International audience The effective conservation of aquatic warbler (Acrocephalus paludicola), one of the most threatened western Palaearctic migratory passerines, requires good knowledge of its ecological needs at stopover sites. In particular, identifying its diet, which controls the accumulation of fat reserves during migration, facilitates the selection and management of adequately protected areas. Further key information includes the relationship between prey species abundance and habitats of aquatic warbler on stopover. We performed standardised mist-netting in the Audierne marshes (western France) during 12 years, which resulted in the capture of 1,200 aquatic warblers, and provided measurements for mass gain and the collection of faeces to infer the birds’ diet. Invertebrate sampling was carried out in the three main Audierne marshhabitats(reedbed, fen mire and meadow). In order to go beyond prey digestibility bias,we also studied two closely related Acrocephalusspecies, present at migration stopover sites during the same period. We found that the diet composition of aquatic warbler observed at migration stopover sites is based on large-sizedprey (Odonata, Orthoptera, Lepidoptera).Likes edge warblers, aquatic warblers put on weight during migration stopovers (daily mass gain =0.38g). This increase in weight suggests that the aquatic warblers might have adopted a strategy for long-distance migration with few stopovers only. Due to great differences in diet, conservation management for the threatened aquatic warbler at stopover sites should not rely on existing knowledge abouts edge and reed warblers. Similarities in the diet of aquatic warbler between nesting areas and migration stopover areas and the relationship between habitat and prey abundance suggest that fen mires play an important role in the quality of the for aging habitat at stopover sites