The annual Monarch butterfly migration is an amazing phenomenon. No other butterfly migrates in such massive numbers over such large distances.
Millions of Monarchs travel every year from the colder regions of North America to warmer sites where they can safely overwinter. They fly, en masse, as high as 10,000 feet, returning to the same groves as preceding generations.
Monarch butterflies usually only live for about six weeks, but the migrating generation can live for up to eight months. So Monarch butterflies manage to travel thousands of miles to arrive at a destination they've never been to before. How do they do it? Scientists have theories, but no one really knows.
Monarch butterflies can be roughly separated into two groups: those east of the Continental Divide (in the Rockies) and those west of the Continental Divide.
The eastern population numbers about ten million and travels up to three thousand miles from as far north as Canada to overwintering sites in central Mexico.
The western population numbers about one or two million and travels up to two thousand miles from as far north as British Columbia to overwintering sites along the California coast. They can be found all along the coast from Mendocino to San Diego, but the largest Monarch butterfly groves are found in the central coast from Santa Cruz to Los Angeles.
In early October, as the days shorten and the weather gets cooler, the Monarch butterfly migration begins. By November, thousands of Monarch butterflies can be seen in groves of pine and eucalyptus at more than 200 sites along the coast. In fact, tens of thousands of individuals may populate a single grove, where they hang from the trees in dense clusters for protection from the winds and to preserve warmth.
During the winter months, the butterflies feed on nectar, which is one reason they often choose the winter blooming eucalyptus for their roosting trees. The larvae, however, feed exclusively on the milkweed plant, which contains bitter chemicals that protect the Monarch from birds and other predators. Come spring, the adults start searching for milkweed on which to lay their eggs.
If you'd like to attract Monarch and other butterflies to your own backyard, plant some milkweed (Asclepias species). Seeds and plants are readily available, and milkweed is colorful and easy to grow.
In late February, the Monarchs become more active and begin to move into California's central valley, the Sierra Nevada foothills, and north to Oregon and Washington. By March, most have left their winter homes, and by the end of summer their offspring can be found as far north as British Columbia. Come fall, a new generation will wend its way back to the groves along the California coast.