Milkweed has poisons called cardenolides. Monarch caterpillars store them and use them as a defense against predators. A monarch's bright colors warn birds that it tastes bad. The viceroy butterfly copies the orange-and-black look of bad-tasting butterflies like the monarch and queen.

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.

Storms and big swings in temperature can kill huge numbers of monarchs, and climate change is making odd weather more common. Forests where monarchs spend the winter are shrinking because of cutting of trees, storms and building. Weed killers kill the milkweed caterpillars need and the wildflowers adults feed on, and insect sprays can kill monarchs.

Monarchs begin the trip south when shorter days, cooler weather and aging milkweed tell them that winter is coming. On the trip south, monarchs gather along coasts and on peninsulas, where the shape of the land funnels them toward the shortest crossing of open water.

The monarchs that fly to Mexico have never been there before, yet they find the way. Monarchs use the sun to stay on course, and they have a backup compass for cloudy days.

As of November 2026, scientists have named about 18,000 to 20,000 kinds of butterflies. There are many more kinds of moths. Butterfly antennae have a club at the tip. Moth antennae are feathery or come to a point. The line between butterflies and moths is fuzzy. Some moths fly by day and some are brightly colored.

A butterfly's wings are covered with tiny scales. 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.

Words to know

• cardenolides: milkweed poisons
• hectares: land areas
• roosting: resting together
• funnels: guides in