Taiwan's Purple Crow Butterfly Migration

Hidden in a mountain valley in southern Taiwan lies one of the world’s great natural wonders. Every winter tens of thousands of butterflies blanket the branches and forest understory of Maolin. If you’re lucky enough to see them take flight, you’ll witness their wings, usually a somber brown with small pink specks, flash blue and violet as the forest itself flickers like a living mosaic of iridescent colors.

The seven valleys of Maolin are Taiwan’s best-known winter refuges for purple crow butterflies: a collection of four similar-looking species known in English as the blue-banded king crow, the double-branded crow, the striped blue crow, and the dwarf crow. Each October, hundreds of thousands of them embark on a months-long journey south to gather in secluded valleys like those in Maolin. Come spring, they’ll leave to lay their eggs all over the island, ready to begin the cycle of life anew.

It’s a similar phenomenon to the monarch butterfly migration in North America, which sees millions of butterflies migrate along the coasts of the United States to spend the winter in natural sanctuaries in central Mexico. Although the North American monarchs are one of the world’s best known migratory species, the purple crow migration is fiercely beloved by nature lovers in Taiwan. The attention that the migration has received in recent years has even inspired some ordinary people to do what they can to ensure future generations can marvel at the migration in all its glory.

And unlike the monarchs, the multispecies purple crow migration is globally rare, particularly one as short as the 300 miles they journey across Taiwan. Scientists are still trying to better understand why these four species behave this way—and the clock is ticking: Like the monarchs, this butterfly migration is imperiled. Saving this wondrous phenomenon starts with understanding it.

Taiwan’s immense biodiversity has earned it the nickname “the butterfly kingdom”: it’s home to over 400 butterfly species, 55 of which are found nowhere else in the world. In that kingdom, there are plenty of butterfly obsessives—yet in the purple crow realm, one man reigns supreme. Chan Chia-lung is an insect scientist and filmmaker who has dedicated his life to studying the purple crow butterflies—and whose tireless work over the last 25 years is what made their winter migration such a cultishly beloved phenomenon. And he’s still at it: Just two years ago, his feature-length documentary, Lost Butterfly, introduced a generation of Taiwanese viewers to their beauty.

The first time Chan himself learned about the purple crow butterflies was in elementary school, he says, when his aunt gave him a photo book about them. But it wasn’t until Chan visited the Maolin butterfly valleys as a college student in the early 1990s that he was humbled by the sight of thousands of them. “As the butterfly flaps its wings, the angle that the sunlight reflects off them and hits your eye changes. Out of 17 different colors, there's only one angle where your eye can see the iridescent purple color," Chan says.

The effect is so pronounced that tree branches, at first glance seemingly covered in dead leaves, reveal themselves to be swarming with hundreds of butterflies, only noticeable when their wings catch the sunlight. When Chan first visited, the butterfly valleys of Maolin were known mainly to insect experts, ecologists, and people in the area. Maolin is the island’s least populated district—at least when it comes to people. Only about 1,800 people live here, mostly from the Indigenous Rukai community which has called these valleys home for countless centuries.

Today, however, nearly a hundred thousand people descend upon Maolin between November and March of each year to take in the incredible sight. That shift is largely thanks to Chan’s work: a mission he took up in 1999 after discovering his most beloved butterfly viewing spot in the region had been turned into a parking lot. “In just that one moment, I realized even the most common species could be endangered,” Chan says.

On a weekend morning in a forest a short drive from the bustling downtown of the northern capital, Taipei, a group of mostly retired volunteers with the Taiwan Butterfly Conservation Society gathers near a local hiking trail. Everyone brings a snack to share. While munching on rice balls and steamed pork buns, they prepare long butterfly nets, clipboards, binoculars, and notebooks. Once everyone’s ready, volunteer Tsai Ke-Hsiong leads the group uphill.

By late November most purple crows have flown south, but a few linger in the north. When Tsai spots a purple crow butterfly, he starts swinging his net, working fast but gently: the goal is to trap the butterfly without harming its delicate wings. Scientists studying bird migrations often use GPS trackers to map a bird’s flight path. But butterflies are too light for that. Instead, once he’s caught a butterfly, Tsai delicately scribbles a series of letters and numbers on its wing. After he releases the butterfly, the hope is that another volunteer will catch it again elsewhere in Taiwan and record its code again. The resulting data will help researchers understand its life cycle and migration path.

As another longtime volunteer Zhuo Ching-bo explains at the basement of the Butterfly Conservation Society’s modest office in Taipei, global conservation institutes like the International Union for Conservation of Nature don’t currently list the purple butterflies as endangered. “But what we can say is that butterfly habitats themselves are disappearing.”

Chan explains that Taiwan’s butterfly habitats are often lost to factories and urban development as the island seeks to maintain its global lead in semiconductors and other high-tech manufacturing. This harms not just overwintering habitats in places like Maolin, but also a complex network of butterfly superhighways: interconnected valleys and mountain paths that the butterflies use to traverse the island.

Hsu Yu-feng, a life-sciences professor at National Taiwan Normal University in Taipei, says that when he started teaching in the city in the mid-1990s, the butterflies still thrived in nearby wetlands, forests and marshes. Decades later, high-density housing, factories, and offices have changed all that. “In the warmer months of the year when the butterflies migrated north, you could see hundreds of them all over campus,” Hsu says. “Now, you can only find them in the mountains or forests outside the city.”

Climate change is a factor as well, says Liao Jin-shan, a conservationist in Maolin. Rising temperatures and unpredictable weather patterns are wreaking havoc on the life cycles of the native plants the butterflies feed on. Rising temperatures may also render the purple crow’s southern migration useless: The Tropic of Cancer splits the island of Taiwan into a subtropical zone in the north and a tropical zone in the south. As northern Taiwan gets warmer, the need to migrate south could vanish.

Like the scientists and volunteers studying the plight of the butterflies in Taipei, the Indigenous Rukai community have become the butterflies’ most passionate stewards in Maolin. Tang Hsiong-Jin, an Indigenous conservationist, says that this is because of their ancient set of values. “Within the Rukai and the broader Indigenous traditions in Taiwan, there has never been a culture of development that alters the landscape dramatically,” Tang says.

In fact, apart from Maolin, all of southern Taiwan’s remaining butterfly valleys are located on Indigenous lands that are generally only known to locals and super-enthusiasts. It’s no accident, Liao Jin-shan says. He and Tang have been working with local officials to evaluate local development plans and help limit butterfly habitat loss in places like Maolin.

In areas where habitat has already been lost, Zhuo and other members of the Butterfly Society have been helping town governments find parks and green spaces where native plants can be re-introduced in a way that naturally supports caterpillars, butterflies, and a variety of other wildlife. And these efforts have real support in Taiwan, where local governments have historically been receptive to protecting the butterflies.

Almost 20 years ago, the filmmaker Chan Chia-lung reported on how migrating butterflies were being killed by cars as they flew across national highways. In response, the Ministry of Transportation directed road crews to put up nets during peak migration periods, and some highways close lanes when enough butterflies are detected crossing the road. The Freeway Bureau also lined the entirety of Taiwan’s National Highway 3—which runs parallel to a prominent butterfly migration path—with native tree and shrub species. It’s a one-of-a-kind policy.

Although Taiwan’s density often brings humans and nature into conflict, “it also means humans and nature are closer together here,” Chan says. “We’re often more directly aware of the impact our actions can have on the natural environment.”