In all the world, no butterflies migrate like the monarchs of North America. They travel up to three-thousand miles twice a year: south in the fall and north in the spring. To avoid the long, cold northern winters, monarchs west of the Rocky Mountains winter along the California coast. Those east of the Rockies fly south to the mountain forests of Mexico. Unlike migrating birds and whales, however, individual monarchs only make the round-trip once. It is their great-grandchildren that return south the following fall.