We show that the low number of aquatic warblers Acrocephalus paludicola ringed in Portugal is explained by the low ringing effort, and that Portugal is an important country for its migration. Most of the seven sites where the species was detected are recognized for their biological importance and protected. There seemed to be a decline in the number of birds caught, particularly of juveniles, which might be associated with a decline in breeding success. All aquatic warblers were captured in August and September, showing a peak at the end of August, and adults migrated significantly earlier than juveniles. Both age classes significantly increased in body condition during the season. The potential non-stop flight range varied substantially between individuals: some birds would be able to migrate to the wintering quarters without refueling, but the low flight range of many individuals suggests that some fueling takes place in Portugal.