Codes are used to identify the birds' species, sex and age where possible, and also to record activity such as song or nest-building. The registrations are then transferred to species maps, which are returned to the BTO for analysis. The pattern of registrations reveals the numbers of territories for each species. By applying rigorous rules while analysing the species maps, we can be sure that there is consistency between our estimates from year to year. Comparison of territory totals with those for the same plots in previous years gives estimates of change between years, and allows the production of a long-running population index for each species.
Observers also provide detailed habitat maps and information from their plots.
This makes it possible to match the distribution of bird territories with habitat features, providing the potential for detailed studies of bird-habitat relationships. The CBC was the first national breeding bird monitoring scheme of its kind anywhere in the world and its value has been widely recognised internationally. The territory-mapping method adopted by the CBC is acknowledged as the most efficient way of estimating breeding bird numbers in small areas. As the benchmark by which other survey methods are compared, it is important that the validity and limitations of the CBC methods are understood. As the CBC relies on data from plots covered by the same observer in consecutive years, this source of bias will not have implications for the CBC's ability to identify population trends.