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.
We are using a bioacoustic approach for the estimation of population sizes of nocturnal birds (rails, bitterns) evaluated during a regular bird census. The study is 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 cardioic 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.