Monarch Butterflies Begin 3,000-Mile Migration This March

Every March, millions of monarch butterflies begin a 3,000-mile migration from central Mexico to Canada, but no single butterfly finishes the journey. It takes three to four generations working in relay, with the "super generation" starting the trip and passing it to the next wave through mating and egg-laying along the way. Scientists now track their movement in real time using devices smaller than a grain of rice, revealing how weather and climate change threaten this incredible relay race.