Fall sets in motion a journey of some 3,000 miles for these monarch butterflies, which migrate from southern Canada to their wintering habitat in central Mexico. There, they cluster together in fir trees, creating the illusion of orange, fluttering foliage. They’ll remain in their winter roosts until March, when the journey back north begins. But no one individual monarch will complete the full roundtrip, which exceeds the normal monarch lifespan; instead it will take four generations of monarchs to complete the full migration, each individual seemingly driven by an internal compass that guides its flight path.