2.1 Common Birds Census
The results from the Common Birds Census (CBC) provide population trends for almost all of the commoner breeding species in Britain. Annual estimates of the number of breeding pairs on between 200 and 300 plots around the country allow comparisons of population levels on a year-to-year basis. Focusing on farmland and woodland habitats, the CBC provides reliable indices of population change for around 60 species.
The CBC has been running since 1962 and was instigated to provide sound information on farmland bird populations in the face of rapid changes in agricultural practice. The same observers survey the same plots using the same methods year after year. Although the original emphasis was on farmland plots, woodland plots were added shortly afterwards. The sample of farmland plots contains most of the main agricultural land-uses, with plots averaging around 70 hectares in extent. Woodland plots are generally smaller, averaging just over 20 hectares. A small number of plots of other habitats, including heathlands and small wetlands, are also surveyed annually. The plots show a rather uneven geographical coverage and are probably representative of lowland England, with relatively few in Wales, Scotland and Northern Ireland. Fieldwork is carried out by a team of dedicated volunteers, currently around 250 strong. On average, plots are censused for around seven consecutive years but a few observers have now been surveying the same sites since the CBC's inception in the early 1960s.
A territory-mapping approach is used to estimate the number and positions of territories of each species present on each survey plot during the breeding season. Volunteers visit their survey plot eight to ten times between late March and early July and all contacts with birds, either by sight or sound, are plotted on large-scale maps. 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. In 1990, the results from the Common Birds Census were brought together in the book Population Trends in British Breeding Birds (Marchant et al. 1990). This landmark publication discussed long-term population trends for the years 1962 to 1988 for 164 species, with CBC population graphs for around two-thirds of these.
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.
Indices are plotted as the thick green line on the graphs, and provide a relative measure of population size on an arithmetic scale with a 1998 value of 100. If an index value increases from 100 to 200, the population has doubled; if it declines from 100 to 50, it has halved. The two dotted blue lines on the graphs, above and below the index line, are the upper and lower 85% confidence limits. A narrow confidence interval indicates that the index series is estimated precisely, a wider interval indicates that it is less precise. The use of 85% confidence limits allows relatively straightforward comparison of points along the modelled line: non-overlap of the 85% confidence limits is equivalent to a significant difference at approximately the 5% level (Anganuzzi 1993). Confidence limits are not provided for farmland or woodland trends unless they show a significant decline >25%. Caveats are provided to show where the data suffers from a "Small sample" if the mean number of plots was <20; and as "Unrepresentative?" if the average abundance of a species in 10-km squares containing CBC plots was less than that in other 10-km squares of the species' distribution in the UK (as measured from New Breeding Atlas data (Gibbons et al. 1993)), or where average abundances could not be calculated, expert opinion judged that CBC data may not be representative.
Where possible, separate indices were calculated for farmland, woodland and all CBC plots, and all three indices from the latter selection are presented graphically in the species accounts. In some cases, however, we were unable to calculate indices for the different habitat types and only the single index for all CBC plots is presented.
The CBC's future
CLICK HERE to go to the CBC section of the main BTO website
2000 British Trust for Ornithology