify if active moulting in migrating Western Marsh Harriers occurs and to quantify the extent of moulting along the season focusing on primary feathers. Methods During a whole post-breeding migration at the Strait of Messina in Southern Italy, we gathered information about symmetrical flight feather moult from 221 adults by taking pictures of raptors passing at close range. Results We found active moulting primaries during autumn migration in 48.4% of our samples. Slight differences on the extension and timing among sex classes were recorded during the season, with adult females showing a more advanced moult stage than adult males. Conclusion The finding that the extension of the suspended moult was already defined in migratory individuals might be explained as an adaptation to minimize the energy required for moulting during migration.