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BBS Research Projects Population Trends Linking the BBS and the CBC An important component of BBS research over the past year has been 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 in the future, long-term population trends that contribute to conservation initiatives such as Birds of Conservation Concern/Importance, will need to incorporate data from both schemes. 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 the Southwest, Wales, Northern Ireland, northern England and Scotland). One example is Green Woodpecker, which is increasing rapidly in the CBC area, but declining in the more peripheral parts of its range. 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. The next step is to analyse the full seven years of data during the overlap period (1994 to 2000) and, in consultation with other conservation organisations, to come up with a way of reporting long-term population trends based on data from both schemes.
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. (2003) Modelling bird population changes
using data from the Common Birds Census and the Breeding Bird Survey.
BTO Research Report 303. British Trust for Ornithology, Thetford. |
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