The spring migration of monarch butterflies

What makes a monarch so special?

The monarch, one of the most beautiful flying insects in the world, is also one of the very few that migrate. Its brain, only the size of a pinhead, is also a marvel, acting like a minicomputer that uses the sun as a kind of compass on its long journey. The monarch weighs no more than a penny, with a bright orange-and-black wingspan about the width of a child’s hand, yet it is capable of migrating up to 3,000 miles. In recent years the migration of the monarchs has taken on a mystique rivaling that of the great whales and African elephants. There is even a campaign to name it the national insect.

Why are monarchs in danger?

The Week

Escape your echo chamber. Get the facts behind the news, plus analysis from multiple perspectives.

SUBSCRIBE & SAVE

Sign up for The Week's Free Newsletters

Join more than 350,000 subscribers and keep yourself informed with a selection of The Week’s most interesting, enlightening and entertaining stories - plus daily puzzles.

Join more than 350,000 subscribers and keep yourself informed with a selection of The Week’s most interesting, enlightening and entertaining stories - plus daily puzzles.

Sign up

Monarchs cannot survive temperatures below freezing. In January, an estimated 225 million butterflies perished in a freak cold front in central Mexico—the worst episode of butterfly mortality in more than 25 years of record-keeping. The previous year, many millions more also died from harsh conditions. Some scientists attribute the high death tolls to the gradual disappearance of the protective forests, which leaves the monarchs exposed to the elements during the winter months. Loggers are destroying the forests at a rate of 2 percent per year. The Mexican government has created a protected area of 216 square miles. Yet local residents ignore restrictions on logging, saying they are too poor to care about butterflies. Another danger is posed by herbicides used in the American farm belt. Though they don’t kill the monarchs directly, they do destroy the milkweed that monarch caterpillars feed on. Insecticides used to kill mosquitoes are also fatal to monarchs, as is pollen from genetically grown corn.

Where do monarchs come from?

During the winter an estimated 250 million monarchs swarm the trunks and branches of fir trees in the Oxymel fir forests, in a volcanic mountain range 150 miles west of Mexico City. There are two main sanctuaries, the Rosario and Sierra Chincua colonies, which together harbor about two-thirds of all butterflies in Mexico, as well as nine other known smaller colonies; these forests act as a kind of umbrella against rain and as a heat-retaining blanket. Monarchs can also be found in other tropical and subtropical parts of the world, including Hawaii and Indonesia.

Why do they migrate?

In the spring the warming sun wakens the monarchs and they begin to mate furiously. Then in a feverish rush the females leave the males behind and migrate north to where the first milkweed is coming up along the U.S.’s Gulf Coast. Monarchs feed on the milkweed, extracting and storing its poison in their tissue. The poison, loudly advertised by the bright color of its wings, protects the butterfly from predators like birds and mice. But this expenditure of energy, during a journey that may take them as far as 700 miles to the north, drains the parents of life. After laying their eggs, they die off. Their offspring, however, continue the trip, following the milkweed as it appears. The gestation of butterflies, from egg to larva (caterpillar) to pupa (a resting phase) to butterfly, takes about a month. Two or three generations live and die during any given summer in the north. Five generations may elapse before a monarch’s great-great-great-grandchildren return in the fall.

How do they know where to migrate?

The migration of monarchs appears to be genetically programmed, not learned, since no monarch ever makes the journey twice. Yet hundreds of monarch studies have so far failed to come up with a definitive answer. One theory is that monarchs use the sun as a compass.

When will monarchs arrive in the U.S.?

This year the migrating monarchs are expected to reach the central United States in mid-April, just after the last frost, and then continue on to most northern border states and southern Canada. The U.S. has two different populations of monarchs, one to the east of the Rockies and another to the west. The western monarchs spend the winter in California; those to the east winter in Mexico.

When do they return to Mexico?

During the summer and fall, monarchs lose all interest in sex. But they do become irresistibly attracted to flowers, cloaking goldenrods and daisies while sucking up nectar to nourish themselves for their long journey south. In late October and November they return to Mexico, arriving at their winter home with shattered wings after a 3,000-mile journey. Their arrival coincides with Mexico’s Day of the Dead celebrations. One ancient tradition holds that the butterflies represent the spirits of the villagers’ ancestors. Back in their pine-forest havens, the monarchs huddle in densities of up to 6 million per acre. Stored body fat helps them to survive the winter.

Discovering their winter home

For centuries, no one knew where monarchs went when they vanished from their northern habitats. Then in August 1976 National Geographic published “Found at Last: The Monarch’s Winter Home,” by Canadian zoologist Fred A. Urquhart. After tracking the migration of monarchs for nearly 40 years, he knew only that they followed a diagonal flight path through Texas and spent the winter somewhere in Mexico. Finally in 1972 his wife, Norah, wrote to newspapers in Mexico, appealing for volunteers to report sightings. Kenneth C. Brugger, an American crisscrossing the Mexican countryside in his motor home, wrote to say he would keep an eye out. On Jan. 9, 1975, he excitedly phoned Urquhart to say, “We have found them—millions of monarchs—in evergreens beside a mountain clearing.” The Urquharts, accompanied by a photographer, visited the Oxymel site early the next year. “At 10,000 feet,” he later wrote, “as we walked along the mountain crest, our hearts pounded and our feet felt leaden.” Then they saw them: millions upon millions of butterflies swirling in the air and carpeting the ground several feet deep, as far as the eye could see.