Western Monarchs

The monarch butterfly is well known for its long-distance seasonal migration and its spectacular winter gatherings. In the western United States, monarchs migrate to the California coast and tagging studies have shown some monarchs from the Southwest even migrate to central Mexico to mix with the Eastern monarch population. However, California is the only place in the country that regularly hosts the awe-inspiring sight of thousands of monarchs gathered for winter. These days, most of California’s monarchs cluster in groves of nonnative blue gum eucalyptus, although they also use native trees such as Monterey pine, Monterey cypress, and redwood.

Each spring, monarchs disperse across California and several western states, searching for milkweed plants on which to lay their eggs. Milkweed is the only plant monarch caterpillars can eat to grow and develop into adults. Several generations are produced throughout the spring, summer, and fall, with each generation spreading further across the landscape. The last generation then migrates all the way back to the overwintering grounds in the fall. Remarkably, monarchs return to the same groves of trees as their ancestors.

See our North American monarch migration map here.

What is the Western Monarch Count?

The Western Monarch Count is an annual effort of volunteer citizen scientists to collect data on the status of monarch populations along the California coast during the overwintering season, which occurs from approximately October through March. The height of this volunteer effort occurs during the Western Monarch Thanksgiving Count, which runs for three weeks surrounding the Thanksgiving holiday. Thanks to the extraordinary effort of a cadre of volunteers, we now have over 20 years of data demonstrating that monarchs have undergone a dramatic decline estimated more than 95% in the western U.S. since the 1980s (Schultz et al. 2017). This effort has been coordinated by Mia Monroe, Shawna Stevens, and Dennis Frey in the past, and is now coordinated by Mia and The Xerces Society for Invertebrate Conservation.

In January 2017, The Xerces Society and Western Monarch Thanksgiving Count volunteers expanded their efforts to monitor western monarchs for a second period each winter. This first New Year’s Count includes a smaller subset of the overwintering sites monitored during the Thanksgiving count period and we could use YOUR help to expanding the number of sites monitored around the New Year’s holiday. Contact wmtc@xerces.org or your regional coordinator to learn more.

Funding for the 2017 Western Monarch Thanksgiving Count and New Years Count was provided by U.S. Fish and Wildlife Service, San Diego Zoo, and Xerces Society Members.

How Can You Participate?

Are you interested in joining this effort? We are looking for volunteers in California who can commit to visiting the same overwintering sites year after year. Most of what we know about the population trends of western monarchs is a result of the incredible work done by volunteers like you. Roughly 200 overwintering sites from Mendocino to San Diego Counties are monitored each year, but we know of over 400 sites that monarchs have either used in the past or are currently using to aggregate. With your help we can start monitoring more sites and get a better picture of the status of these sites. All the tools you need to get started as a volunteer can be found on this website. You can start by reading the Step-by-Step Monitoring Guide and exploring overwintering sites near you.

Don’t live near an overwintering site? There are other things you can do to help!
  1. Plant native milkweed. This can be in your backyard, at your workplace, or at your school. However, if you live within 5-10 miles of the Pacific coast (outside of milkweed’s historic range), we recommend planting fall, winter, and spring nectar sources INSTEAD of milkweed. Find sources of local, native milkweed seed in your state using our Milkweed Seed Finder.
  2. Plant native flowers. Monarchs need nectar to provide energy to migrate, breed, and overwinter. Flowers can be planted anywhere, including overwintering sites.
  3. Support organic and GMO-free agriculture.
  4. Avoid using insecticides and herbicides. These may kill butterflies or caterpillars, or kill the plants that monarchs use for nectaring or breeding.
  5. Get involved in another citizen science project.
  6. Support the Xerces Society’s monarch conservation efforts.


  • Mia Monroe, Founder and regional coordinator (Sonoma and Marin counties)
  • Rick Hansen, Regional coordinator (Mendocino county)
  • Bill Shepard, Co-regional coordinator (Alameda county)
  • Christina Garcia, Co-regional coordinator (Alameda county)
  • Martha Nitzberg, Regional coordinator (Santa Cruz county)
  • Nick Stong, Regional coordinator (Monterey county), PG museum
  • Jessica Griffiths, Regional coordinator (San Luis Obispo county)
  • Charis van der Heide, Regional coordinator (Santa Barbara county)
  • Rachel Williams, Regional coordinator (Inyo county)
  • Saul Riatiga, Regional coordinator (Baja California)
  • Alison Watson, Previous regional coordinator (Monterrey County), PG museum
  • Barbara Rice, Previous regional coordinator (Mendocino County)
  • Liam O’Brien, Previous regional coordinator (San Francisco County)
  • Dennis Frey, Monarch researcher – previously at Cal Poly 
  • Shawna Stevens, Monarch researcher – previously at Cal Poly 
  • David Marriott, Monarch researcher
  • Candace Fallon, The Xerces Society
  • Stephanie McKnight, The Xerces Society
  • Emma Pelton, The Xerces Society
  • Katie Hietala-Henschell, The Xerces Society
  • Sarina Jepsen, The Xerces Society
  • Jen Zarnoch, The Xerces Society (previously) – developed the OW database
  • Carly Voight, The Xerces Society (previously) – monitored OW sites
  • Plus all of the WMTC volunteers, past and present