Migration

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This map was created in collaboration with the Center for Global Environmental Education at Hamline University with generous support from U.S. Forest Service - International Programs, U.S. Fish and Wildlife Service, and the Missouri Department of Conservation.

Eastern Monarchs

Decreasing day length and temperatures, along with aging milkweed and fewer nectar sources trigger a change in monarchs; this change signifies the beginning of the migratory generation. Unlike summer generations that live for two to six weeks as adults, adults in the migratory generation can live for up to nine months. Most monarch butterflies that emerge after about mid August in the eastern U.S. enter reproductive diapause (do not reproduce) and begin to migrate south in search of the overwintering grounds where they have never been before. From across the eastern U.S. and southern Canada, monarchs funnel toward Mexico. Along the way, they find refuge in stopover sites with abundant nectar sources and shelter from harsh weather. Upon reaching their destination in central Mexico beginning in early November, monarchs aggregate in oyamel fir trees on south-southwest facing mountain slopes. These locations provide cool temperatures, water, and adequate shelter to protect them from predators and allow them to conserve enough energy to survive winter. In March, this generation begins the journey north into Texas and southern states, laying eggs and nectaring as they migrate and breed. The first generation offspring from the overwintering population continue the journey from the southern U.S. to recolonize the eastern breeding grounds, migrating north through the central latitudes in approximately late April through May. Second and third generations populate the breeding grounds throughout the summer. It is generally the fourth generation that begins where we started this paragraph, migrating through the central and southern U.S. and northern Mexico to the wintering sites in central Mexico.

Western Monarchs

In any given year, adult monarchs west of the Rocky Mountains leave overwintering sites along the California coast (with a small number of sites in Baja California and Arizona) in February and March and head inland in search of milkweed on which to deposit their eggs. Once first-generation monarch eggs reach adulthood, they disperse east across the Central Valley and north across most of the western states. Second- and third-generation monarchs live and die throughout spring and summer, generally staying in the same areas where they hatched. The fourth generation (along with late bloomers from the third generation) emerges in late summer to fall. This migratory generation lives 6-9 months, compared to the 2-5 weeks of earlier generations. Western migratory monarchs also differ biologically from non-migratory generations; they are in a state of reproductive diapause, meaning their reproductive organs do not mature until later in the adult stage (after winter). Instead of looking for milkweed, fourth-generation western monarchs require nectar to build lipid reserves as they migrate south and west to the overwintering sites along the California coast (also in Baja California and Arizona), arriving around late October. Once they arrive, they roost for the winter in eucalyptus, Monterey cypress, Monterey pine, and other trees, sometimes in aggregations of thousands of individuals. In February and March, reproductive diapause ends and the annual cycle starts anew. Milkweed and nectar plant availability throughout the spring, summer, and fall will benefit western monarchs. In areas of the desert southwest, monarchs use nectar and milkweed plants throughout much of the year. For western monarch information and resources, visit the Western Monarchs category of our Downloads and Links page.

How do monarchs find the overwintering sites?

Orientation is not well understood in insects. In monarchs, orientation is especially mysterious. How do millions of monarchs start their southbound journey from all over eastern and central North America and end up in a very small area in the mountains of central Mexico? How do western monarchs find the same overwintering groves year after year? We know that they do not learn the route from their parents since only about every fourth to fifth generation of North American monarch migrates. Therefore, it is certain that monarchs rely on their instincts rather than learning to find overwintering sites. What kind of instincts might they rely on? Other animals use celestial cues (the sun, moon, or stars), the earth’s magnetic field, landmarks (mountain ranges or bodies of water), polarized light, infra-red energy perception, or some combination of these cues. Of these, the first two are considered to be the most likely cues that monarchs use, and consequently have been studied the most.

Sun Compass: Since monarchs migrate during the day, the sun is the celestial cue most likely to be useful in pointing the way to the overwintering sites. This proposed mechanism is called a sun compass. Monarchs may use the angle of the sun along the horizon in combination with an internal body clock (like a circadian rhythm) to maintain a southwesterly flight path. The way this would work is illustrated below. For example, if a monarch’s internal clock reads 10:00 AM, then the monarch will fly to the west of the sun to maintain a southern flight direction. When the monarch’s internal clock reads noon (12:00 PM), the monarch’s instincts tell it to fly straight toward the sun, while later in the day the monarch’s instincts tell it to fly to the east of the sun.

However, this would have to be combined with the use of some other kind of cue. If all the monarchs in eastern and central North America maintained a southwesterly flight, they could never all end up in the same place. It has been proposed that mountain ranges are important landmarks used by monarchs during their migration. For example, when eastern monarchs encounter a mountain range, their instincts might tell them to turn south and follow the mountain range. This kind of instinct would serve to funnel monarchs from the entire eastern half of North America to a fairly small region in the mountains of central Mexico.

Magnetic Compass: Scientists have suggested that monarchs may use a magnetic compass to orient, possibly in addition to a sun compass or as a “back-up” orientation guide on cloudy days when they cannot see the sun. Studies of migratory birds have indicated that they register the angle made by the earth’s magnetic field and the surface of the earth. These angles point south in the Northern Hemisphere and north in the Southern Hemisphere.

James Kanz (1977) conducted experiments to test the orientation of migratory monarchs held in cylindrical flight chambers. He reported that the monarchs flew in southwesterly directions on sunny days, but flew in random directions on cloudy days. He concluded that monarchs primarily use the sun to orient, and that magnetic orientation was unlikely, since the monarchs did not appear to be able to orient when they could not use the sun. However, Klaus Scmidt-Koenig (1985) reported conflicting evidence. He recorded the vanishing bearings (the direction in which a monarch disappears from sight) of wild, migratory monarchs, and found that even on cloudy days, most monarchs still flew in a southwesterly direction. Scientists attempted additional tests of magnetic orientation, but were not able to determine whether monarchs use the Earth’s magnetic field to orient.

However, researchers from the Reppert Lab (2014) showed that migratory monarchs indeed possess a magnetic compass that aids in orienting migrants south towards their overwintering grounds during fall migration. Remarkably, the use of the magnetic compass requires short wave UV-light (previous magnetic compass experiments failed to account for light at this range). With UV-light being allowed to enter the flight simulator, eastern migratory monarchs consistently oriented themselves south. The light-sensitive magnetosensors reside in the adult monarch’s antennae. While the expert consensus remains that the sun compass is the monarch’s primary compass for navigation, the authors suggest migratory monarchs use the magnetic compass to augment their sun compass.

Genetics: Upon dispersal, the Central and South American, Atlantic, and Pacific populations lost the ability to migrate. This prompted researchers to identify the gene regions in North American monarchs that appeared highly differentiated from non-migratory populations. Kronforst et al. (2014) identified 536 genes significantly associated with migration. One single genomic segment appeared to be divergent in the non-migrating populations and was extremely different from the North American population. One gene, collagen IV alpha-1, showed high divergence between migrating and non-migrating populations. Collagen IV alpha-1 is an important gene for muscle function, and divergence of this gene implicates selection for different flight muscles between migrating and non-migrating populations. Surprisingly, Collagen IV alpha-1 was down regulated in migratory monarchs, perhaps preparing them for lengthy flight. Furthermore, migrating monarchs had low metabolic rates compared to non-migrants as a consequence of flight muscle performance, lowering energy expenditure in migrating monarchs muscles. This evidence led researchers to conclude that changes in muscle function afforded migrating monarchs the ability to fly farther and use their energy more efficiently. Dr. Kronforst used the analogy of a marathon runner vs. a sprinter, "Migrating butterflies are essentially endurance athletes, while others are sprinters."

Overwintering Biology

Where do the monarchs go?

It is thought that monarchs were originally tropical butterflies that underwent range expansion. Scientists are not sure how long the monarch’s spectacular annual migration to Mexico has been occurring; it may be as old as 10,000 years (when the glaciers last retreated from North America) or as young as a few centuries. The earliest reports of overwintering clusters of monarchs in the United States were from California in the 1860s.

Monarch overwintering sites in Mexico and along the California coast have particular characteristics that enable monarch survival. These characteristics are important because they provide the monarch with the right overwintering conditions. Trees on which to cluster are one of the most important elements of the sites. The climate and the entire surrounding area are also important. Nearby trees, streams, underbrush, and fog or clouds all form an intricate natural ecosystem comprising the monarchs’ winter habitat. Monarchs need a cool place to roost so that they don’t use up their energy reserves as quickly. They also need to be protected from snow and winds. The surrounding trees serve as a buffer to the winds and snow.

Although monarchs are found in many areas of the world, the most spectacular migration occurs in North America.

Monarchs that spend the summer breeding season in eastern North America (including states and provinces east of the Rocky Mountains: central and eastern Canada, midwestern and eastern United States) migrate to the Transvolcanic mountains of central Mexico. Many millions of monarchs from these regions fly south to Mexico each fall. Their flight pattern is shaped like a cone as they come together and pass over the state of Texas on their way south. In massive butterfly clouds, they sweep up into the mountain ranges of central Mexico. Indigenous Mexican cultures of the Transvolcanic Range have welcomed the monarch migration since time immemorial; in 1975 scientists from Canada and the U.S. recorded the overwintering sites for the first time.

In Mexico, monarchs roost in oyamel fir forests, which occur in a very small mountainous area in central Mexico. Overwintering sites are about 3000 meters (almost 2 miles) above sea level and are on steep, southwest-facing slopes. Because monarchs need water for moisture, the fog and clouds in this mountainous region provide another important element for the winter survival of the monarchs. The butterflies choose spots that are close to, but not quite, freezing. They cluster together, covering whole tree trunks and branches, and cling to fir and pine needles. The tall trees make a thick canopy over their heads. Protective trees and bushes soften the wind and shield the butterflies from the occasional snow, rain, or hail. Each of the above elements is important to the butterflies, making up the monarch habitat – trees in which to roost, other trees and shrubs to protect them, the cool air, and the presence of water.

Monarchs do not reproduce until the spring, so they do not rely on milkweed during this time. They do seek out water and drink nectar from flowers in the overwintering sites. These two documents list nectar plants identified within the overwintering reserves in Mexico (documents are in Spanish): PLANTAS de la Reserva de la Biosfera Mariposa Monarca, PLANTAS.

Monarchs that spend the summer breeding season in western North America (including states west of the Rocky Mountains: Washington, Oregon, California, Idaho, and Montana) migrate to specific overwintering groves along the California coast roughly between Mendocino County in the north, and San Diego County in the South. Here, monarchs roost in eucalyptus trees, Monterey pines, and Monterey cypresses that are located in bays sheltered from wind, or farther inland where they're protected from storms. There are over 400 historic overwintering aggregations in California in addition to many temporary clusters. Scientists estimate that the California monarchs make up about 5% of the overall worldwide monarch population. The Xerces Society for Invertebrate Conservation has mapped out where to see congregations of overwintering monarchs in California here.