Mute swans (Cygnus olor) are an invasive species that can adversely affect aquatic vegetation and native wildlife. Their recent expansion in California prompted the passage of Assembly Bill 764 in 2025, authorizing take beginning in 2026. We quantified the geographic and numerical expansion of mute swans using records from eBird, Audubon’s Christmas Bird Count (CBC), and the California Department of Fish and Wildlife (CDFW) Waterfowl Breeding Population Survey. eBird data indicate initial establishment in urban areas of Los Angeles and San Francisco, followed by rapid expansion into the San Francisco Bay–Delta and Central Valley. The proportion of occupied 1-km grid cells increased at 16.0% annually (1995–2024). CBC data showed a similar temporal pattern, with an 8.8% annual increase in counts reporting mute swans, though geographic expansion plateaued after 2016. Both CBC and CDFW surveys indicate exponential population growth, with annual growth rates of 17.1% and 33.2%, respectively. These trends parallel patterns observed in the eastern United States and suggest increasing risks to California’s native waterfowl and wetland ecosystems.

Rachel-Ann Arias is a 4th year undergraduate at University of California, Davis studying wildlife Biology. She developed an interest in conservation and human-wildlife interactions volunteering with the Urban Wildlife Information Network in Los Angeles. At UC Davis, she is a research assistant with the Museum of Wildlife and Fish Biology and co-president of The Wildlife Society student chapter. Her field experiences include a shrew study with USFWS and leadership in a UCD songbird banding program. Rachel-Ann hopes to continue pursuing a research career in wildlife conservation, with plans to further her work through graduate study or professional research opportunities.