Every year, from January to the end of March, millions of monarch butterflies make a 4000 kilometre migration to breed in El Rosario where the world's largest colony of monarch butterflies are found. When the sun starts to warm up the butterflies they wake up and around noon they invade the forest and its surroundings, returning to the forest in the late afternoon to find the trees where they will rest for the night. The butterfly's El Rosario sancturary was only discovered in 1976.