Monarch butterfly migration is the phenomenon, mainly throughout North America, where the monarch subspecies Danaus plexippus plexippus migrates each autumn to overwintering sites near the west coast of California or mountainous sites in central Mexico. Other populations from around the world perform minor migrations or none at all. This massive movement of butterflies has been recognized as "one of the most spectacular natural phenomena in the world".
The North American monarchs begin their southern migration in September and October. Migratory monarchs originate in southern Canada and the northern United States. They then travel thousands of kilometers to overwintering sites in central Mexico. The butterflies arrive at their roosting sites in November. They remain in roosts atop volcanic mountains on oyamel fir trees (Abies religiosa) during the winter months and then begin their northern migration in March, back to North America and southern Canada.
Two to three generations of monarchs complete the migration north. Female monarchs lay eggs for a subsequent generation during the northward migration. Four generations are involved in the annual cycle. The generation undertaking the southbound migration lives eight times longer than their parents and grandparents due to a regulatory age-inducing hormone. Similarly, the western populations migrate annually from regions west of the Rocky Mountains to overwintering sites near the coast of California.
Not all monarch populations make major migrations. Monarchs migrate short distances in Australia and New Zealand. There are some populations of D. p. plexippus, such as in Florida and the Caribbean, as well as another subspecies (D. p. megalippe) distributed in the Caribbean, Central America and northern South America, that do not migrate. Additional overwintering sites have been identified in Arizona and northern Florida. In encouraging news, the eastern monarch butterfly population nearly doubled in 2025, according to a report announced in Mexico. The population wintering in central Mexico's forests occupied , up from during the previous winter. While monarchs occupied nearly twice as much forest habitat as they did during the previous year, populations remained far below the long-term average.
Historical accounts As late as 1951, monarchs were mistakenly thought to overwinter as adults or pupae. Roosts of thousands were observed in the southern regions of North America.
Migrating western populations of Danaus plexippus and their overwintering sites were known long before the Mexican winter sites were discovered by scientists in 1975. Pre-Hispanic Native Americans, the Purépecha and Otomi once occupied this area and tied the harvest of corn to the arrival of the butterflies. Monarchs appear in legends of the people that live near overwintering areas. In the areas surrounding the overwintering sites in Mexico, local residents were quite aware of the overwintering behavior of monarch butterflies long before 1975. The local people, called the Mazahua, have lived near the overwintering sites for centuries. The arrival of the monarch butterflies is closely tied to the traditional Dia de los Muertos celebrations. Local residents today easily recall seeing the migrating butterflies prior to 1975.
For at least a century, monarchs were observed overwintering in California. Historical records kept by lepidopterists do not mention the presence of monarchs in their current western range extending northward through Washington, Oregon and Canada. Female Monarchs lay their eggs exclusively on milkweeds, which provide the caterpillars and later adults with protection from predators, and it is speculated that milkweed may not have been available until western lands were cultivated, resulting in the expansion of the butterfly.
More formal migration studies began when Fred Urquhart graduated from the University of Toronto in 1935 and accepted a graduate fellowship in the field of entomology. In 1937, Urquhart began to plot the route taken by the migrating butterflies. He was the first to record that monarchs move in a south to southwest direction during the North American fall and that these movements were correlated to high pressure systems. He began the first successful tagging program which returned data. He and his volunteers recognized the existence of roosting behavior.
Fred Urquhart advertised for 'interested persons' in the Mexican press to assist him in locating the roosting sites. Catalina Trail and Kenneth C. Brugger responded and in January 1975 they led him to one of the major overwintering sites. Urquhart, William Calvert, John Christian, and Lincoln P. Brower collaborated to publish the details of this discovery in 1976. There was some concern at the time that public knowledge would endanger the butterflies.
Since 1976, 12 roosting sites have been identified to the northwest and west of Mexico City in the Mexican states of Michoacán and México. At least 70% of the monarchs were typically concentrated in only three of those sites. At least twenty colonies existed in Mexico during 1997.
As of 2023, one of the larger known overwintering sites was in the Joya Redonda Monarch Butterfly Sanctuary, located southeast of Mexico City, in the foothills of the volcanoes Popocatépetl and Ixtaccíhuatl in the state of México's municipality of Atlautla. Clusters of monarchs roost in the sanctuary's oyamel fir trees each winter. Although area residents had earlier seen small numbers of monarchs, more butterflies began to establish colonies in Atlautla during the early 2000s as deforestation to the northwest destroyed old hibernation grounds. Assisted by local volunteers, the Joya Redonda's colonies increased tenfold in one year. During the 2021–2022 season, only four colonized trees existed; during the following season, there were 42. At the time, tourists were charged 100 pesos (around U.S. $5.50) for access to the sanctuary.Multiple sources:
Migrations
Southward
Although the exact dates change each year, by the end of October, the population of monarchs east of the Rocky Mountains migrates to the sanctuaries of the Mariposa Monarca Biosphere Reserve within the Trans-Mexican Volcanic Belt pine-oak forests in the Mexican states of Michoacán and México. They also overwinter in areas that are privately owned. Some monarchs migrate to other locations such as Cuba and Florida in the fall.
Two migratory flyways exist through North America. One in the Central states leads to the Mexican overwintering areas and a smaller flyway along the eastern North American seaboard. The timing of the eastern flyway lags behind the more central flyway. Monarchs migrating along the coast are less likely of being recovered in Mexico. This suggests that butterflies migrating along the eastern seaboard are migrating to locations other than Mexico, or they have a higher rate of mortality than those migrating inland.
Monarch butterflies are thought to respond to different cues that promote the fall season southern migration. These include the angle of light coming from the sun, the senescence of larval host milkweed plants, the decreasing day period and temperature drop. The migration begins at the northernmost summer range approximately in August. Migrating monarchs are thought to rely heavily on the nectar of fall flower composites that lie along the migration path.
Generally speaking, the eastern monarch population migrates from southern Canada and the midwestern United States, almost directly south towards Mexico. Monarchs from the Northeast tend to migrate in a southwest direction, and those transplanted from the Midwest to the East Coast migrated directly south but then reoriented their path to the southwest in one study. Geographical features affect the migration route.
In general, the North American western population of monarchs migrates from land west of the Rocky Mountains, including northern Canada to California. Australian monarchs that migrate travel from the west to eastern regions closer to the Pacific.
Monarch diapause
In most individual adult butterflies, diapause begins with its southern migration, but unlike other insects in this state, it remains active.The Insects; Structure and Function, 4th Edition. R.F. Chapman, Cambridge University Press, 1998. , p 404.The Insects; Structure and Function, 4th Edition. R.F. Chapman, Cambridge University Press, 1998. , p 403.Tauber, M.J., Tauber, C.A., Masaki, S. (1986) Seasonal Adaptations of Insects. Oxford University Press, 414 pp. When diapause is initiated the butterflies accumulate and store lipids, proteins and carbohydrates. Monarchs migrating to Mexico accumulate more lipids than those migrating to California. Fats and lipids reduce water to provide energy reserves and prevent desiccation.Epicuticular Wax Secretion in Diapause and Non-diapause Pupae of the Bertha Army worm, Author: HEGDEKAR, B. M., Source: Annals of the Entomological Society of America, Volume 72, Number January 1, 15, 1979, pp. 13–15(3)Publisher: Entomological Society of America These substances are used to maintain the insect throughout diapause and to provide fuel for development following diapause termination. It occurs genetically well in advance of environmental stress. In diapause, high-metabolic activities are reduced, resulting in reduced oxygen consumption.The Insects; Structure and Function, 4th Edition. R.F. Chapman, Cambridge University Press, 1998. , pp. 403–404
Monarchs in diapause of the fall migrating population are physiologically distinct from those in active reproduction behavior in the spring and summer. In diapause, fats and lipids levels can be as high as 34%. The fat storage organ is substantially larger in migrating and overwintering monarchs compared to the summer generations. Samples of tissue excluding the fat body also show higher levels of free lipids in the hemolymph. Females in diapause show little evidence of mature eggs. Mating is repressed and only occasionally observed among overwintering monarchs. This is thought to increase the survivability of winter populations and maintain fat reserves that will promote spring northward migration. At one site, the population stayed in diapause until the middle to the end of January. By the beginning of February the day length increases to just over 11 hours, the point at which monarchs come out of diapause.
Diapause has distinct phases. Decreasing day period and dropping temperatures inhibitHerman, 1981, Biological Bulletin Vol 160, No1, pp 89-106 the production of juvenile hormone. This represses the development of gonadal activity, mating behaviors, and egg-laying. New behaviors emerge such as the development of social nectaring groups and late afternoon formation of night-time clusters or roosts. Roosting reduces water loss, probably due to decreased surface-area-to-volume ratios reducing evaporative water loss.
Northward
There is a northward migration in the spring. Female monarchs lay eggs for the next generation during these migrations. Northward migration from Florida normally occurs from mid-March to mid-May and the initial wave of the migration may be the offspring of monarchs that have overwintered in Florida and along the northern Gulf Coast, not in central Mexico. Tagged monarchs from Tallahassee were recovered in Virginia and Georgia.
The distance and length of these journeys exceeds the normal lifespan of monarchs, which is less than two months for butterflies born in early summer. The first generation leaving the overwintering sites only migrates as far north as Texas and Oklahoma. The second, third and fourth generations return to their northern breeding locations in the United States and Canada in the spring.