Monarchs east of the Rocky Mountains breed through spring and summer in a succession of short-lived generations, each adult living roughly two to five weeks. Late in the season a physiologically different generation emerges: reproductively delayed, longer-lived, and primed to fly south. These autumn migrants can survive some eight months, reaching overwintering colonies in the oyamel fir forests of Michoacán and the State of Mexico.
The return north is a relay. Overwintered adults fly back into the southern United States in spring, breed on newly emerged milkweed, and die. Their offspring continue northward, and it takes several further generations to repopulate the northern range. The butterflies arriving in Mexico each autumn are therefore separated from the previous year's arrivals by multiple generations.
How they find the same small forest patches remains partly unresolved. Experiments support a time-compensated sun compass supplemented by a magnetic sense, but the precision of the final approach is not fully explained. Monarch populations west of the Rockies overwinter along the Californian coast instead, on a shorter and differently structured route.