The relationship between Mute Swan Cygnus olor population trends in Great Britain and environmental change

ABSTRACT

Capsule

The increase in autumn sowing of crops is probably an additional contributory factor to the national ban on lead angling weights in influencing the large increase in British Mute Swan Cygnus olor numbers since the 1970s.

Aims

The national population of Mute Swans in Great Britain has more than doubled since the 1970s, and previous correlative analyses of national population changes identified a national ban on lead angling weights in 1987 as the main driver of this change. We examine regional variation in Mute Swan population changes to test the contribution of additional environmental covariates to the observed increase.

Methods

We explore regional and national variation in Mute Swan population trends to changes in climate, agriculture, water quality, and angling to assess whether the same patterns emerge at different scales.

Results

Changes in the extent of oilseed rape and wheat, which provide winter food for Mute Swans, showed a consistent positive association with the spatial and temporal pattern of Mute Swan population trends, while a proxy for the expected change in the exposure of swans to lead weights from angling contributed much less.

Conclusion

The lead weight ban occurred alongside rapid changes in arable cropping area, with swans probably benefitting from both increased food resources and reduced rates of lead ingestion. Our study highlights the value of exploiting both spatial and temporal variation in abundance when exploring potential drivers of population change. Future changes in agricultural policy and practice in Great Britain may influence Mute Swan populations.

Acknowledgements

We thank T. Finch and C. Morrison for much useful discussion, N. Calbrade, S. Gillings, and F. Worrall for their help with data; W. Peach and P. Grice for advice on coarse fishing; and P. Fox for guidance on measurements of fishing intensity. We thank the amazing citizen scientists who contributed swan counts to WeBS; without them this project would not have been possible. We thank the editors and our two reviewers for their invaluable comments, which have greatly improved the manuscript. This work was undertaken as part of an MSc in Applied Ecology and Conservation at the University of East Anglia (UEA), for which T.L.T.K. received kind funding support from the UEA International Excellence Scholarship and the Santander Postgraduate Scholarship.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

We obtained the raw data used to calculate derived environmental change variables in our analyses from published sources cited in the text. We obtained the raw count data on Mute Swan populations from the BTO's WeBS programme and are available from the BTO on application. We have archived the values for environmental change and swan population change variables we derived from these raw data on Zenodo (https://doi.org/10.5281/zenodo.8162813).