　The documentary depicts monarch butterflies’ great and mysterious migration. Each year’s autumn, millions of monarch butterflies start their incredible southward migration from Canada, cross America, to Mexico. They fly above 2,000 miles to arrive the mountain of Mexico .....The film opens with caterpillars munching milkweed in southern Canada in late summer. Soon each caterpillar transforms itself into a silky chrysalis. Roughly 10 days later, a delicate four-winged monarch emerges.Then, at some unknown signal, the monarchs take to the air on a two-month, 2,000-mile flight over fields, forests, cities, plains, open water, deserts, and finally mountains to congregate in a tiny, high-altitude region of central Mexico where they've never been before. Incredibly, they arrive by the millions at the same time each year.Shedding light on this natural wonder are some of the world's leading monarch researchers, including Lincoln Brower of Sweet Briar College, independent biologist Bill Calvert, and Orley "Chip" Taylor of the University of Kansas.Putting the monarch phenomenon into perspective, Taylor says, "You've got a butterfly that's originating in Toronto, or it's originating in Detroit, Michigan, or it's coming down from St. Paul or maybe even Winnipeg, and it's moving south. Somehow it finds its way to Mexico. Could you do that?"No one yet knows how the butterflies do it, but Taylor's research reveals that they are expert navigators. In one experiment, he transported Mexico-bound monarchs from Kansas to Washington, DC, and then set them loose. At first, they flew south as if they were still in Kansas—a course that from Washington would miss Mexico entirely. But after a few days, they corrected their flight path, as if some inborn GPS unit had alerted them to the true direction of their destination.In another sequence, NOVA accompanies celebrated monarch watcher Bill Calvert around backcountry Texas as he looks for signs of the monarch migration. Sure enough, they show up en masse and on time, heading toward the Sierra Madre mountains across the border—the last leg of their flight.And in the Mexican state of Michoacán, NOVA joins mountain villagers as they celebrate the arrival of the monarchs in the first week of November. The butterflies' arrival marks the start of a celebration called the Day of the Dead, since the local people have traditionally associated the monarchs with the returning souls of their departed ancestors.Unfortunately, illegal logging in the Mexican butterfly sanctuaries threatens the unique habitat that monarchs depend on for their survival. Monarchs may not yet be an endangered species, but their annual migration is an endangered phenomenon that could dwindle to insignificance if the giant firs that they cling to during the winter disappear.Gone also would be the colorful festival that closes the program—a fireworks display welcoming the hardy fliers to Mexico, with orange bursts against the black sky, looking almost like the beautiful cloaks of the monarchs..[收起部分]