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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 mainly representative of lowland England, with relatively few in Wales, Scotland and Northern Ireland. Fieldwork is carried out by a team of dedicated volunteers, until recently 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.

Validation studies
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 to which other survey methods are compared, it is important that the validity and limitations of the CBC methods are understood. Snow (1965) compared CBC mapping and intensive nest-finding, and concluded that mapping censuses are good indicators of breeding population size for 70% of species. Experiments to test differences between observers' abilities to detect birds found that, although there was considerable variation between individual abilities, the observers were consistent from year to year (O'Connor & Marchant 1981). 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. It has also been confirmed that the sample of plots from which CBC results are drawn has not changed in composition or character over the years (Marchant et al. 1990) and that the results of territory analysis are not affected by changes in analysts, once trained (O'Connor & Marchant 1981). Fuller et al. (1985) found that farmland CBC plots were representative of ITE land-classes and cropping patterns in lowland England.

Data analysis
Population changes are modelled using a generalised additive model (GAM), a type of log-linear regression model that incorporates a smoothing function (Fewster et al. 2000). This replaces the Mountford model that employed a 6-year moving window (Mountford 1982, 1985; Peach & Baillie 1994) and was used from the mid-1990s until 1999, but the principles are similar. Counts are modelled as the product of site and year effects on the assumption that between-year changes are homogeneous across plots. "Smoothing" is used to remove short-term fluctuations (e.g. those caused by periods of severe weather and measurement error) and thus reveal the underlying pattern of population change. This is achieved by setting the degrees of freedom to one-third the number of years in the series. Confidence limits on the indices are estimated by bootstrapping (a resampling method; Manley 1991) and thus do not make any assumptions about the underlying distribution of counts.

Indices are plotted as the thick blue 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 green 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 suffer from a "Small sample" if the mean number of plots was <20. Data are regarded 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 1988-91 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
The CBC is recognised as having many strengths and has been a keystone of bird population monitoring within the United Kingdom for more than three decades. However, all monitoring programmes are subject to compromises between the theoretical ideal and methods that are practicable and cost-effective. The weaknesses of the CBC are largely related to the fact that both fieldwork and analysis are very time-consuming. This inevitably limits the numbers of volunteers who are able to participate in the scheme, with the result that areas with a low density of birdwatchers are under-represented. Due to the constraints imposed by the relatively small sample size, it was felt necessary to concentrate on farmland and woodland habitats. Bird population trends in built-up areas and the uplands are therefore little known. Moreover, as the plots are chosen by the observers, it may be that plots are not always representative of the surrounding countryside and there may be some bias towards bird-rich habitats. It is for these reasons that the Breeding Bird Survey (see below) was introduced in 1994. Both surveys were run in parallel for several years to allow calibration between the schemes. The 2000 field season was the last year of operation of the full CBC. During 2001 a reduced set of CBC plots was operated, with the aim of providing information on the relationships between bird locations and features of their habitats, and providing monitoring information for a small number of specific habitat types. CBC monitoring will continue to take this format in future years.

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This report should be cited as:
Baillie, S.R., Crick, H.Q.P., Balmer, D.E., Beaven, L.P., Downie, I.S., Freeman, S.N., Leech, D.I., Marchant, J.H.,
Noble, D.G., Raven, M.J., Simpkin, A.P., Thewlis, R.M. and Wernham, C.V.
(2002) Breeding Birds in the Wider
Countryside: their conservation status 2001. BTO Research Report No. 278. BTO, Thetford. (

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