One of North America's most dazzling natural phenomena, the annual winter migration to central Mexico by millions of monarch butterflies from the northern United States and Canada, has shrunk to record lows and is in danger of ending. The monarch migration has been documented in books and movies and attracts thousands of tourists to a nature preserve about 100 miles west of Mexico City. The black-and-orange butterflies hang from the trees there like shaggy beards. In the 20 years since environmentalists began keeping detailed records of the monarch's winter habitats, the butterflies have covered as much as 45 acres of forest in the Mexican state of Michoacan. But the most recent winter count showed how far the migrating monarch population has fallen: As of December, they blanketed just 1.6 acres of forest, the smallest area yet. The butterflies face numerous threats across North America. In Michoacan, illegal logging has cut into their winter habitat in the oyamel fir trees, although government conservation efforts have slowed the rate of deforestation. In the United States and Canada, herbicides used in industrial-scale farming have destroyed the milkweed plants where they lay their eggs. Omar Vidal, the director of the World Wildlife Fund's Mexico office, said he wants North American leaders to agree on a plan to protect the monarch, saying the migration symbolically unites our three countries."