The Iowa Monarch Conservation Consortium
Most people are aware of the monarch’s migration to overwintering sites in central Mexico. There are about 11 mountain areas west of Mexico City where the monarchs spend the winter (Figure 1). The elevation of these sites range from 8000 to 12,000 ft, with temperatures ranging from 32 to 59 F. The monarchs cluster in trees to retain heat, occasionally branches break due to the weight of the monarchs. The size of the monarch population is estimated by measuring the area of trees covered with adult butterflies.