Monarch Butterfly population declining dramatically The butterflies migrate to their overwintering habitat along the coast where this year the numbers are so low, experts are concerned they could disappear completely. Author: Shawn Styles (Weather) Updated: 6:34 PM PST February 22, 2021 SAN DIEGO — In the 1980's there were millions of Monarch Butterflies overwintering on the coast of California, now those numbers have declined dramatically. Sarina Jepsen is the direct of endangered species at the Xerces Society for Invertebrate Conservation. "Within North America they primarily breed in the United States a little in Southern Canada and Southern Mexico and the Monarchs spend the winter in two places, the mountains of Mexico and the California coast," Jepsen said.