While in Mexico, the monarchs gather in very large numbers to rest. They perch on suitable trees and bushes by clinging (hold on to tightly) with their feet. They cling to each other making layers and layers of resting Monarchs overwintering. This was discovered by Fred Urquhart of the University of Toronto, who studied monarch butterflies for nearly forty years. He knew the Monarchs migrated because he would stop seeing them after a certain time of the year. He put a label on each Monarch he found. On this label was his name, address, and a request to send him the butterfly if it was found.