Surveys of Nightjars have been carried out in 1957-58, 1981, 1992 and 2004. Sampling units have varied slightly between surveys but the 2004 one was based on a stratified random sample of 1-km squares of the National Grid. Two visits to each chosen square were made between mid-May and mid-July and all Nightjars seen or heard were mapped. Subsequently these records were used to determine the number of territories on the area.