Every autumn, a small but mighty traveler takes flight across North America: the monarch butterfly. Their striking black and orange bodies escape cold temperatures in northern climates, and embark on annual journeys spanning thousands of miles, landscapes and generations. Unlike any other butterfly, monarchs are the only species known to make a true two-way migration, just as birds do.
In the United States, there are two populations of monarchs: the Eastern and the Western. The Eastern population journeys from southern Canada to central Mexico, while the Western population (Danaus plexippus) follows a shorter, east–west route. The summer season is spent breeding in states west of the Rocky Mountains, including California, Oregon, Washington, and Idaho, and migrating to overwintering groves along California’s coast, from Mendocino County down to San Diego County.
A Western Monarch’s Multi-Generational Migration
No single monarch makes the full round-trip migration. Instead, the journey is a relay of sorts, carried out over several generations. The overwintering generation, which is the last one of the year, is known as the “super generation.”
Come August and September, these monarchs are born, and are physiologically different from their parents. They enter a suspended reproductive development known as diapause, and live up to eight months—much longer than the typical two to six weeks—because they conserve energy and build fat reserves.
Some of these butterflies fly as far as 3,000 miles to reach overwintering sites, and their precise navigation is guided not by memory or from previous generations, but by innate instinct and environmental cues.
Studies reveal the position of the sun becomes their compass, and an internal clock in the monarch’s antennae helps them maintain a consistent path and miraculously determine time of day. Other factors such as air currents and genetic coding help these super migrators stay on course.
As they arrive on the California coast, they cluster together in sheltered groves of eucalyptus, Monterey pine, or Monterey cypress, entering a hibernation-like state in which their bodies conserve energy through the winter months.
Some overwintering sites in California’s state parks include Morro Bay State Park, Montaña de Oro State Park, Pismo State Beach, and Gaviota State Park. The eucalyptus and cypress trees of the Pismo State Beach Monarch Butterfly Grove, for instance, offer shelter and canopy.
As early spring welcomes longer days and warmer temperatures, the dormant monarchs awaken, mate and head inland to lay the first generation of eggs on milkweed plants.
By spring and summer, two or three successive generations will have continued their north-bound journey, each living only a short time to reproduce. By late summer, a new super generation emerges, destined to return to the California coast.
Why it matters
This unique phenomena and incredible journey is also deeply fragile. Since the 1980s, the number of western monarch butterflies overwintering along the Pacific Coast has fallen by more than 95%. And unlike the larger eastern population, western monarchs are at greater risk of extinction, threatened by habitat loss, pesticide use, climate change, and the decline of native milkweed.
Protecting nectar-rich flowers, breeding grounds, and safe overwintering groves is critical for their survival.
The upside is that everyday actions make a difference. Planting native milkweed –– not to be confused with the potentially dangerous tropical milkweed –– in gardens, reducing pesticide use, and supporting pollinator-friendly landscapes are powerful first steps. Organizations like the Xerces Society and Journey North also provide opportunities for community members to track and support the migration.
So the next time you see a monarch gliding past you in late summer or clustered in the trees along the coast in winter, take a moment to revel in the magic of this natural cycle –– an incredible journey carried out across miles and generations.shells.
