Common Birds Census

The Common Birds Census (CBC) was a major volunteer fieldwork project of the BTO during almost forty years between 1962 and 2000, funded by JNCC. Its main function of monitoring trends in the UK populations of common breeding birds has now passed to the BTO/RSPB/JNCC Breeding Bird Survey (BBS).

The CBC used a full version of the territory-mapping census method, as described in depth in the CBC Instructions of 1983. As well as numbers of territories, from which population trends were assessed, this method produces a map for each species, census plot and year to show exactly where the birds were (and were not) holding territory.

CBC was enthusiastically supported by BTO members, several of whom achieved more than thirty continuous years of mapping censuses. It was the work of CBC volunteers that drew attention to a number of serious declines among common and widespread species, such as Skylark and Willow Tit, and provided the evidence by which they (alongside a number of rare birds) have become high priorities for conservation effort. CBC also documented how, for species such as Sparrowhawk and Stock Dove, environmental change brought recovery and population expansion. Sharp drops in population, for Wrens and other small birds after cold winters and for Whitethroats after the failure of rains in West Africa, helped to demonstrate the importance of these factors in determining the abundance of these species. More than 10,000 CBC surveys were completed, at more than 1,500 different sites. Results to 1988 were documented fully in a popular book, Population Trends in British Breeding Birds.

Although the CBC is no longer running as a wide-scale project, data from the CBC remain constantly in use for the assessment of long-term population changes, and are displayed – mostly in the form of joint CBC/BBS trends – in our report on Breeding Birds of the Wider Countryside. Maps of well over a million bird territories are archived in new storage areas at the BTO HQ in Thetford, and are accessed frequently for a variety of research projects. We continue to add to the databases where new surveys are conducted at established CBC sites.

The CBC method is no longer used for large-scale population monitoring in the UK because it requires intensive observation at each site – the less intensive transect methods of BBS cover more sites and detect trends more efficiently. CBC mapping remains, however, the most accurate practical way to determine the numbers and distribution of breeding birds within a relatively small study site, and is widely used by BTO staff and other researchers for studies where a high level of detail is required. The original CBC Instructions (12 A4 pages) are out of print but can be supplied from BTO HQ as a photocopy. Plans are in hand to add these to the BTO web site, thus documenting how CBC volunteers operated in the past, along with a more concise, updated version to guide future use of the method.