Monarch butterfly migration is the event in which the monarch subspecies Danaus plexippus plexippus travels each autumn to places where they spend the winter. These locations are near the west coast of California or in mountainous areas of central Mexico. Other monarch populations around the world may travel short distances or not travel at all. This large movement of butterflies is considered "one of the most spectacular natural events in the world."

North American monarchs begin their journey south in September and October. These butterflies come from southern Canada and the northern United States. They travel thousands of kilometers to reach their winter resting places in central Mexico. They arrive at these sites in November and stay on volcanic mountains covered with oyamel fir trees (Abies religiosa) during the winter. In March, they begin their return trip north to North America and southern Canada.

Two to three generations of monarchs complete the journey north. Female butterflies lay eggs during the trip, which will hatch into the next generation. Four generations are part of the yearly cycle. The generation that travels south lives eight times longer than their parents and grandparents because of a hormone that controls aging. Similarly, western monarch populations move every year from areas west of the Rocky Mountains to wintering sites along the coast of California.

Not all monarch populations travel long distances. In Australia and New Zealand, monarchs travel short distances. Some populations of D. p. plexippus, such as those in Florida and the Caribbean, and another subspecies (D. p. megalippe) found in the Caribbean, Central America, and northern South America, do not migrate. Additional wintering sites have been found in Arizona and northern Florida. According to a report from Mexico, the eastern monarch population nearly doubled in 2025. The area covered by monarchs in central Mexico's forests was 4.42 acres (1.8 hectares), up from 2.22 acres (0.9 hectares) the previous winter. Although monarchs occupied nearly twice as much forest habitat as the year before, their numbers were still much lower than the long-term average.

Historical accounts

As late as 1951, people incorrectly believed that monarch butterflies spent the winter as adults or pupae. Large groups of monarchs were seen resting in the southern parts of North America.

Western monarch butterflies and their winter homes were known to people long before scientists discovered the Mexican winter sites in 1975. Before European arrival, the Purépecha and Otomi people lived in this area and linked their corn harvests to the butterflies’ arrival. Stories about monarchs appear in the traditions of people living near winter sites. In the areas around the Mexican winter sites, local people, such as the Mazahua, knew about the butterflies’ winter behavior for many years before 1975. The arrival of monarchs is connected to a traditional celebration called Dia de los Muertos. Today, people in these areas remember seeing the butterflies before 1975.

For at least 100 years, monarchs were seen spending the winter in California. Records kept by butterfly experts do not mention monarchs in their current western range, which includes areas in Washington, Oregon, and Canada. Female monarchs lay eggs only on milkweed plants. These plants protect caterpillars and adult butterflies from predators. It is thought that milkweed may not have been available until western lands were farmed, which may have helped the butterflies expand their range.

Serious studies about monarch migration began when Fred Urquhart graduated from the University of Toronto in 1935. He joined a program to study insects. In 1937, Urquhart started mapping the path monarchs take during migration. He was the first to record that monarchs fly south and southwest during fall in North America, and that these movements are connected to weather patterns. He created the first successful tagging program that collected data. He and his helpers discovered that monarchs rest in groups during winter.

Urquhart asked for help in Mexican newspapers to find winter sites. Catalina Trail and Kenneth C. Brugger answered and led him to one of the major winter sites in 1975. Urquhart, William Calvert, John Christian, and Lincoln P. Brower shared their discovery in a scientific paper in 1976. Some people worried that public knowledge might harm the butterflies.

Since 1976, 12 winter sites have been found northwest and west of Mexico City in the Mexican states of Michoacán and México. At least 70% of monarchs usually gathered in three of these sites. At least 20 colonies existed in Mexico in 1997.

As of 2023, one of the largest known winter sites is in the Joya Redonda Monarch Butterfly Sanctuary. This site is 50 miles (80 km) southeast of Mexico City, near the volcanoes Popocatépetl and Ixtaccíhuatl in the state of México’s Atlautla municipality. Monarchs gather in oyamel fir trees in the sanctuary each winter. Earlier, local people saw small numbers of monarchs, but more butterflies started living in Atlautla in the early 2000s after old winter sites were destroyed by deforestation. With help from local volunteers, the number of butterflies in Joya Redonda increased ten times in one year. In the 2021–2022 season, only four trees had butterfly colonies. The next season had 42 trees. At that time, tourists paid 100 pesos (about $5.50) to visit the sanctuary.

Migrations

Monarch butterflies east of the Rocky Mountains begin migrating to the Mariposa Monarca Biosphere Reserve in the Mexican states of Michoacán and México by late October. This reserve is located in the Trans-Mexican Volcanic Belt pine-oak forests. Some monarchs also overwinter on private land. Others travel to places like Cuba and Florida during the fall.

Two main migration routes exist in North America. One route runs through the central United States and leads to the Mexican overwintering areas. A smaller route follows the eastern seaboard. Monarchs traveling along the eastern route tend to arrive later than those on the central route. Fewer monarchs from the eastern route are found in Mexico, which may mean they migrate to other locations or face higher risks of dying during their journey.

Monarch butterflies begin their fall migration in response to several signals, including changes in sunlight angles, the aging of milkweed plants, shorter days, and cooler temperatures. The migration usually starts in August from the northernmost summer areas. Monarchs rely on nectar from fall flowers along their path for energy.

The eastern monarch population typically travels from southern Canada and the U.S. Midwest directly south to Mexico. Monarchs from the Northeast often travel southwest, while those moved from the Midwest to the East Coast initially go south but then adjust their path to southwest. Natural features like mountains and rivers influence migration routes.

The western monarch population migrates from areas west of the Rocky Mountains, including parts of northern Canada, to California. Australian monarchs travel from western to eastern regions near the Pacific Ocean.

During migration, most adult monarchs enter a state called diapause, which is a period of reduced activity. Unlike other insects, monarchs remain active during diapause. They store energy in the form of fats, proteins, and carbohydrates. Monarchs migrating to Mexico store more fat than those going to California. These stored substances help them survive the winter and prepare for spring migration. Diapause begins before environmental stress occurs and reduces energy use.

Monarchs in diapause during fall are physically different from those reproducing in spring and summer. Fat levels in diapause monarchs can reach 34%. Their fat storage organs are larger than those of summer monarchs. Tissue samples show higher lipid levels in their blood. Female monarchs in diapause rarely have mature eggs, and mating is rare among overwintering butterflies. This helps protect winter populations and conserve energy for spring migration. Monarchs in diapause typically remain in this state until mid- to late January. By February, longer days signal the end of diapause.

Diapause has different stages. Shorter days and colder temperatures stop the production of a hormone that controls reproduction. This reduces mating and egg-laying behaviors. New behaviors, like forming groups to feed and gathering in clusters at night, develop during diapause. Roosting helps reduce water loss.

In spring, monarchs begin a northward migration. Female butterflies lay eggs during this time. Monarchs from Florida typically start their northward journey between mid-March and mid-May. Some of these monarchs are offspring of butterflies that overwintered in Florida and along the Gulf Coast, not in Mexico. Tagged monarchs from Florida have been found in Virginia and Georgia.

Monarchs travel long distances during migration, often farther than their normal lifespan of less than two months. Butterflies born in early summer only reach as far as Texas and Oklahoma. Later generations return to northern areas in the United States and Canada.

Spring migration is triggered by cues like rising temperatures and longer days. Western monarchs move northwest during this time. Roosting sites may shift, and monarchs move to lower elevations. Mated females leave overwintering sites before males. Monarchs traveling north do not form roosts.

The rate at which monarchs return to breeding areas has remained steady since 1997. Recolonizing breeding grounds takes two to three generations. The pattern of returning to northern areas has not changed since monitoring began in 1997.

During migration, eastern and western monarch populations often travel together and gather at overwintering sites. Scientists use roosting locations to map migration paths. Fred Urquhart observed roosting in Mexico and Florida. In California, monarchs roost in places like Fremont, Natural Bridges State Beach, golf courses, and suburban areas. These roosts differ from those in Mexico and are often found inland on non-native trees.

Overwintering sites in California, Mexico, Arizona, the Gulf Coast, and Florida share similar conditions: stable temperatures, high humidity, access to water, and trees for roosting. California has over 200 overwintering sites. Monarchs have also been found overwintering in South Carolina and as far north as Virginia Beach.

California overwintering sites are often in developed areas, not forests. These sites usually have uniform vegetation, such as Monterey pine or eucalyptus trees, and are common in urban areas. Monarchs move between roosts during winter. Monarchs on the Gulf Coast and in Florida do not enter diapause and breed throughout the year.

Western monarchs overwinter in coastal areas of central and southern California, such as Pacific Grove, Santa Cruz, and Grover Beach. They also overwinter in Baja California, the Central Valley, and the Sierra Nevada foothills.

Not all monarchs migrate. Some populations remain in their habitats year-round.

Population and migratory study methods

Initially, direct observation was the main way to study monarch butterfly migration. Since 1975, more advanced methods have been developed. The number of monarchs changes a lot from year to year.

Since 1993, the World Wildlife Fund and its partners have measured the area of Mexican forests where monarch butterflies hibernate each winter during the last two weeks of December. The largest recorded area was in 1996, and the smallest was in 2013. In 2021, the area was 35% larger than in 2020. It decreased by 22% in 2022 compared to 2021 and by 59% in 2023 compared to 2022. In December 2024, the area increased by 92% from the previous December, but it was still smaller than the area recorded two years earlier. The 2024 area was 10% of the 1996 area and 29% of the area measured when recording began in 1993.

Changes in monarch numbers are caused by natural events, different counting methods, and human changes to their habitat. Scientists question the accuracy of population counts at overwintering sites in North America. Differences between migrating monarchs and those at overwintering sites suggest many adults die during migration. The Commission for Environmental Cooperation says long-term and large-scale monitoring is needed. Population estimates of adults, eggs, larvae, and milkweed should match counts at overwintering sites. Currently, no data is available to confirm these connections, but a study by The Monarch Larva Monitoring Project aims to compare population counts in Mexico with those in the Midwestern United States and Canada.

Monarch tagging began in 1796 with silk moths. Fred Urquart started tagging monarchs in the 1940s, using methods like wing cuts, colored spots, and painted letters. These early methods failed because no instructions were given to return tagged butterflies or report their recovery. Today, many groups study migration through tagging. New methods include using VHF transmitters, commercial aircraft, and isotopic tagging.

Tagging and recapturing butterflies helps estimate total population numbers. The number of recaptured butterflies is proportional to the total population. Scientists calculate the total population by dividing the number of tagged butterflies by the proportion of tagged butterflies found in a second sample. Other similar methods include capture-recapture, mark-recapture, and the Lincoln method. These methods have been used to study monarch migration from Florida and roosts in California.

Monarch migration is studied during annual butterfly counts. Observatories like the Cape May Bird Observatory, Peninsula Point Light in Michigan, and Point Pelee National Park in Ontario, Canada, monitor monarch concentrations during their southward migration. Counts also occur in Quebec. Other methods include driving censuses, walking censuses, roosting counts, and hawk-watch observations. Monarchs often gather in roosts on peninsulas that face south. Monitoring programs track roost numbers along migration routes. Data from multiple sites are compared. The ratio of monarchs to other species during counts helps scientists understand habitat changes. Yearly changes are linked to weather, El Niño Southern Oscillation, and volcanic eruptions.

Most people who study monarch migration are not scientists but are called "citizen scientists." Some anecdotal reports have been criticized as not "good science" or "science at all." Conservation groups and scientists use these observations in their research. People who join organized butterfly counts provide reliable data. In Texas, observers record patterns, distributions, and populations during migration.

Satellite images have been used to study changes in Mexican overwintering areas. Researchers say satellite images cannot accurately count butterflies, but aerial surveys show areas where butterflies might live. High-altitude aerial photography was found to be too costly and time-consuming.

Direct observation involves recording data when butterflies are at one stage of their migration. This data includes:

Over the years, a large amount of data has been collected and used by scientists. Sometimes, scientific observations are kept private and not shared with others. Observers now use Google Maps to record sightings.

In 2025, Project Monarch placed over 400 solar-powered ultralight transmitters on monarchs during their southward migration. These transmitters tracked butterflies from across North America and the Caribbean to their overwintering sites in Mexico.

Migratory theory mechanisms

There are many theories that try to explain how monarch butterflies migrate. Scientists have not yet fully explained how this happens. Researchers often suggest that several different methods help the butterflies find their winter homes. Not all scientists who study monarch migration agree on which methods are most important.

The sun plays an important role in how monarchs travel. They fly during the day and use a natural clock based on the sun’s position in the sky to help them find the right direction. Since the sun moves across the sky throughout the day, monarchs use their internal clock to adjust for these changes. This allows them to use a special tool called a time-compensated sun compass. Studies have shown this behavior in both wild and lab settings, but scientists still do not fully understand how monarchs interpret the signals that guide their migration. Even with a time-compensated sun compass, it is unclear how monarchs from different starting points all reach the same winter location.

In addition to using the sun, monarchs also use a magnetic compass to help them travel south during migration.

When monarchs were trained to follow a laboratory light-dark cycle and placed in flight simulators, they could use sunlight and their internal clock to consistently choose a southward direction. However, if the simulators changed the time by six hours, the monarchs’ preferred direction changed. This happened because their internal clock was confused by the altered time. Monarchs rely on their internal clock to match the sun’s position with the time of day. If the sun appears in a different place at the same perceived time, their navigation is disrupted. In these cases, monarchs may begin to travel in directions other than south.

Scientists have studied how the internal clock works in monarchs. They have found that monarchs use a system similar to those in fruit flies and mammals. This system involves a process called a transcription-translation feedback loop (TTFL), which controls the levels of certain genes and proteins in the clock. The monarch clock is unique because it uses two proteins called CRY1 and CRY2, which have different roles. CRY1 acts like a sensor for blue light, helping the clock adjust to day and night cycles. CRY2 works like a protein in mammals, helping to control the timing of the clock.

In the main part of the monarch clock, proteins called CLOCK (CLK) and BMAL1 work together to turn on genes that produce proteins involved in the clock. These proteins form a group in the cell and later return to the nucleus, where they stop the clock from working. After some time, these proteins break down, and the clock starts again. Blue light can also cause the breakdown of another protein called TIM, which restarts the clock. This is how the monarch clock adjusts to the 24-hour day.

Another part of the monarch clock is similar to that of fruit flies. This part involves genes that help control the activity of CLK, which is important for the clock’s function.

Monarchs use polarized light, which is light that bends in a specific direction, to help them navigate. This light is first seen by the monarch’s compound eyes. A special part of the eye called the dorsal rim area detects this polarized light. This information is sent to the brain, where it is combined with the position of the sun and the time of day. These signals help the monarchs fly in the right direction during migration. More research is needed to understand how the brain processes these signals.

While the brain processes information, scientists have found that the actual clock that controls migration is in the monarch’s antennae. When monarchs lose their antennae, they no longer travel in the same direction as a group, even though they may still fly in a general direction individually. Studies show that the antennae have their own internal clock, even when removed from the butterfly. This means the antennae can create the clock’s rhythm without the brain. If only one antenna works, the monarch can still navigate correctly. However, if both antennae give conflicting signals, the monarch becomes disoriented.

Monarchs use their time-compensated sun compass during both their fall migration to the south and their spring return to the north. The change in direction needed for the spring return depends on the cold temperatures monarchs experience while overwintering in Mexico. This change is not caused by the shorter days during winter but may affect when the spring migration begins.

An experiment showed how cold temperatures help monarchs change direction. Monarchs that were exposed to cold in the lab during winter could change their sun compass to point north in the spring. Monarchs that were not exposed to cold continued to point south in the spring and did not change their compass direction. This proves that cold temperatures are important for monarchs to prepare for their spring migration.

Conservation

Researchers and citizen scientists who study monarch butterfly migration have different opinions about whether the species might disappear from certain areas. The International Union for the Conservation of Nature (IUCN) listed the monarch as endangered in 2022 because its population has dropped by 23 to 72 percent over the past decade. Scientists are worried that the eastern North American population’s migration might be in danger. Some media reports about the monarch’s possible extinction have been criticized by scientists. Lincoln Brower, a top monarch conservation researcher, says, "Monarchs are not in danger of extinction."

Groups focused on monitoring and protecting monarchs often work to support the four stages of the monarch’s life cycle.

Winter roosts in Mexico and California were listed as threatened by the IUCN in the IUCN Invertebrate Red Data Book. Conservation efforts began in 1890 when Pacific Grove, California, passed a rule to stop people from disturbing monarch butterflies during their winter rest.

In 1986, the Mexican government created the Monarch Butterfly Biosphere Reserve to protect 62 square miles (16,000 hectares) of forest in four monarch sanctuaries. The reserve was expanded to cover 217 square miles (56,000 hectares) in 2000.

A parasite called Ophryocystis elektroscirrha can harm many migrating monarchs. It weakens their ability to fly, making it harder for them to reach their wintering sites.

Efforts to protect monarchs in Mexico started before 1975, led by local people. Monarch numbers in Mexico dropped significantly between the 1992 counts and more recent ones. Overwintering sites are found along the Gulf Coast, Arizona, and Florida.

Disasters, such as storms, at Mexican overwintering sites are often linked to population declines. Some sites lost 30 to 90 percent of their monarchs during storms. Conservation efforts include planting native trees where monarchs prefer to roost.

Monarchs in California usually roost on native trees but also choose introduced eucalyptus trees even when native trees are available. Their roosting sites are often near the coast, though some are inland.

Some scientists say the loss of milkweed in the U.S. Midwest is a major reason for fewer monarchs reaching Mexico. Since 1996, 167 million acres of monarch habitat have been lost. These scientists argue that the decline of milkweed in agricultural areas is a key cause of fewer monarchs reaching Mexico. However, other scientists disagree, as long-term monitoring programs in the U.S. show no decline in the number of breeding adult monarchs since the 1990s. Some scientists also point to pesticide and herbicide use as a cause of declining overwintering monarchs. They note that before genetically modified (GMO) crops were introduced, milkweed was common in crop fields. The link between GMO crops and monarch declines is described as "suggestive but not conclusive," because other factors, like deforestation and weather, may also be involved. Milkweed habitat is also lost due to urban and suburban expansion.

By 2018, a study linked monarch declines to the use of genetically modified corn and soybean seeds resistant to glyphosate herbicide. This allowed farmers to spray glyphosate twice—once before planting and again by air when weeds grew. This practice reduced the unplowed areas between fields and roads, where milkweed and nectar plants once thrived.

Some researchers now believe the decline in monarchs seen in Mexico is due to losses during migration, not breeding or wintering. Conservationists also highlight the loss of habitat for nectar plants. Other factors that may harm migration include cold winters in central Mexico, droughts in Texas, invasive plants that are not milkweed (on which monarchs lay eggs), and the increased use of synthetic insecticides that break down slowly in the environment.

Many organizations and programs work to protect monarchs and their migration. 3-letter codes, explained in the linked footnotes, are used to label the table clearly.

Economics

Tourism near the places where butterflies spend the winter in Mexico and California helps people earn money by offering services such as guiding, providing food and lodging, and selling crafts and souvenirs.

Some people living near these areas worry that their children do not have enough food. Because of this, they sometimes cut down trees illegally to make money. Other people use the time when butterflies are near their homes to earn money by offering services like guiding, providing meals, and selling handmade items. Even though they are poor, they can sometimes earn enough money for the whole year through these activities.

Butterflies that spend the winter in Mexico and California rest in trees on land owned by private individuals. Laws and rules about protecting these areas and their surroundings are more important than the wishes of landowners, farming groups, or local leaders.

In 1986, Mexico created protected areas for butterflies during the winter. Parts of the forest were closed to people who relied on cutting trees for income. Small groups of people continued to cut trees illegally. Conservation groups pay local people to watch over the forest and protect it.

People are asked to give money to support programs that help protect butterflies. Some of the money given to these programs is used to raise more funds for the organizations that work to save butterflies.

Politics

The United States, Canada, and Mexico work together to protect the monarch butterfly. This partnership created the North American Monarch Conservation Plan. Some people believe conservation efforts in Mexico need improvement.

People who live near where monarchs spend the winter, such as Indigenous groups, farmers, and landowners, have expressed concerns about how Canadian and American conservationists are managing land near protected areas. Sustainable development, which means using resources in ways that help the environment and the economy, is important for protecting monarchs. Mexican communities worry about rules that limit how they use land and natural resources. Conservation plans are more likely to succeed if they include the needs and ideas of local people.

Studying animals helps conservation, but it is not enough unless problems related to society, money, and politics are solved.

Access to areas where monarchs spend the winter is carefully controlled by Mexico. Groups like Profepa, UNAM, IPN, and the Monarch Butterfly Biosphere Reserve, along with local and international volunteers, monitor these areas. The World Wildlife Fund helps pay for workers who protect these sites.

Groups in Mexico, local communities, and organizations have created policies to protect monarchs. The North American Monarch Conservation Plan (NAMCP) brings together Mexico, Canada, and the United States to protect habitats along the monarchs’ migration path, breeding areas, and wintering sites. One policy includes planting milkweed and nectar plants in large numbers.

Mexico has also created policies to help protect monarchs. These include paying local people to monitor forests and encouraging tree planting in wintering areas. Efforts have been made to reduce activities, like logging and tourism, that could harm monarchs.

The University of Minnesota studies monarchs across North America to track their health and numbers. Other groups work with lawmakers, companies, and government agencies to protect habitats.

On June 20, 2014, President Barack Obama created a plan called "Creating a Federal Strategy to Promote the Health of Honey Bees and Other Pollinators." This plan formed a group called the Pollinator Health Task Force, led by the Secretary of Agriculture and the Administrator of the Environmental Protection Agency.

The U.S. General Services Administration (GSA) sets rules for building designs in its P100 documents. These rules require planting designs that provide food for pollinators, such as bees and butterflies.

In May 2015, the Pollinator Health Task Force released a "National Strategy to Promote the Health of Honey Bees and Other Pollinators." This strategy aimed to achieve three goals, two of which were to improve pollinator habitats and reduce threats to them.

Many projects in the strategy focused on the I-35 highway, which runs from Texas to Minnesota. This highway passes through areas where monarchs breed during spring and summer.

At the same time, the Task Force released a "Pollinator Research Action Plan." This plan outlined five areas for action, including studying pollinators, reducing environmental dangers, restoring habitats, supporting people involved in conservation, and sharing knowledge.

In May 2015, the U.S. Department of Agriculture (USDA) and the U.S. Department of the Interior (USDI) created a 52-page guide called "Pollinator-Friendly Best Management Practices for Federal Lands." This guide explains how to protect pollinators when managing federal lands. It also lists steps to restore monarch habitats, such as ensuring native wildflowers and milkweed are available.

On December 4, 2015, President Obama signed the Fixing America's Surface Transportation (FAST) Act. This law added new rules to support pollinators. It asked the U.S. Secretary of Transportation to:
1. reduce mowing on roadsides and other transportation areas to help plants grow; and
2. plant native flowers and milkweed to create habitats for monarchs and other pollinators.

The FAST Act also said that projects to help pollinators could receive federal funding if they are part of transportation projects.

In February 2016, the Office of the Secretary of the Interior shared a plan to use pollinator-friendly landscaping on federal lands. This plan included steps for new buildings, existing sites, and education programs. It asked agencies like the Bureau of Land Management and the National Park Service to follow these steps.

In June 2016, the Pollinator Health Task Force released a "Pollinator Partnership Action Plan." This plan showed examples of how the federal government and other groups have worked together to help pollinators.

The USDA’s Farm Service Agency helps increase monarch populations through its Conservation Reserve Program’s State Acres for Wildlife Enhancement (SAFE) Initiative. This program gives farmers money to plant species that improve the environment and protect wildlife.