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 4.42 acres (1.8 ha), up from 2.22 acres (0.9 ha) 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 50 miles (80 km) 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.
Migrations
Southward
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