Long-term changes in the abundance of passerines in Britain and Ireland as measured by constant effort mist-netting

Abstract

Changes in the abundance of 28 species of common passerines in scrub and wetland habitats were assessed from changes in annual catch sizes on Constant Effort Sites between 1983 and 1995. At sites where annual catching effort fell below the required level, annual catch sizes were inflated according to experience during years with complete coverage. Changes in abundance were assessed through application of log-linear Poisson regression models, which allow tests for temporal trends in abundance and for differences in trends in abundance between habitat types. For most species, long-term changes in adult catches were similar to changes in territory counts on Common Birds Census plots, suggesting that standardized mist-netting is a reliable method for assessing extensive changes in songbird populations. Changes in catch sizes tended to be more positive and less negative in wetter habitats than in drier habitats. Catches of most insectivorous resident species either increased or remained stable, while catches of resident thrushes, small finches, buntings and some trans-Saharan migrants declined. The largest increases in catches of adult birds were recorded for Robin, Wren, Greenfinch, Long-tailed Tit and Chaffinch, while the largest decreases in adult catches were recorded for Linnet, Redpoll, Spotted Flycatcher, Yellowhammer, Reed Bunting and Willow Warbler. Relatively large declines in the catches of young Greenfinches and Goldfinches may reflect declining productivity. Possible environmental causes of these changes in abundance are discussed.