Seventy standardized ringing sessions were carried out from April 1997 to August 2000 in a reedbed in Ria de Aveiro at Salreu, Portugal. The variation in the abundance of the common species is described as well as some aspects of their weight dynamics. Both, number of birds and species were higher during summer and autumn, when passage migrants occur and the number of juveniles is high. The pre-breeding passage was inconspicuous. The majority of the species were migratory and insectivorous. Savis Warblers Locustella luscinioides arrived and started breeding one month before the other reedbed passerine migrants. Juvenile Reed Warblers Acrocephalus scirpaceus migrate nearly one month later than adults, a pattern not shown by Savis Warblers; adult Savis Warblers seem to delay migration in order to moult at least some flight feathers.The weight and fat of Reed Warblers increased significantly during autumn. The frequency of occurrence of Bluethroats Luscinia svecica of various wing lengths is very different from the one observed in southeast Spain; in Salreu, birds with smaller wing-lengths, mainly L.s. namnetum, were the commonest. Despite being resident, Cettis Warblers Cettia cetti only used reedbeds during winter, where females were far more abundant than males. Due to the long breeding season of reedbed passerines at Salreu, particularly Reed and Savis Warblers which breed at least until the middle of August, cutting of the vegetation should only be carried out from September onwards.