Monarchs migrate to Mexico from across eastern North America.  They spend the winter in a very small region that's only 73 miles wide. Within the region, only 12 places have the habitat the butterflies need to survive. The sites are at a high elevation of 10,000 feet. There the oyamel forest habitat provides the microclimate conditions the monarchs need. The butterflies arrive in November and depart in March. Here's what takes place during the winter season: