Monarch butterflies wintering in Mexico occupied roughly 4.4 acres of forest during the 2024-2025 season, nearly double the previous year’s total, according to survey results released by the World Wildlife Fund-Telcel Foundation Alliance. The increase offered a rare piece of encouraging news for a species whose numbers have trended downward for decades.

Researchers track the eastern monarch population each winter by measuring how much forest area the butterflies cluster in in central Mexico, a proxy for overall population size that scientists have monitored since the early 1990s. This year’s rebound is largely attributed to favorable weather conditions during the fall 2024 migration, which appears to have helped more butterflies complete the thousands-of-miles journey from breeding grounds across the eastern United States and Canada down to their overwintering sites.

The news stood in sharp contrast to the species’ western population, which saw its second-lowest overwintering count on record in California, plunging to just over 9,100 butterflies from more than 233,000 counted two years earlier. That divide has added urgency to a federal proposal, issued in December 2024, to list the monarch as a threatened species under the Endangered Species Act — a designation that would extend legal protections to habitat critical to both populations.

Despite the encouraging uptick, researchers caution that monarch populations are naturally cyclical and that a single strong year doesn’t reverse the long-term decline driven by habitat loss, pesticide exposure and climate-related disruptions to migration patterns. Conservation groups say sustained gains will depend on continued efforts to restore milkweed habitat and protect the forests monarchs rely on each winter.