Scientists do not count each eastern monarch. They estimate the population by measuring the area of Mexican forest covered by roosting monarchs, in hectares. As of November 2026, a hectare of roosting monarchs holds an estimated 21 million butterflies, though the number varies from year to year. As of February 2024, in winter 2023-24, eastern monarchs covered only 0.9 hectares of Mexican forest, less than half the winter before. As of March 2025, in winter 2024-25, eastern monarchs covered 1.79 hectares, nearly double the winter before. As of November 2026, in winter 2025-26, eastern monarchs covered 2.93 hectares (about 7.24 acres) of forest, up about 64 percent from the winter before. Mexico protects the mountain forests where monarchs spend the winter in the Monarch Butterfly Biosphere Reserve.

As of November 2026, western monarchs have fallen by more than 95 percent since the 1980s. As of November 2026, eastern monarchs have fallen by 80 percent or more since the 1990s. Monarchs have fewer places to lay eggs because milkweed has been lost from many fields and roadsides. Weed killers kill the milkweed caterpillars need and the wildflowers adults feed on, and insect sprays can kill monarchs. In December 2024 the U.S. Fish and Wildlife Service proposed to list the monarch as threatened under the Endangered Species Act; as of November 2026 it is a proposal. People can help monarchs by planting milkweed in yards and gardens, and the best milkweed to plant is the kind that grows naturally where you live.

Monarchs begin the trip south when shorter days, cooler weather and aging milkweed tell them that winter is coming. Monarchs use the sun to stay on course, and they have a backup compass for cloudy days. The sun moves across the sky but the monarch's goal stays put, so monarchs use an inner clock to correct their sun compass for the time of day. Butterflies can sense polarized light, a pattern in skylight that people cannot see, and monarchs may use it with the sun to steer. A monarch's sense of direction for the trip south is inherited, not learned, because each migrant is on its first trip. Monarchs fly north in spring over three to five generations, because each generation lays eggs and then dies.

Milkweed has poisons called cardenolides. Monarch caterpillars store them and use them as a defense against predators. The viceroy butterfly copies the orange-and-black look of bad-tasting butterflies like the monarch and queen, and scientists once thought the viceroy was only a harmless copy of bad-tasting butterflies. A study found that viceroys in northern Florida, where queens are rare, have become bad-tasting too. Some butterfly colors come from tiny structures in the scales that bend and reflect light, and not from pigment; the blue of a Blue Morpho is one example. The ridges on Blue Morpho scales are stacks of thin layers that reflect blue light strongly, like a tiny mirror tuned to blue.

Words to know

• hectares: land areas
• polarized: lined-up light
• inherited: passed down
• migrant: traveling animal