Those for Schedule 1 species are kept confidential. Data are computerised according to priorities for population monitoring and for specific research projects. Currently the BTO receives a total of more than 30,000 records each year for around 180 species. Typically, the BTO receives more than 150 records for 55 species and more than 100 for a further 10-15 species. The quality of records improved substantially in 1990 with the introduction of a new recording card, which promotes greater standardisation and clarity in the information recorded by observers.
Habitat coverage is broad since the scheme receives records from all the UK's major habitats.
Most records come from woodland, farmland and freshwater sites, but the scheme also receives data from scrub, grassland, heathland and coastal areas. Each calculation assumes that failure rates were constant during the period considered. Violations of this assumption of the Mayfield method can lead to biased estimates if sampling of nests is uneven over the course of each period. In this report, therefore, we present only temporal trends in daily nest failure rates. Regressions through annual mean laying dates, clutch sizes, brood sizes were weighted by sample size.