Local annual survival of booming male Great Bittern Botaurus stellaris in Britain, in the period 19901999

ABSTRACT

Male Great Bitterns Botaurus stellaris have individually distinctive boom vocalizations which have been used since 1990 to count the British population accurately. We used vocal individuality to identify birds between years and analyse the survival of the British booming population. We used six instances of Great Bitterns known to be the same birds (from radio tracking and re-sighting of darvic rings) in successive years to provide a means of identification independent from vocalizations. All of these birds remained in the same territory from one year to the next. Seven spectrogram measures were chosen as quantitative descriptors of Great Bittern booms. Boom similarity was expressed in terms of Euclidean 'acoustic' distance between descriptors of pairs of birds. Great Bitterns that were known to be the same birds had more similar booms across years than those known to be different. The acoustic distances and knowledge of site fidelity were used to construct parsimonious rules on which to base re-identification decisions and to build survival histories. Great Bittern local survival in Britain as a whole was estimated as 70% (± 5.1 se) with survival in north-west England (at Leighton Moss) higher (82.8%, ± 7.3 se) than that in East Anglian sites (65.2%, ± 5.4 se) (₁² = 3.4, P = 0.07). Movements of males between years were apparent between sites within the Suffolk and Norfolk coastal regions but not on a large scale. Survival of adult males in East Anglia was positively related to winter rainfall, but there was no significant relationship with winter temperature. There was a positive relationship between the annual population change in East Anglia and the annual local adult survival rate.