The authors analysed how body mass, migration distance, taxonomic family and breeding on small islands vs. continents affect wing length in 72 species of closely related Old World reed and bush warblers (Acrocephalidae and Locustellidae), based on literature data. The species they analysed share similar morphology, habitat, food preferences, feeding habits and breeding systems, but their migratory behaviour varies from sedentariness to long-distance migration. The mean wing length of these species was strongly correlated with their body mass, migration distance and taxonomic family. The wing was on average 23.4% longer for each doubling ofbody mass and 2.7% longer for each 1,000 km of migration distance. Breeding on small islands was not significantly related to wing length. Species of Acrocephalidae had on average 11.7% longer wings than Locustellidae with the same body mass and migration distance. After they partly controlled for the effects of different habitats and behaviours, the results showed that at inter-specific level migration affects wing length in proportion to migration distance.