Recorded vocalizations from 18 known Great Bitterns (Botaurus stellaris) were analyzed, both quantitatively and qualitatively, to determine the amount of intra- and inter-individual variation. All parameters examined showed significant variation among males, and vocalizations of the same male were not stable over time. Variability within males over time makes re-identification of individuals on the basis of vocalizations difficult. The high degree of uncertainty in recognizing individuals limits the usefulness of vocalizations for censusing of bitterns.