The Butterflies’ Great Migration

Each year, a new generation of Monarchs flies south for the winterbut habitat loss is making the journey harder. An Object Lesson.

Monarch butterflies take flight at the Piedra Herrada sanctuary in Central Mexico. (Felipe Courzo / Reuters)

The town of Valle de Bravo is one of Mexico’s beloved “Pueblos Magicos” or “Magical Towns,” a place full of sensory delights. White stucco walls trimmed in rust-colored paint hug narrow, cobbled streets. The market bursts with the colors of fresh citrus and plump avocados. Church bells ring inside twin bell towers in the bustling town square. Nearby, the cool, clear water of Lake Avandaro shimmers in the constant winter sunshine. A sense of nostalgia and magic pervades the town, but a few miles away, in the lush oyamel fir forests, the real magic takes flight.

The Piedra Herrada Butterfly Sanctuary outside Valle de Bravo provides a temporary home to more than 2 million Monarch butterflies that find refuge each winter in its forested microclimate. The air in the forest is thin, cool and peaceful; on early winter mornings, horses carry visitors up the steep dirt path to the Monarchs’ roosting place.

Because of the butterflies’ sensitivity to cold temperatures, tens of thousands of them cling together for warmth on the branches and trunks of the oyamel trees. Though the Monarchs weigh less than half of a gram each, the branches sag under the weight of these clusters. Thanks to the dull underside of the insects’ wings, they’re nearly unrecognizable, camouflaged against the bark and foliage—but when the sun rises and light filters through the branches, the butterflies awaken, flutter, and expose their vibrant colors. As the temperature rises throughout the morning, the butterflies begin their journey downhill, where they’ll drink water from the salty mud ponds and eat nectar from blossoming flowers before flying back up to their trees. When they fly, thousands at a time, the flutter of their wings sounds like a soft, soothing rain or leaves rustling in a light wind.

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Every year in late summer and early fall, the Monarchs that live in eastern Canada and the United States escape the impending winter weather and migrate to Central Mexico, where the wintertime temperatures and humidity levels are more agreeable. The (Monarchs in western North America travel to the similarly temperate southern California, where they roost in fir and eucalyptus trees.) Researchers aren’t sure what triggers the butterflies’ migration, but every year, some instinct propels them towards their southern homes, where they live from October to March.

The Monarchs’ annual journey covers around 2,500 miles each way. They travel about 25 to 30 miles per day, using tail winds, thermals, and air currents to glide and conserve energy. Monarchs are unique among insects because they migrate both north and south, like birds. But unlike birds, an individual butterfly never makes a full two-way trip. Instead, a succession of four to five generations must interlink to complete the round-trip migration.

Towards the end of winter, as the days become slightly warmer and longer, the Monarchs prepare for their northern return. Once they reach the southern United States from Mexico, they search for milkweed on which to lay their eggs. The butterflies that emerge from these eggs represent the start of a new life cycle, and they are known as the year’s first generation of butterflies.

The Monarch’s metamorphosis from egg to caterpillar to chrysalis to adult takes approximately one month to complete. When the butterflies of the first generation reach adulthood, they lay eggs of their own further along on the path back north, sparking a spring-summer cycle; the process will repeat itself for three subsequent generations of butterflies, each of which will carry the migration forward over a lifespan of two to five weeks.

But the butterflies of the last generation in the cycle, the ones that emerge in late summer, have a different job to do. These butterflies, often called the final generation, must turn around and make the migration south to the Monarchs’ winter sites, finding a patch of trees thousands of miles away when they’ve never traveled there before. The butterflies in this final generation are also physiologically different from the previous generations, emerging from the chrysalis in a state of reproductive diapause that prevents them from breeding. As a result, these butterflies are able to conserve their much-needed energy for the migration. It’s only after the winter has passed, shortly before their journey back north, that the diapause is released and they begin breeding again.

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The Monarch butterfly—with its characteristic orange wing, patterned by a web of black veins and small white dots—represents nature at its most bewitching. But it also embodies nature’s fragility and ephemerality. With the growing threats of climate change and habitat loss, the Monarch’s numbers continue to drop every year, and its fate looks increasingly uncertain. In fields across the U.S., milkweed has been displaced by corn and soybean seeds, and herbicide use has further inhibited milkweed growth. The plant’s dwindling numbers, in turn, have adversely impacted the Monarch’s habitat along the migratory route. According to The New York Times, the number of Monarchs migrating from the U.S. has dropped by 90 percent in the past 20 years.

Michael Fiala / Reuters

Not all threats can be controlled. Because they travel and cluster together in concentrated populations, Monarchs are very susceptible to extreme weather conditions, from drought to strong storms. One early frost or unprecedented downpour could annihilate a whole concentration of roosting butterflies. Still, additional conservation efforts can help, and some organizations have begun taking steps to protect these fragile animals.

In November 2008, UNESCO declared the 139,019-acre Monarch Butterfly Biosphere Reserve in Mexico a World Heritage Site. Before this point, the logging industry destroyed much of the Monarch’s Mexican habitat; now, logging is tightly controlled, and maintaining the forests at sites like the Piedra Herrada Sanctuary is more of a priority. The state police even enforce a speed limit of nine miles per hour near the sanctuaries.

More recently, in February 2014, President Barack Obama, Mexican President Enrique Pena Nieto, and Canadian Prime Minister Stephen Harper met for a one-day summit to discuss a shared commitment to conserve the Monarch and restore its dwindling habitats from Canada to Mexico. A few months later, First Lady Michelle Obama planted the White House’s first pollinator garden, which contains milkweed and other plants that Monarchs feed on. The following spring, the White House released a plan to promote similar gardens to foster the insects’ nourishment and reproduction across the country. Gradually, as more milkweeds are planted and pollinator gardens proliferate, the Monarch—along with bees and other butterflies—will find new opportunity to flourish.

This article appears courtesy of Object Lessons.