Population sizes of birds in large wetland areas are often difficult to estimate by traditional methods of bird census. Some of the characteristic species in reed areas (bittern, rails) are predominantly active during the night. Often there are difficulties to access the areas and regular visits to the reed zones can lead to damages of the vegetation and disturbances for breeding birds. Here we present a bioacoustic approach for the estimation of population sizes of nocturnal birds (rails, bitterns) evaluated during a regular bird census. The study was carried out in an extended half-bog renaturation area in the Northeastern part of Germany. The nocturnal acoustic environment was recorded in the absence of an observer by an array of four four-channel-recorders using cardioid microphones arranged in a cross configuration. This configuration allowed for estimation of the direction of the caller. The recording units were positioned at a distance which assured that a caller was recorded at least by two of them. Pattern recognition algorithms were used to find call sequences in the long term recordings. The position of the caller was estimated by an acoustic triangulation on the base of travel time differences at the different microphones. Our results on estimation of population density of rails and bitterns were in good agreement with the data obtained from field ornithologists. We could show that the bioacoustic approach to bird census can be successfully applied to nocturnal birds in open landscapes. The study was supported by the Deutsche Bundesstiftung Umwelt.