Mute Swan Abundance and Habitat Use in the Central Sierra Nevada Foothills, California

Abstract

The Mute Swan (Cygnus olor) has recently colonized California, and its population is reported to be growing rapidly, but little information is available on its occurrence and habitat use there. I conducted surveys for the species at waterbodies in the foothills of the central Sierra Nevada during the 2025 breeding season. I summarize information on occurrence by waterbody size and whether emergent vegetation and islands were present. Swans occupied 23 (20%) of 114 surveyed waterbodies. They avoided using waterbodies smaller than 1 ha (only 4% used) and larger than 100 ha (at low densities and only in shallower reservoirs) and occurred at 37% of 57 waterbodies 1–100 ha in size. Mute Swans used waterbodies with emergent vegetation and islands more than expected if use were random. More than one pair of swans were found only at three waterbodies that ranged in size from 4.6–76 ha. This study shows that the Mute Swans are widespread at stock ponds and other waterbodies in the Sierra foothills. This information provides a baseline for future status surveys and can be used to more thoroughly assess the regional population, the potential for further expansion in the region and in other similar areas in the state, and the effectiveness of control efforts in the region.