Migrating western monarch butterflies rebounded in the Bay Area and at West Marin’s overwintering sites this season. Though a cold snap in late November and January’s storms put a dent in monarchs’ survival rate through the winter, the year still brought greater numbers to an area that hasn’t seen a strong population in years. “Small populations are particularly vulnerable to being snuffed out by extreme weather, so we are lucky these storms occurred in a relatively good year,” said Emma Pelton, a conservation biologist at the Xerces Society for Invertebrate Conservation. “We don’t want to count on luck alone to ensure the survival of the western monarch migration.” The Xerces Society holds two counts, one around Thanksgiving and one around New Year’s. In the 2020-21 counts, numbers were at an all-time low of just 2,500 across all the species’ overwintering sites. But the following year, numbers increased tenfold, with 250,000 overwintering monarchs, mostly in Southern California. Biologists point to habitat destruction to explain why numbers have dropped. Whether it’s wildfire, downed trees or drought, all are in some way linked to climate change. Chip Taylor, a professor emeritus of ecology and evolutionary biology at the University of Kansas and the director of Monarch Watch, also suspects monarchs may be establishing a new migration pattern. The low numbers in 2020 may have reflected a dispersal that led butterflies away from where scientists and volunteers expected to find them. This year’s Thanksgiving count, which ran from Nov. 12 to Dec. 4, yielded 330,000 monarchs. Conservationists are happy with these numbers but say it is not the time to be complacent. “One of the most important things we can do is to enhance the overwintering sites to directly benefit monarchs,” said Mia Monroe, a Golden Gate National Recreation Area ranger and co-founder of the Xerces Society’s monarch count. “When Marin had its cold snap, they didn’t have the protection they needed.” In early November, over 2,000 monarchs were observed in Bolinas, more than half of them at a site by Kale Road on the Big Mesa. After temperatures dropped later that month, around 1,400 butterflies were observed at a grove north of Poplar Street that hasn’t been in such abundance since 2005. Days later, almost 400 had migrated to the sewer ponds, an area sheltered from winds by a neighboring eucalyptus grove that many residents are calling to remove. (Ms. Monroe said that residents “can remove most of the trees for human safety, but protect the small grove that’s important to the monarch habitat. I believe there’s a win-win there.”) After the New Year’s cold weather and intense storms, only 50 butterflies were spotted in Bolinas, 30 in Muir Beach—the first time that community had seen more than 10 monarchs since 2017—and nearly 60 in Stinson Beach. When a Stinson resident called Ms. Monroe one morning in January, he described a 56-strong cluster of butterflies on a flowering plum glittering in the sun. The discovery renewed Ms. Monroe’s optimism in the resiliency of the insects and their ability to find new habitats—and the potential for overwintering sites to be hidden on private or inaccessible coastal lands. Like many wildlife counts that take place in West Marin, the monarch count is dependent on weather. This winter, many volunteers couldn’t come out to count, but Ms. Pelton said at least the storms coincided with a high-population year. She said rehabilitating overwintering sites should be a priority so storms don’t decimate monarchs when they are in low abundance. Monarch sites in the recreation area are receiving support in the form of fuel reduction and the planting of native trees to create wind blocks, Ms. Monroe said. Western monarchs overwinter along the Pacific Coast from Mendocino to northern Baja California and in parts of Arizona. In recent years, many of the historic sites in Sonoma, Marin and Mendocino that once harbored thousands of butterflies have been empty, leaving Bolinas sites as the northernmost in the country.