Every year, one of the most spectacular natural phenomena in Mexico takes place in the forested mountains west of Mexico City. During the fall, colonies of monarch butterflies migrate here from colder northern climates to find warmth and begin their breeding season. These huge flying colonies can contain as many as 20 million monarchs, which make use of air currents to travel as far as 100 nautical miles per day. The monarchs remain in Mexico from around November to March.