Monarch butterflies are not able to survive the cold winters of most of the United States so they migrate south and west each autumn to escape the cold weather. Also, the larval food plants do not grow in their winter overwintering sites, so the spring generation must fly back north to places where the plants are plentiful. The monarch migration usually starts in October, but can start earlier if the weather turns cold sooner than expected.
The monarch butterflies will spend their winter hibernation in Mexico and some parts of Southern California where it is warm all year long. If the monarch lives east of the Rocky Mountains, it will usually migrate to Mexico and hibernate in oyamel fir trees (photo courtesy U.S. Fish and Wildlife Service). If the monarch butterfly lives west of the Rocky Mountains, then it usually hibernates in and around Pacific Grove, Calif., in eucalyptus trees.