Where Do Monarch Butterflies Migrate? Mexico and California

Where do eastern monarchs spend the winter?

Most monarchs that breed east of the Rockies, from southern Canada to Texas, head for a small area of volcanic mountains on the border of the Mexican states of Michoacán and México, northwest of Mexico City. There they settle in forests of oyamel fir (Abies religiosa) at around 3,000 m (10,000 ft) above sea level.

The colonies are remarkably compact. In most winters the butterflies occupy only a few hectares of forest in total, spread over roughly 8 to 14 colonies. Scientists estimate that each hectare can hold tens of millions of monarchs, packed so densely that branches sag under their weight.

The high forest works like a natural refrigerator with a blanket. It is cool enough for the butterflies to rest and save their stored fat, while the dense tree canopy shelters them from frost, rain and snow. Most of these sites are protected within the Monarch Butterfly Biosphere Reserve. Smaller numbers of eastern monarchs winter along the Gulf Coast and in Florida, where many keep breeding through the winter.

Where do western monarchs go?

Monarchs that breed west of the Rocky Mountains, in states such as Washington, Oregon, Idaho, Nevada, Utah, Arizona and inland California, mostly fly to the Pacific coast of California. More than 200 winter sites are known along the coast, with some in Baja California in Mexico. Well-known groves include Pacific Grove, Natural Bridges State Beach in Santa Cruz, and Pismo Beach.

Many western monarchs roost in eucalyptus trees introduced from Australia, as well as in native Monterey pines and Monterey cypresses. The two populations are not completely separate: tagged monarchs from Arizona have been found wintering both in California and in Mexico.

How far do monarchs fly?

Most eastern monarchs travel roughly 2,000–4,500 km (1,200–2,800 mi), and the longest journeys, from southern Canada and New England, reach about 4,800 km (3,000 mi). Western monarchs usually fly shorter distances.

The southward migration begins in August in the north and peaks in September and October. Monarchs reach Mexico in late October and early November, around the Day of the Dead celebrations.

Monarchs are excellent gliders. On sunny days they rise on thermals, columns of warm rising air, then glide long distances with little flapping. Tagging and tracking suggest average progress of very roughly 30–80 km (20–50 mi) a day, with longer days when tailwinds help. Along the way they feed on late-season flowers, build up fat for the winter, and gather at night in roosts of hundreds or thousands of butterflies, often on south-pointing peninsulas such as Point Pelee in Ontario and Cape May in New Jersey.

The super generation

The butterflies that fly south are not ordinary monarchs. They emerge in late summer, when shorter days, cooler nights and aging milkweed switch off their reproductive development, a state called reproductive diapause. Instead of mating and dying within a few weeks, they put their energy into flying and storing fat.

This generation, often called the super or Methuselah generation, lives about eight months. Summer monarchs, by comparison, usually survive only two to five weeks as adults. For more on how long butterflies live, see how long do butterflies live?

How do monarchs find their way?

No monarch has made the trip before, so the directions must be inherited. Experiments show that monarchs use a time-compensated sun compass: they track the sun’s position and the pattern of polarized light in the sky, and correct for the time of day using an internal clock.

Remarkably, a key part of that clock sits in the antennae. In a 2009 study, monarchs whose antennae were removed or painted black lost their consistent southward heading. Later experiments suggested that monarchs also carry a magnetic compass, which may help when the sun is hidden.

Mountain ranges and coastlines help funnel the migrants toward Mexico. How the butterflies pinpoint the same small patches of forest each year is still not fully understood. Birds face similar puzzles, explained in how do migrating birds navigate?

The return north in spring

As the days lengthen in February and March, the overwintering monarchs mate and leave the mountains. They fly north into Texas and nearby states, lay eggs on fresh spring milkweed and die. Their offspring continue north, and over two or three more generations monarchs recolonize the northern United States and southern Canada by early summer.

The cold of the Mexican winter seems to flip the sun compass from south to north. In laboratory experiments, autumn migrants that were not exposed to winter cold kept heading south the following spring.

The monarch journey is famous, but it is not the longest insect migration. The painted lady’s relay between Africa and Europe covers even more ground; see how far do painted ladies migrate? and our list of the longest animal migrations. The monarch butterfly profile covers the rest of its life story.

When and where to see migrating monarchs

In Mexico, several sanctuaries welcome visitors during the winter season, roughly from November to March, including El Rosario and Sierra Chincua in Michoacán and Piedra Herrada in the State of Mexico. The butterflies are most active on sunny afternoons, when they fly down to drink.

In California, the coastal groves usually host monarchs from about October or November to February. During the autumn migration, peninsulas such as Cape May, Point Pelee and Peninsula Point in Michigan can attract large numbers of roosting monarchs in September.