Each fall, monarch butterflies retreat to groves along the Pacific coast in California to settle in for the winter. Every year, the Xerces Society, an organization dedicated to conserving all kinds of invertebrates, gets together to count how many arrive for their annual vacation.

The butterflies nest in about 400 locations from Mendocino County here in California, all the way down to Baja California in Mexico, according to Emma Pelton, a biologist with the society.

For three weeks surrounding Thanksgiving, the Xerces Society will work with volunteer citizen scientists to count butterflies at nearly 300 of those nesting spots.

The annual count has led to the publication of State of the Monarch Butterfly Overwintering Sites in California, which found that the monarch butterfly population in California’s overwintering sites is down 74 percent.

To find out how to volunteer with the Western Monarch Thanksgiving Count or what else you can do to help create new habitats for monarch butterflies and other invertebrates, visit the Xerces Society.