Population trends of Swallows Hirundo rustica breeding in Britain

Abstract

Capsule There are regional and habitat differences, but no long-term decline in numbers. Aims To review the evidence for declining Swallow populations in Britain, based on the British Trust for Ornithology's long-running Common Birds Census (CBC) and suggest possible causes. Methods Trends were fitted to CBC data for 1964–98 using a generalized linear modelling framework and analysed with regard to specific regions and habitats. Current habitat use, particularly of different farming types, was characterized using data from the Breeding Bird Survey. Correlations between Swallow population size and loss of nest-sites and rainfall in the non-breeding grounds were examined. Results Although Swallow numbers show large annual fluctuations, there is no evidence for a long-term decline on CBC plots. Numbers seem to have declined on farms in eastern England and in non-farmland habitats, but were not correlated with changes in cattle numbers or with an index of regional changes in nest-site availability. The number of Swallows breeding each year in Britain was not correlated with rainfall on their wintering grounds but was correlated with rainfall in the western Sahel on spring migration. Conclusion Changes in farming practice do not appear to have had a long-term effect on Swallow numbers. Although Swallows show high breeding site fidelity, changes in breeding location may mean local declines are not reflected in regional or national population trends.