(For information about how BBS trends are calculated, see the Methodology section)
Linking the BBS and the CBC
An important component of BBS research initially was the development of methods to link the indices derived from the Common Birds Census (CBC) since 1962 to those derived from the BBS since 1994. This is important, because long-term population trends that contribute to conservation initiatives, such as Birds of Conservation Concern, will need to incorporate data from both schemes. Joint CBC/BBS trends are published annually in the BirdTrends Report.
We compared annual indices from the BBS and the CBC for 75 species that were covered by both schemes, over the period 1994 to 1999. We corrected for the geographical bias of the CBC by restricting the comparison to plots in an area bounded by northing 500 (km) and easting 300 (km) in the British National Grid. The most important result is that despite the differences in the methods employed in the two schemes and the fact that the CBC estimates the number of territories and the BBS is an index of the number of individuals encountered along a line transect, the vast majority of species exhibited very similar trends. In fact, formal testing revealed statistically significant differences in indices for only five species – Wren, Blackbird, Robin, Chiffchaff and Pheasant – and for most of these species, the differences are unlikely to be biologically important.
The other important finding from this work is the confirmation that population trends based upon the geographically-biased CBC may not reflect changes in the population at the national level. Of the 75 species tested, 40% exhibited statistically significant differences between BBS population trends within the CBC area (defined above) and those outside the area (essentially South West England, Wales, Northern Ireland, Northern England and Scotland). Although we do not know whether population trends prior to the 1990s would show the same geographic patterns, this work highlights the importance of regional effects on bird population trends.
Comparison between BBS (open square) and CBC indices (solid diamond) for Southern Britain for two species routinely indexed by both surveys for the period 1994 to 2000. The dashed lines represent upper and lower 95% confidence intervals of the BBS indices. Indices are measured relative to the year 2000, which is set to 1.
Freeman, S.N., Noble, D.G., Newson, S.E., Baillie, S.R. (2007) Modelling population changes using data from different surveys: the Common Birds Census and the Breeding Bird Survey. Bird Study 54, 61-74.