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 groves of trees on the Pacific coast from Mendocino to Northern Baja, 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, sycamore, and redwood.

Each spring, monarchs leave overwintering sites and disperse across California first, and eventually migrate to all 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 on the Pacific coast in the fall. Remarkably, monarchs return to the same groves of trees each year!

See our North American monarch migration map here.

What is the Western Monarch Count?

The Western Monarch Count is an annual effort of volunteer community scientists to collect data on the status of the western monarch population along the Pacific coast from Mendocino to Northern Baja, Mexico during the overwintering season, which occurs from approximately October through March. In addition, in recent years, the count has expanded to include annual counts at inland overwintering sites in the Saline Valley of California and in Arizona. The height of this volunteer effort occurs during the Western Monarch Thanksgiving Count, which runs for three weeks surrounding the Thanksgiving holiday, and during an additional count added in 2017, the New Year’s Count which occurs during a three week period beginning around New Year’s. Thanks to the extraordinary effort of a cadre of volunteers, we now have 23 years of data demonstrating that monarchs have undergone a dramatic decline estimated more than 99% in the western U.S. since the 1980s (Pelton et al 2019).

History of the Western Monarch Count

Community scientists began monitoring the abundance of monarch butterflies at overwintering sites in California in the 1980s, but the concerted effort of the Thanksgiving Count didn’t begin until 1997. Walt Sakai and other monarch scientists proposed the idea of an annual count, and in 1997 the Monarch Program initiated the official Thanksgiving Count. Early count coordinators included The Monarch Program, Shawna Stevens, and Dennis Frey. In 2000, as a volunteer with the Xerces Society for Invertebrate Conservation, Mia Monroe took over coordinating the count, and the count continues to be coordinated by Mia and Xerces staff (Oberhauser et al. 2015).

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 County, California and Northern Baja, Mexico 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. If you are interested in just seeing monarchs and visiting a public monarch overwintering site, you can see the list of sites open to public viewing on the Xerces Society website here.

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 community science project.
  6. Support the Xerces Society’s monarch conservation efforts.


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

  • Mia Monroe, Thanksgiving count co-founder and regional coordinator (Sonoma and Marin counties)
  • Rick Hansen, Regional coordinator (Mendocino county)
  • Christina Garcia, Regional coordinator (Alameda county)
  • Martha Nitzberg, Regional coordinator (Santa Cruz county)
  • Connie Masotti, 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, Co-regional coordinator (Inyo county)
  • Joanna Gilkeson, Co-regional coordinator (Inyo county)
  • Robert Allen, Regional coordinator (Orange county)
  • Richard Rachman, Regional coordinator (Los Angeles county)
  • Karen Sinclair, Regional coordinator (Ventura county)
  • Melissa Stepek, Regional coordinator (San Diego county)
  • Saul Riatiga, Regional coordinator (Baja California)
  • Bill Shepard, Co-regional coordinator (Alameda county), deceased, read about Bill’s legacy here
  • Eric Porter, Previous Regional coordinator (San Diego county)
  • Lara Drizd, Previous regional coordinator (Ventura and Los Angeles counties)
  • Alison Watson, Previous regional coordinator (Monterey County), PG museum
  • Nick Stong, Previous regional coordinator (Monterey County), PG museum
  • Barbara Rice, Previous regional coordinator (Mendocino County)
  • Liam O’Brien, Previous regional coordinator (San Francisco County)
  • Dennis Frey, Thanksgiving count co-founder and monarch researcher, deceased – previously at Cal Poly 
  • Shawna Stevens, Monarch researcher – previously at Cal Poly 
  • David Marriott, Thanksgiving count co-founder and monarch researcher, deceased
  • 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