Information and survey forms are distributed to organisers, who contact volunteers willing to survey the squares every year, and after the field season, forms are returned to BTO headquarters via the Regional Organisers. Fieldwork involves three visits to each survey square each year. The first is to record details of habitat and to establish the survey route, the second and third to count birds. A survey route is made up of two roughly parallel lines, each 1 km in length, although for practical reasons routes typically deviate somewhat from the ideal. Each of these lines is divided into five sections, making a total of ten 200 m sections, and birds and habitats are recorded within these units.
Volunteers record all the birds they see or hear as they walk along their transect routes.
Recording birds within distance bands is important because it provides a measure of bird detectability in different habitats and allows population densities to be estimated. The total numbers of each species, excluding juveniles, in each 200 m transect section and distance category, are recorded on summary forms, as well as the timing of the survey and weather conditions. The average time observers spend per visit is around 90 minutes. Squares are distributed throughout the UK, and cover a broader range of habitats than the CBC, including uplands and urban areas. For a small number of species, which are colonial or flocking in habit, it is unclear how well they are monitored by the BBS but they are not currently monitored by other BTO schemes, and have therefore been included.