Why Do Chestnut Tiger Butterflies Cross 3,000km of Ocean? The Record Flight to Hong Kong

The single butterfly that proved a 3,017km journey

A butterfly named for a pale blue-green color

The chestnut tiger butterfly (scientific name Parantica sita niphonica) is a large butterfly in the family Nymphalidae, subfamily Danainae, with a wingspan of around 10cm. Its Japanese name, "Asagimadara," comes from the translucent, pale blue-white pattern on the inside of its wings — a traditional Japanese color called "asagi." It is found across Japan, the Korean Peninsula, China, Taiwan, and as far as the Himalayas, and tends to live in high-altitude mountain areas. Its distinctive gliding, leaf-like flight is said to leave a lasting impression on anyone who has witnessed its migration in person.

According to a report published in 2026 by a research team from the University of Hong Kong (HKU) together with researchers in Japan, the tagging took place on August 18, 2024, at the Grandeco Ski Resort in Kitashiobara, Fukushima Prefecture. Over just six days, the two researchers responsible for the tagging, Masayoshi Shimizu and Hiroki Takizawa, marked 481 chestnut tiger butterflies. One of them, a male marked "Deco 8/18," was recaptured four months later, on December 21, at Repulse Bay in Hong Kong.

A 3,017km flight lasting 124 days

The straight-line distance was 3,017km, over 124 elapsed days. This significantly updates the previous record of an individual tagged in Wakayama Prefecture in 2011 and recaptured in Hong Kong 82 days later (about 2,423km), and marks the third confirmed case of a chestnut tiger butterfly moving from Japan to Hong Kong. The research team published the finding as a paper in Volume 80 (2026) of The Journal of the Lepidopterists' Society.

An insect just a few centimeters long, weighing less than a gram, reaches all the way to Hong Kong by flapping and gliding alone, crossing Okinawa and Taiwan along the way. This is not a fluke — it could only be "proven" thanks to the mark-release-recapture network built up over nearly half a century.

124 days does not mean 124 days of continuous flight

Dividing 3,017km by 124 days gives an average of only about 24km per day. This shows that the butterfly did not fly nonstop, but rather rested its wings for days at a time in flower fields and highlands along the way, gradually working its way south while building up energy reserves. Researchers describe the reality of the journey as closer to a multi-day mountain traverse — resting and refueling repeatedly — than a marathon. Without a chain of relay stops along the way, a distance like this simply would not be possible.

This discovery is also significant as the third confirmed case of a chestnut tiger butterfly being recaptured in Hong Kong after departing Japan. A single case cannot rule out mere chance, but as independent records accumulate, they confirm that crossing the East China Sea is not an "exceptional stray flight" but a migration pattern that a certain number of individuals of this species attempt every year.

The paper's authors include three researchers from the Hong Kong side (Yuet Fung Ling, Professor Timothy C. Bonebrake, and Emily E. Jones), two taggers from Japan, and Itaru Kanazawa, who has long led Japan's mark-release-recapture surveys. The very fact that the paper was co-authored across national borders illustrates that this migration is a phenomenon no single country can fully capture on its own.

What is mark-release-recapture surveying? 55 years of citizen science

Suspicion that chestnut tiger butterflies migrate is said to date back to the early 1960s, when Kunio Nagamine observed on Okinawa's main island that "in summer, neither larvae nor adults could be found." Organized mark-release-recapture surveys began in earnest around 1980, and by the very next year, long-distance southward and northward movements had already been confirmed.

How the marking is done

• Catch a chestnut tiger butterfly in the field and write the capture location, date, and a contact address (or handle name) directly on the translucent part of its wing using an oil-based marker
• Release it at the same spot and confirm that it resumes normal behavior
• When it is recaptured at a different location by a different surveyor, the written information reveals when, from where, to where, and over how many days it traveled
• Recapture reports are shared through blogs, social media, and surveyor networks, accumulating into a nationwide and international database

The first shock the surveys revealed

Records indicate that experimental marking became full-scale around August 1980. From that very first season, and already by the following year, long-distance movements spanning the length of Honshu had been confirmed. At the time, the very idea of a "migratory butterfly" had not yet taken hold in the public imagination, and each new report of a recapture tens or hundreds of kilometers away spread astonishment among researchers and enthusiasts alike. Continued without interruption for more than 40 years since, the surveys have revealed migration not as a one-off curiosity but as a regular phenomenon repeated every season.

The scale of data accumulated so far

A defining feature of this survey is that it has been carried out not by a single research institution, but by nature-observation groups, local governments, and individual enthusiasts across the country. Records published in the journal of the Lepidopterological Society of Japan report tens of thousands of butterflies tagged and over 2,000 recaptures confirmed to date, and in 2025 Itaru Kanazawa and colleagues published a compiled survey history that includes 130 cases of movement across national borders.

Why only citizen science could solve this puzzle

Chestnut tiger butterflies are found across the whole of Japan, from Okinawa and Taiwan in the south to Hokkaido in the north. It is nearly impossible for a single research team to comprehensively track where and when they appear over the course of a year. Marking proved well suited to broad, long-term data collection: it requires no special equipment and lets anyone take part, while any recapture leaves reliable evidence of movement. As nature observers, local governments, and schools across the country worked independently yet were connected by the common language of recapture reports, a dataset far beyond what any single researcher could gather has built up over half a century.

This structure resembles the way amateur astronomers discover new comets or supernovae. Experts handle the theory and data analysis, while citizens scattered across a wide area serve as observation points, together producing discoveries no individual could achieve alone. Chestnut tiger butterfly migration research is counted among the leading examples of citizen science in Japan.

From Hokkaido to Okinawa and the Nansei Islands, there are as many "witnesses" as there are survey points spread across Japan's length. The very structure of recapture — in which a butterfly released in one place happens to be found by a surveyor in an entirely different region — presupposes a network connecting people across great distances. This broad grassroots reach is a strength unique to mark-release-recapture surveying, one rarely achievable in tracking studies of other species.

Why do they migrate? A north-south relay across generations

The migration of the chestnut tiger butterfly is not a round trip made by a single individual, but a relay across multiple generations. A generation that emerges in spring in the Nansei Islands or Taiwan begins moving north, changes generations while spending summer in Honshu or Hokkaido, and the new generation born in autumn then heads south again. Unlike migratory birds, a defining feature is that the same individual never "returns" to its birthplace.

Summer: north to the cool highlands

• Around May and June, the generation that migrated up from the south lays eggs, and their offspring spend the summer in the highlands of Honshu or in Hokkaido
• Chestnut tiger butterflies are sensitive to heat and tend to move toward high-altitude mountains and cooler climates
• Highland resorts such as Grandeco in Fukushima Prefecture serve both as summer retreats and as relay or departure points for the migration

Autumn: south to the warm Nansei Islands and Taiwan

Between September and November, the new adults that emerged over the summer begin heading south, building up energy by feeding on autumn flowers such as boneset. Their destinations include Okinawa and Taiwan, and sometimes as far as Hong Kong and southern China. Because temperatures drop and the larval host plants wither in Honshu, the region cannot support overwintering, so this movement is thought to be essential for the species' survival. Just as whales migrate to balance feeding and reproduction, the migration of large marine animals over long distances and that of the chestnut tiger butterfly share a common ecological strategy: shifting habitat to follow seasonal resources.

Unlike migratory birds, the same individual never returns

Many migratory birds, such as swallows and geese, show "site fidelity" — the same individual returns to its birthplace or wintering ground. The adult chestnut tiger butterfly, by contrast, is generally thought to live only a few months, making it unlikely that an individual that moved north in spring would then fly back south in autumn. The generation that moves north and the generation that moves south are different generations — in effect, a multi-generation "relay race" that links together the annual migration cycle spanning the length of the Japanese archipelago. This ecology is known to closely resemble the structure of the monarch butterfly's migration across North America, which likewise proceeds through generational change.

The generation that overwinters in the south

Because the generation that spends summer in Honshu or further north cannot survive the winter there, individuals that move south remain active through the winter in warmer regions such as Okinawa and Taiwan, breeding there to leave the next generation. The observation made by Kunio Nagamine on Okinawa's main island in the early 1960s — that "in summer, neither larvae nor adults could be found" — is now regarded as the first clue that this southern population moves north during the summer months. Migration should not be understood as a one-way phenomenon in either direction, but as an entire annual cycle shuttling between the warm south and the highlands of the north, carried forward across generations.

The power to cross the ocean: airflow and energy

Migrating from Honshu to Taiwan or Hong Kong inevitably requires crossing stretches of open water hundreds of kilometers wide, such as the East China Sea. Continuous flapping alone would exhaust the butterfly's energy reserves, so it is thought to rely heavily on gliding, catching updrafts and seasonal winds to travel long distances with minimal energy expenditure.

A light body and large wings

The chestnut tiger butterfly's wings are translucent, letting light pass through, and are large relative to its body. This build is well suited to gliding on the wind while minimizing the number of wingbeats — a feature it shares with other migratory Danainae butterflies, such as North America's monarch.

A cold-blooded insect with no choice but to go south

Butterflies are ectothermic, meaning their body temperature and activity level are strongly governed by ambient temperature; once it drops, they can no longer power their flight muscles adequately. Winter temperatures in Honshu fall below the chestnut tiger butterfly's activity threshold, and the larval host plants wither as well, so simply staying put and enduring the winter is not an option. Moving to a warmer region, even if it means crossing the sea, is a migration driven less by the performance of its wings than by the physiological constraints of being a cold-blooded animal — in a sense, a journey it has no choice but to make.

Storing energy from autumn flowers

Before heading south, chestnut tiger butterflies feed heavily on nectar from composite flowers such as boneset, Eupatorium chinense, and thistles, building up the energy reserves needed for long-distance flight. Whether they can adequately refuel at relay stops is thought to determine whether they can successfully cross the sea.

Seasonal winds as a tailwind

During the autumn southward season, northerly winds from the continent prevail, while southerly winds tend to blow during the spring northward season. The view that chestnut tiger butterflies use these seasonal prevailing winds as a tailwind is supported by the close match between the seasonal reversal of migration direction and the corresponding shift in weather patterns around Japan. The idea that a sea-crossing insect makes use of wind is not unique to this species — it has also been noted in dragonflies that migrate long distances over the ocean, such as the wandering glider, suggesting a shared strategy among small flying animals crossing wide expanses of sea.

The well-known migration of the monarch butterfly in North America, spanning thousands of kilometers from southern Canada to central Mexico, is often compared by researchers to that of the chestnut tiger butterfly, since both rely on gliding via updrafts and a relay of multiple generations. Though closely related within the subfamily Danainae, each appears to have independently evolved its own mechanism for crossing a continent.

Routes pieced together over time: a record spanning borders

The longest-distance record is not the only thing mark-release-recapture surveys have built up. From short domestic movements to long-distance migrations crossing national borders, countless recapture cases have drawn out the overall picture of the routes involved.

A survey history compiled in 2025 by Itaru Kanazawa and colleagues reports 130 confirmed cases of movement between Japan and locations abroad, including Taiwan, China, and Hong Kong. Since each case depends on a chance recapture, the actual number of individuals that have made such crossings is presumed to be far greater. Just as the spawning grounds of the Japanese eel were narrowed down over decades of tracking research, the search for where eels spawn, the unraveling of chestnut tiger butterfly migration routes has likewise progressed through the steady accumulation of data.

What we see is "the tip of the iceberg"

Mark-release-recapture surveys have a structural limitation. Only a small fraction of tagged individuals are ever actually recaptured and reported; most simply live out their lives unrecorded. In other words, the published migration records represent only the tiny portion of actual migrations that happened to be caught by human eyes. Even so, continuing the same method over decades has gradually revealed statistically meaningful patterns — the timing of southward and northward movement, the main routes, and average migration distances. Behind every single record-setting flight lie countless unrecorded migrations.

The Pacific coast route and the "Sea of Japan route" hypothesis

Attention has long focused on the route heading south from Honshu's Pacific coast toward the Nansei Islands and Taiwan, but a 2013 paper by Itaru Kanazawa and colleagues in the journal Nature Study, titled "The Possibility of a Sea of Japan Route in the Migration of the Chestnut Tiger Butterfly," pointed to the existence of a migration route passing along the Sea of Japan side toward the continent. The possibility that multiple routes — along the Pacific coast and the Sea of Japan coast — are used in parallel suggests that migration is not a single path, but a collection of different routes shaped by weather conditions and each individual's starting point.

Islands in the Sea of Japan such as Tsushima and the Goto Islands are also thought to serve as stepping stones for individuals traveling to and from the continent, potentially forming "stepping stones for crossing the sea" scattered along both the eastern and western sides of the Japanese archipelago, alongside Pacific-side relay points such as Himeshima. Just how important each island is as a relay point is a question that will continue to be clarified gradually as more recapture data accumulates from mark-release-recapture surveys.

International cooperation across borders

As the international movement of chestnut tiger butterflies has become clearer, information-sharing with researchers and enthusiasts in Taiwan, Hong Kong, and China has advanced, developing into an East Asia-wide citizen science network with standardized marking notation and instant recapture reporting via social media. The 3,017km record itself could only be confirmed by cross-referencing Japan's tagging data with the recapture data gathered by the University of Hong Kong team — a reminder that cross-border cooperation itself is what improves the precision of migration research.

Boneset flowers and male chemistry: the secret of the sex pheromone

Every autumn, the sight of chestnut tiger butterflies gathering on boneset and Eupatorium chinense flowers can be seen at observation sites across the country. This behavior, it turns out, means more than simply feeding on nectar.

Pyrrolizidine alkaloids, the key compound

Boneset and some other composite-family plants contain a defensive compound called pyrrolizidine alkaloid (PA). Male chestnut tiger butterflies actively ingest this PA and use it internally as a raw material for sex pheromones. It is thought that when a male releases this pheromone while courting a female, it enables successful reproduction.

The Apocynaceae-family plants eaten during the larval stage — such as Marsdenia tomentosa, Cynanchum, Marsdenia tinctoria, and Hoya carnosa — supply toxic compounds (cardiac glycosides) that render both larvae and adults poisonous, protecting them from predators such as birds. In other words, the chestnut tiger butterfly obtains "defense" from its larval host plants and the chemical compound for "courtship" from boneset as an adult, drawing on entirely different plants for each purpose.

Only males collect the alkaloids

It is mainly males that gather on boneset to actively ingest pyrrolizidine alkaloids, converting them into sex pheromones used in courtship. Females are thought to choose their mates based on attraction to this pheromone, making boneset colonies effectively both a "perfume source" for males and a gathering place where males and females meet. The fact that most chestnut tiger butterflies observed on boneset flowers in autumn appear to be male reflects this difference in behavior.

Pharmacophagy: seeking out "medicinal" plants

The behavior of actively seeking out plants containing toxins or medicinal compounds for a purpose other than nutrition is known in entomology as "pharmacophagy." It is widespread among butterflies of the subfamily Danainae: just as North America's monarch butterfly accumulates cardiac glycosides from milkweed during its larval stage, the adult chestnut tiger butterfly actively collects pyrrolizidine alkaloids from boneset as a "weapon of courtship." Behind the seemingly peaceful act of sipping nectar lies a chemical strategy staked on reproductive success.

In evolutionary biology, this kind of behavior is sometimes explained as a form of "honest signaling." The idea is that a male able to gather enough PA to synthesize a high-quality pheromone is likely also a capable individual — one that can fly widely and gather resources efficiently — giving females a reliable cue for evaluating a potential mate. Even the simple act of sipping nectar carries within it the logic of reproductive evolution.

Islands and coastlines as relay stations: the role of Himeshima

The ability of the butterfly itself is not the only thing that sustains a migration across the sea. Relay points where it can rest its wings and refuel along the way are essential. A leading example is Himeshima, an island in Oita Prefecture in the Seto Inland Sea.

Himeshima, visited twice a year in spring and autumn

In spring (early May to early June), individuals migrating up from the south arrive at Himeshima seeking nectar from Heliotropium along Mitsuke Beach, resting before heading north to cooler climates. In autumn (around October–November), individuals from the generation that changed over the summer stop by on their way south, drawn to the boneset flowers that grow in colonies in the island's central hills. The number of butterflies arriving ranges from several hundred in a low year to several thousand in a high year.

Himeshima has a distinctive geology, formed when four small volcanic islands were joined into one by connecting sandbars, and in 2013 it was certified as a Japan Geopark under the name "Oita Himeshima Geopark." The fact that terrain and vegetation shaped by volcanic activity also happen to function as a crucial resting place for the chestnut tiger butterfly is a good example of the connection between geology and ecosystems.

Locally, observation events and guided tours are held to coincide with the migration season, sustaining activities that balance tourism with nature conservation. Preserving colonies of host and nectar plants within the limited area of a small island is no easy task, but positioning the entire island as a "migration relay point" — a community asset — appears to have helped sustain the arrivals over many years.

If the flower colonies shrink, the migration thins out too

Boneset is also a plant whose wild habitat has been shrinking due to development of riverbanks and grasslands and to weed clearing. If boneset colonies at relay points are lost, more individuals may attempt to cross the sea without adequate refueling, potentially affecting the overall success rate of the migration. At places such as Himeshima and Akagi Nature Park, local residents and nature parks continue efforts to plant and preserve boneset.

Akagi Nature Park, a base in the north

If Himeshima is an "ocean relay point" visited en route during southward and northward migration, Akagi Nature Park in Gunma Prefecture is one of the leading mark-release-recapture bases on inland Honshu. It continues to observe the arrival of chestnut tiger butterflies on its summer highlands and to carry out tagging and release, and when butterflies released there are recaptured months later on a distant southern island or overseas, the inland highland and the sea-crossing migration become connected by a single line of evidence. Only once data has accumulated at both southern and northern relay points does the full picture of the migration come into view.

"Ocean rest stops" are scattered across Japan

Beyond islands of the Seto Inland Sea like Himeshima, chestnut tiger butterfly relay points are scattered along sandy beaches, capes, and remote islands all along the coast. What they share is the presence of clustered host and nectar plants such as boneset and Heliotropium, and a location where the butterfly can pause immediately before or after crossing the sea. These unassuming stretches of coastal vegetation quietly form the foundation that supports this grand phenomenon of migration.

What we can do: join the surveys, plant the flowers

Unraveling the migration of the chestnut tiger butterfly has been an achievement of citizen science, carried out not only by professional researchers but by enthusiasts, local governments, and schools nationwide. Even without any special equipment, anyone can get involved in the following ways.

• Take part in mark-release-recapture events run by local nature-observation groups and survey teams, and experience the tagging process firsthand
• Plant boneset or Eupatorium chinense in a home garden or on a balcony, and observe and record any arrivals
• If you find a marked individual, report the recapture information to the contact address or social media account written on it
• Visit observation sites such as Himeshima or Akagi Nature Park during their arrival seasons and learn about local conservation efforts

The appeal of mark-release-recapture surveying is that even non-experts can become part of a discovery. If a butterfly you marked is recaptured months later on a distant island, that becomes a genuine piece of survey data. Conversely, simply finding and reporting an individual someone else released adds a new point to the migration record. Even the 2024 record of 3,017km only came together as meaningful data through the connection between people in distant places — the taggers Shimizu and Takizawa, and the Hong Kong team that confirmed the recapture.

A natural fit for school education

Because mark-release-recapture surveying lets students experience several ecological themes at once — growth, metamorphosis, and migration — it has been adopted as teaching material for integrated studies at elementary and junior high schools across the country. Growing boneset in a school garden or nearby flowerbed and waiting for arrivals lets children experience, through a full cycle of planting, observing, and recording, a real sense of connection to distant places. Tracking how far a butterfly they released has traveled becomes a gateway to understanding connections on a planetary scale, extending even to ecosystems across the sea.

The fact that a butterfly weighing less than a gram can cross more than 3,000km stands alongside the albatross's ability to glide thousands of kilometers without flapping its wings, far exceeding our intuitive sense of what living creatures are capable of. Even after half a century of patient tagging surveys, mysteries of this migration remain unsolved.