THE LIFE CYCLE OF THE NORTH AMERICAN MONARCH BUTTERFLY Monarch butterflies born in Canada and the United States begin their southward migration in the fall. Those west of the Rockies head for various sites in California, while those east of the Rocky Mountains fly all the way to Mexico. Some fly as many as 2,800 miles. There is emerging evidence that some Western monarchs reach Mexico as well. They start out one by one, but soon are joined by tens, then scores, hundreds, thousands and tens of thousands of fellow monarchs. Roosting together at night in trees and feasting on nectar from favorite flowers, they build their fat reserves as they head toward Mexico, a place to which they have never been before.