A Breathtaking Swarm of 100 Million Butterflies | Earth's Great Seasons | BBC Earth

In Mexico, there's a tiny creature whose spring awakening is on a truly epic scale. In the forests of the Sierra Madre mountains, the trees are festooned with monarch butterflies. They've spent the winter asleep, huddled together for warmth. There are millions of them. These trees have been the perfect roost. The branches shelter them like a blanket, keeping out the freezing nights. On a sunny spring morning, the butterflies begin to stir. When the sun warms their bodies above 13 degrees centigrade, they start to fly. More than a hundred million take to the air. It's the biggest gathering of butterflies in the world. They're getting ready for a long and arduous journey. They stream from the colony and make their way to water to drink. Then they bask in the spring sun to get their muscles warmed up for flight. Soon, they'll be off. They're all going in search of one very special plant. And it only flowers in spring. Milkweed. The butterflies won't lay their eggs on any other plant, so they must time their wake-up perfectly for when the milkweed blooms. Generations of butterflies fly nearly 5,000 kilometres to Canada, carefully following the flowering of this one particular plant. It's the greatest insect migration on Earth, and it only starts when the sun wakes them up in spring.