The migration of the monarch butterfly is one of the wonders of the natural world. Each autumn, a new generation of monarch butterflies is born in the northern United States and southern Canada. Hundreds of millions of these butterflies then fly to the mountains of Central Mexico, between 4,000km and 4,800km away. There, they overwinter in forests of the sacred fir Abies religiosa at high altitudes. Without these sacred firs, the monarchs couldn't survive their grueling migration.