Instead, they spend the winter in roosting spots. There the Mexican Government has set-up a Monarch sanctuary. Monarchs sometimes cover it's preferred host, the Oyamel Fir, top to bottom. The migration is driven by seasonal changes. Day length and temperature changes influence the movement of the Monarch.
Monarchs are the only butterflies to make such a long, two-way migration every year.
Amazingly, they fly in masses to the same winter roosts, often to the exact same trees. It will take about five generations to make the complete round-trip. This one-way movement is properly called emigration. In tropical lands, butterflies do migrate back and forth as the seasons change. At the beginning of the dry season, the food plants shrivel and the butterflies leave to find a moister climate.