Optimal sampling of booming Bitterns Botaurus stellaris

Abstract

The Great Bittern Botaurus stellaris is a secretive bird that lives in dense reed marshes. Population surveys of this vulnerable species are based on the characteristic vocalization (booms) of males during the breeding season . Probability of detecting a male is highly dependent upon the occurrence of booms, which is assumed to be densitydependent and highly variable over time . We studied booming frequency in relation to time of day, time of season, weather conditions, and bittern density at ten sites in the Camargue, south of France. At each site, twice monthly from 6 March through 18 July 2000, booms were recorded over two periods of 6 h, one centred on sunset, the other centred on sunrise, and both divided into 72 sampling units of 5 min each. The number of bitterns heard within a single 6-h period varied from 0 to 10. Booming frequency was highest in April and May. The proportion of males booming peaked 0-30 min after sunset (68%), and 30-60 min before sunrise (78%). Booming activity decreased significantly under cloudy and rainy conditions, but low temperatures had no effect . Optimal sampling protocols (point count duration and frequency) are suggested for various situations at dawn and dusk, during the peak and low booming periods, and for sites comprising a single or multiple bitterns . Given the daily and seasonal variation in booming frequency, only standardized protocols can provide afaithful estimate of the number of male bitterns across years.