As the days get shorter and monarchs make their way to the forested groves along the California coast to settle in for the winter, volunteers for the Xerces Society Western Monarch Thanksgiving Count are preparing to head out to observe and monitor this migratory phenomenon. This year is particularly exciting because it marks the 20th time the count has been done!
Overwintering monarch clusters first became known to Western science 200 years ago when a Russian expedition landed in San Francisco Bay in October 1816 and collected a single butterfly. Since then, monarchs have been observed overwintering at over 400 locations up and down the Pacific coast stretching from Mendocino County, California to Baja, Mexico. Monarchs from as far away as Idaho, Washington, and Arizona converge to cling to the branches of Monterey cypress, Monterey pine, and nonnative eucalyptus in groves along the coast during October and November. They rely on the calm, humid environment the groves provide to weather winter storms. Because they cluster in groups of tens, hundreds, thousands, or even tens of thousands, the overwintering season also provides a convenient opportunity to count the butterflies—a useful metric of the size of western monarch population.
By Emma Pelton, the Xerces Society