VOCAL INDIVIDUALITY AS A CENSUS TOOL - PRACTICAL CONSIDERATIONS ILLUSTRATED BY A STUDY OF 2 RARE SPECIES

Vocalizations have the potential to allow the identification of indivi dual birds. The practicalities of the technique were assessed for Euro pean Bitterns (Botaurus stellaris) and Black-throated Divers (Gavia ar ctica), two species that present different monitoring problems. The di urnal and seasonal pattern of booming behavior is predictable for bitt erns; the less predictable pattern of Black-throated Diver vocalizatio ns proved a significant limitation on the usefulness of the technique with this species. Spectrograms of the yodel vocalization of Black-thr oated Divers showed clear qualitative. differences between individuals , whereas quantitative analysis of bittern booms was required to demon strate individual distinctiveness. Within- and between-year identifica tion of bitterns is complicated by a degree of variability in boom str ucture of some birds, but bitterns with relatively more stable booms w ere successfully re-identified. Black-throated Diver yodels showed str iking between-year similarity. The factors that affect and limit the s uccess of vocal individuality as an alternative to more conventional m arking techniques are discussed.