Monarch butterflies have returned to forested groves along the California coast for the winter, and once again volunteers for the Xerces Society Western Monarch Thanksgiving Count are heading out to observe and monitor this phenomenon. This year is particularly exciting because it marks the 20th time the count has been done!

The Western Monarch Thanksgiving Count (WMTC) is the longest running and most comprehensive effort to monitor overwintering monarchs in California. During a three-week period centered on Thanksgiving, volunteers count the butterflies and assess the condition of the habitat they rely on. 

Monarchs from as far away as Idaho, Washington, and Arizona converge during October and November on forested groves along the coast from Mendocino County, California to Baja, Mexico. Clinging to the branches of Monterey cypress, Monterey pine, and nonnative eucalyptus, they rely on the calm, humid environment of the groves as shelter from storms.

“Because monarchs cluster in groups of tens, hundreds, thousands, or even tens of thousands,” said Emma Pelton, a conservation biologist with the Xerces Society who helps coordinate the count, “the overwintering season provides an opportunity to count the butterflies—a useful measure of the size of the western monarch population, and one that is even more important at a time when monarch populations are falling.”

A report based on data gathered through the WMTC, State of the Monarch Butterfly Overwintering Sites in California, was published by the Xerces Society this summer. The analysis of the data shows a 74% decline in the number of monarchs which overwinter along the California coast since the late 1990s.

There are currently 299 sites that are part of the WMTC with up to two thirds of them monitored in any single year. Visiting one to two hundred sites over five hundred miles of coast in a three-week period is no easy feat, and it takes the efforts of over a hundred volunteers.

The WMTC was started in the 1990s due to the dedication of three passionate biologists, Dennis Frey, David Marriott, and Mia Monroe, who had become concerned that monarch populations appeared to be declining. Over the years, the WMTC has grown in size and scope, but it still retains its grass-roots style of citizen science—and is still led by Mia Monroe!

As Mia Monroe puts it, “I am proud to see the counts but also the immediate connection to the monarch, the place, and the other devoted field biologists who make commitments to get out to the chilly coast to look for a cloud of fluttering orange and black butterflies, take the time to count, then turn around and share the methodology with others.”

This year, the Western Monarch Thanksgiving Count runs from Saturday, November 12, through Sunday, December 4. Anyone interested in joining as a citizen scientist can find a nearby overwintering site and sign up at www.westernmonarchcount.org.