Monarch butterfly migration

Monarch butterfly migration is the phenomenon, mainly across North America, where the subspecies Danaus plexippus plexippus migrates each summer and autumn to and from overwintering sites on the West Coast of California or mountainous sites in Central Mexico. Other subspecies perform minor migrations or none at all. This massive movement of butterflies has been called "one of the most spectacular natural phenomena in the world". The monarchs begin their southern migration from September to October. Eastern and northeastern populations, up to 500,000 monarch butterflies, migrate at this time. Originating in southern Canada and the United States, they travel to overwintering sites in central Mexico. The butterflies arrive at their roosting sites in November. They remain in their roosts during the winter months and then begin their northern migration in March. No individual butterfly completes the entire round trip. Female monarchs lay eggs for a subsequent generation during the northward migration. Four generations are involved in the annual cycle and the generation undertaking the southbound migration live eight times longer than their parents and grandparents. Similarly, the western populations migrate annually from regions west of the Rocky Mountains to overwintering sites on the coast of California. Not all monarch populations make major migrations. Monarchs migrate short distances in Australia and New Zealand. There are some populations, for instance in Florida and the Caribbean, that do not migrate, as well as another subspecies distributed in the Caribbean, Central America and northern South America. Additional overwintering sites have been identified in Arizona and northern Florida. As late as 1951, monarchs were mistakenly thought to overwinter as adults or pupae. Roosts of thousands were observed in southern regions of North America. Migrating western populations of Danaus plexippus and their overwintering sites were known long before the Mexican winter sites were discovered in the 1970s.