The Bittern Botaurus stellaris is one of the UK's rarest breeding birds, with, in recent years, a population of around 20 booming males restricted to 12 or so breeding sites (Smith & Tyler 193; Gilbert et al. 1999). Bitterns were extinct in Britain at the end of the 19th century but returned early in the 20th century, and were proved to breed in the Norfolk Broads in 1911 (Turner 1924).