Where do monarch butterflies migrate?

Each autumn, monarch butterflies from eastern North America travel as far as 4,800 kilometres to spend the winter in the oyamel fir forests of the Sierra Madre mountains in the Mexican states of Michoacán and México. Hundreds of millions of butterflies cluster so densely on the trees that branches bend under their weight, concentrating in an area of just a few hectares. No single butterfly completes the round trip: it takes three to four generations to return north in spring, yet each autumn generation somehow navigates to the same overwintering groves its great-grandparents used. Western monarchs, by contrast, winter along the California coast near Pismo Beach and Pacific Grove.