Using repeated winter surveys to estimate changes in abundance of seed-eating passerines

Capsule Winter Atlas surveys of 16 species on lowland farmland revealed significant changes in count for four species. Aims To estimate changes in abundance between the early 1980s and late 1990s, of wintering seed-eating passerines, in 'core' areas of lowland Scotland. Methods Ninety-five Scottish 10-km squares were selected that held high numbers of seed-eating passerines in the 1981-84 Atlas of Wintering Birds in Britain and Ireland. The same survey methods were used to resurvey these in winters 1997/98 and 1998/99, and visits were matched as closely as possible for duration and date. Analyses compared counts between the two survey periods for 16 species of seed-eating passerines and, for 12 of these, differences were also compared with national breeding population trend information for the same period. Results Mean Corn Bunting Emberiza calandra count per visit declined by 62% between the early 1980s and late 1990s, a difference which was statistically significant (P = 0.026). Significant increases were recorded for Yellowhammer Emberiza citrinella (up 62%), Common Linnet Carduelis cannabina (up 3.4-fold) and European Goldfinch Carduelis carduelis (up 13-fold). For 12 species for which national breeding population trend data were available, trends were weakly positively correlated (r(s) = 0.43, P = 0.08) with those from our results, but several species trends were more positive in our study. This difference was particularly marked for Eurasian Tree Sparrow Passer montanus, Goldfinch and Linnet. Conclusion Repeating Winter Atlas surveys offers a useful additional method for assessing population trends. They are particularly useful in a region with low observer coverage and for species that are poorly covered by long-term bird monitoring data sets. It would be valuable to validate this approach at a regional level, especially in a region for which detailed long-term bird monitoring data are available.