Monitoring bird populations is essential for the proper management of natural areas, since birds serve as indicator species to assess the conservation status of ecosystems. Therefore, it is important to estimate population sizes using methods of the highest possible accuracy and reliability. In this study, the populations of three marsh-dwelling passerines were sampled in the Tablas de Daimiel National Park (central Spain) during the breeding season, using a double independent observer technique. Data analysis was performed using a hierarchical Bayesian model with covariates that can simultaneously determine the population size and detectability, as well as factors that significantly affect both parameters. The presence of the two most threatened species, the reed bunting Emberiza schoeniclus witherbyi and the moustached warbler Acrocephalus melanopogon, was significantly and positively related to the coverage of fen sedge Cladium mariscus, an indicator species for waters of high quality and low fluctuation in depth. The bearded parrotbill Panurus biarmicus was the most generalist species with no positive association with any of the studied variables. The moustached warbler was more readily detectable until the beginning of May and early in the morning.