The North American monarch butterfly migration spans four generations per year. No single monarch completes the entire round trip between the northern breeding grounds and the overwintering sites in Mexico; the journey is passed through successive generations.
Monarchs east of the Rocky Mountains generally overwinter in the mountains of central Mexico. In spring, the first generation begins moving north and reproduces along the way. Its offspring continue the journey, followed by additional generations that eventually recolonize the northern United States and southern Canada.
The fourth generation is especially unusual. The fall-migrating generation enters reproductive diapause, allowing it to live far longer than ordinary summer monarchs. It can survive the southbound journey, winter in Mexico, and begin the next northward cycle.
A common mix-up is saying that four generations all migrate the same distance. In reality, the migration is multi-generational: the second, third, and fourth generations help return north, while the long-lived fall generation travels south and survives the winter. Western monarchs follow a separate route to California.