A number of insect species migrate long distances, but monarch butterflies stand out for their epic North American migrations. Each spring and fall, monarchs travel thousands of miles to and from their winter roosts. A western population overwinters in California, and travels inland in warmer months. An eastern population flies from Mexico to Canada and back again. It takes the butterflies several generations to complete the journey—they reproduce along the way, and a new group continues on. But even a portion of the trip is serious mileage for a flying insect that weighs as little as a paper clip.

Before the year 2000, hundreds of millions of eastern monarch butterflies overwintered in central Mexico each year; at least four million western monarch butterflies did so in California. But since then, numbers in both populations have fallen dramatically, and though they fluctuate from year to year, a downward trend is clear. The reasons are many: monarch caterpillars eat only milkweed plants, but these are being plowed under, or sprayed with herbicide. Insecticides also play a role in declines, as do illegal logging near the butterflies’ roosts in Mexico and development in California near overwintering sites. But people all over North America are stepping up to plant native milkweed and nectar plants, and government agencies in Canada, Mexico and the U.S. are working to protect overwintering sites and to restore breeding habitat.