About the Western Monarch Count
Western Monarch Migration
LINK: North American Monarch Migration Map
What Is the Western Monarch Count?
LINK: Saline Valley
LINK: Pelton et al 2019
Why Is This Effort Important?
Diversity & Inclusion
LINK: Read More
Project History
LINK: Oberhauser et al. 2015
Contributors
Thank you to our western monarch conservation funders, who make this work possible:
Folks from the Past:
And, a huge thank you to the 400+ dedicated volunteers who collect data at overwintering sites every year!
Notice: Below is a list of 5 important links included on this page.
1. North American Monarch Migration Map
4. Read More
Please note that while screen readers have made significant strides, they may still lack full support for optimal web accessibility.
Western Monarch Migration
The monarch butterfly, a native species of North America, is well known for its long-distance seasonal migration and 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 phenomenon in which thousands of monarchs gather 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 their overwintering sites and disperse across California first, and eventually migrate to all western states, searching for milkweed plants on which to lay their eggs. Milkweeds are host plants for monarchs; they are the primary plant that monarch caterpillars eat, allowing them to grow and develop into adult butterflies. 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 fall. Remarkably, monarchs return to the same groves of trees each year! North American Monarch Migration MapWhat Is the Western Monarch Count?
The Western Monarch Count (WMC) is an annual effort of volunteer community scientists and conservation organizations to collect data on the migratory western monarch population along the Pacific coast from Mendocino County, California, to Northern Baja, Mexico, during the monarch overwintering season, which occurs from approximately October through February/March each year. In recent years, the count has expanded to include annual surveys at inland overwintering sites in the Saline Valley of Inyo County, California, and in Arizona. The peak of this survey effort is the mid-season count (formerly the Thanksgiving Count), which runs for three weeks beginning in mid-November. The early-season count, introduced in 2024 to capture the arrival of monarchs at overwintering sites, takes place over two weeks in October. The late-season count, added in 2017, runs for two weeks starting in late December. Thanks to the extraordinary effort of a cadre of participants, we now have several decades worth of data demonstrating that monarchs have undergone a dramatic decline (Pelton et al 2019).Why Is This Effort Important?
Monarchs aren’t doing well. Their population has declined significantly since the 1980s and 1990s when millions of butterflies were observed at overwintering sites each year. The number of overwintering monarchs has plummeted due to factors such as: loss of habitat, pesticide contamination and climate change. Long-term efforts like the Western Monarch Count help us keep track of monarch trends over time. Data from projects like ours indicate that the monarch populations in western North America have declined by approximately 95% since the 1980s (Espeset et al. 2016, Schultz et al. 2017) while monarch populations in eastern North America have declined by 80% since the 1990s (Semmens et al. 2016). Monarchs need conservation action now, before they are at the brink of extinction, and that starts with an understanding of their presence and abundance across the landscape.Diversity & Inclusion
The Xerces Society is committed to establishing and maintaining a robust community that collectively supports our mission: to protect the natural world by conserving invertebrates and their habitat. Read More.Project History
Community scientists began monitoring the abundance of monarch butterflies at overwintering sites in California in the 1980s, but the concerted effort of the Western Monarch Count didn’t begin until 1997. Walt Sakai and other monarch experts proposed the idea of an annual count, and in 1997 the Monarch Program initiated the official Thanksgiving count, now called the mid-season count. Early count coordinators included Mia Monroe, Dennis Frey, and David Marriott. In 2000, as a volunteer with the Xerces Society for Invertebrate Conservation, Mia Monroe took over managing the Western Monarch Count, and today the initiative continues to be managed by the Xerces Society staff (Oberhauser et al. 2015).Contributors
Thank you to our western monarch conservation funders, who make this work possible:
United States Forest Service International Programs, California Association of Resource Conservation Districts, California Wildlife Conservation Board, Elizabeth Weber, The Maxwell/Hanrahan Foundation, Monarch Joint Venture, Nature’s Path Foods Inc., San Diego Zoo Wildlife Alliance, The Taggart Saxon Schubert Fund, and Xerces Society members.Folks from the Past:
- Bill Shepard, Previous regional coordinator (Alameda county)
- Saul Riatiga, Previous Regional coordinator (Baja California)
- Connie Masotti, Previous Regional coordinator (Monterey County)
- 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, monarch researcher – 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 (formerly)
- Emma Pelton, The Xerces Society
- Katie Hietala-Henschell, The Xerces Society (formerly)
- Sarina Jepsen, The Xerces Society
- Jen Zarnoch, The Xerces Society (formerly)
- Carly Voight, The Xerces Society (formerly)
- Richard Rachman, previous regional coordinator (Los Angeles County)
- Robert (“Bug Bob”) Allen, Previous regional coordinator (Orange County)
- Melissa Stepek, Previous regional coordinator (San Diego County), California Department of Fish & Wildlife
- Kirthana Pisipati, Previous regional coordinator (San Diego County)
