The role of season and social grouping on habitat use by mute swans (Cygnus olor) in a lowland river catchment

Capsule: The Mute Swan, a large generalist herbivore, showed patterns of habitat use influenced by social grouping and season. Territorial swans showed strong preferences for river and lake habitat in all seasons, while the non-territorial birds known to cause grazing conflicts preferred river in summer–autumn and pasture in winter–spring. Aims: To quantify the habitat preferences across different seasons of two types of Mute Swan social group, territorial and non-territorial, and assess the implications for the grazing conflict between swans and valuable plant communities. Methods: Repeated surveys of the River Frome catchment, Dorset, UK, over a two-year period allowed us to record the numbers of swans in each habitat type. An electivity index was used to calculate habitat preference scores for territorial and non-territorial swans across different seasons. Results: We found strong seasonal switches in habitat use for both territorial and non-territorial swans, but preferences for some habitats differed between these social groupings. In particular, non-territorial swans preferred pasture in winter and spring, and river in summer and autumn, while territorial swans preferred river and lake habitats throughout the year. Conclusion: The Mute Swan, a large generalist herbivore that can cause grazing damage to river plant communities, showed patterns of habitat use influenced by social grouping and season. These seasonal patterns of habitat use suggest that the grazing conflict with the river plant community caused by flocks of non-territorial swans is currently limited to summer and autumn.