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BBWC Home > Contents > Methodology > Combined CBC/BBS trends

2.3 Combined Common Birds Census (CBC) and Breeding Bird Survey (BBS) trends

The field protocols for the two surveys are described in sections 2.1 and 2.2. As previously noted, the CBC has been an enormously influential project, providing the main source of information on national population levels in the UK since its inception. For all its importance and impact, however, certain biases in coverage have long been known. Coverage is predominantly in lowland south-eastern Britain, where the numbers of potential volunteers are greatest. Coverage in more sparsely populated upland regions has always been much more patchy. Even within the well-covered regions, sites are situated in a limited number of habitats, predominantly farmland and woodland. Within this region, the results are nevertheless believed to be broadly accurate (Fuller et al. 1985). However, several species such as Wood Warbler and Meadow Pipit have the greater part of their numbers in the north or west of the country, outside the area adequately covered. For these species, the CBC may not accurately reflect national trends.

The BBS, on account of its more rigorous, stratified random sampling design, and its simplicity in the field, produces data that better cover the previously under-represented areas. In previous editions of 'Breeding Birds in the Wider Countryside’ (e.g. Baillie et al. 2002), indices have been published both from CBC and BBS data, for those species with sufficiently large sample sizes. The CBC was discontinued in 2000; from now on, BBS data will be used in the production of national population trends dating back to its year of establishment, 1994.

For many purposes, however, the presentation and analysis of longer time-series will be required, dating back to before the establishment of the BBS but coming right up to the present day. The calculation of 25-year alert designations, as in this report, provides just one example. This need has led to the BTO recently carrying out research into the compatibility of indices from BBS and CBC data in various years and regions, and the possibility of deriving trustworthy long-term indices from the two data sources in combination (Freeman et al. 2003). This research suggested that for the vast majority of species considered there was no significant difference between population trends, calculated from the two surveys, based on that part of the country where CBC data are sufficient to support a meaningful comparison. Where a statistically significant difference was found, this was sometimes for very abundant species for which the power to detect even a biologically insubstantial difference was considerable. Within this region, therefore, long-term trends based on CBC and BBS data can be produced for almost all species previously monitored by the CBC alone. In Freeman et al. (2003) this was the area covered by Fuller et al. (1985), because CBC plots in that region were shown to be representative of lowland farmland there. As this region covers the bulk of England, and for consistency with the rest of this report we have produced joint indices for CBC/BBS for the whole of England (called CBC/BBS-England index), rather than just the Fuller rectangle.

A second question then is whether one can obtain reliable trends over the same period for the entire UK. That is, since prior to 1994 only CBC data are available, are the population trends within the region well-covered by the CBC typical of those for the UK as a whole? The shortage of CBC data in the north and west means that the only way of investigating this is via the BBS data. Significant regional variation in trends was found for approximately half the species (see Freeman et al. 2003 for full details). For such species, the regional bias in CBC data means that no reliable UK index can be produced prior to 1994. In summary, joint population indices dating back to the start of the CBC can continue to be produced for that part of the country well served by the CBC for almost all common species. A similar UK index can be produced for only about 50% of species (CBC/BBS-UK index).

The present ‘Breeding Birds in the Wider Countryside’ is the first since the close of the CBC and the first to present joint CBC/BBS indices, in place of those derived solely from the CBC. The model fitted to these combined data is that historically employed for the BBS, a Generalized Linear Model with counts assumed to follow a Poisson distribution and a logarithmic link function. Standard errors were calculated via a bootstrapping procedure. For presentation in the figures, both the population trend and its confidence limits were also subsequently smoothed using a thin-plate smoothing spline with 11 degrees of freedom.

Indices are plotted as the blue line on the graphs, and provide a relative measure of population size on an arithmetic scale with a 2000 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 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).

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This report should be cited as:
Crick, H.Q.P., Marchant, J.H., Noble, D.G., Baillie, S.R., Balmer, D.E., Beaven, L.P., Coombes, R.H.,
Downie, I.S., Freeman, S.N., Joys, A.C., Leech, D.I., Raven, M.J., Robinson, R.A. and Thewlis, R.M. (2004)
Breeding Birds in the Wider Countryside: their conservation status 2003.
BTO Research Report No. 353. BTO, Thetford. (http://www.bto.org/birdtrends2003)

Pages maintained by Susan Waghorn and Iain Downie: Last updated 16 February, 2009