The Seven-Acre Winter
Every autumn, eastern North America's monarch butterflies fly to a few mountaintops in central Mexico. Scientists measure the whole population by how much fir forest its clusters cover. That area has shrunk a lot.
The forest area the entire eastern migration covered in winter 2025–26: about 7.2 acres, or four football pitches. That's 64% more than the year before, but 84% less than the 18.19-hectare record of 1996–97.
Two migrations, two winter homes
East of the Rockies, monarchs fly up to ~4,800 km to central Mexico. West of them, they head for groves on the California coast.
Eastern population. These butterflies breed across the U.S. Midwest, East and southern Canada. In winter they pack into a few dozen colonies in the oyamel fir forests of Michoacán and the State of Mexico. The 2025–26 survey found just nine colonies.
Western population. These butterflies breed across the West and spend the winter in eucalyptus, pine and cypress groves along the California coast. Volunteers count them one by one every November.
The whole migration, measured in hectares
Each winter, WWF and Mexico's protected-areas agency map the forest the colonies cover. Every eastern season since 1993 falls on this strip.
Thirty-three winters in Mexico
Year to year, the area swings widely. The long-run direction is still down, and no season since 2003–04 has reached even 7 hectares.
Hectares of forest occupied by overwintering colonies, by season. The dashed line is the 2015 U.S. Pollinator Health Strategy goal of ~6 ha (about 225 million monarchs).
From millions to a few thousand
The western population has fallen further still. The 2025 count of 12,260 is 99% below the first count in 1997.
Monarchs counted at California and western overwintering sites during the annual mid-season count. The 1980s figure (hatched) is a reconstruction from earlier counts, not a direct tally.
Fewer milkweeds, fewer caterpillars
Monarch caterpillars eat only milkweed. As herbicide-tolerant crops spread, the milkweed growing in Midwestern fields largely disappeared.
Habitat loss isn't the only cause. Researchers also point to extreme heat and drought along the flyway, insecticides, and the thinning of the high-altitude fir forests that shelter the colonies. In the Pleasants & Oberhauser study, each year's Midwest production tracked the size of the next winter's colonies in Mexico.
A migration near its threshold
Both populations are now small enough that one bad winter or storm could push them below the point of recovery.
Estimated chance that the eastern migration falls to "quasi-extinction" within 20 years, depending on the threshold used (Semmens et al., 2016).
Experts set 30,000 butterflies as the western quasi-extinction threshold. The counts in 2018, 2019, 2020, 2024 and 2025 all fell below it.
The U.S. proposed listing the monarch as threatened in Dec 2024. The decision was delayed, and a court settlement now sets a deadline of June 2030.
Every patch of milkweed counts
- Plant native milkweed and nectar flowers that bloom in late summer. Choose species native to your region. In the South and along the California coast, avoid tropical milkweed, which can spread disease.
- Skip insecticides in gardens and yards, including systemic neonicotinoids that make the plant itself toxic to caterpillars.
- Count them. Volunteers run the Western Monarch Count and programs like Journey North and the Integrated Monarch Monitoring Program. Their records are the main source of this data.
- Back the forests. Support groups that protect the Monarch Butterfly Biosphere Reserve and California's overwintering groves. Losing these small sites would mean losing the whole migration.