The migratory phenomenon of the Monarch butterfly takes place under conditions in which there have been great changes in the landscape brought about by humankind. Changes to the vegetation in the United States and Canada began in 1600 with the transformation of large areas of coniferous, deciduous and mixed forest and pastureland until the beginning of agricultural intensification at the beginning of the 1920s. Currently, some areas have been reforested, but urban expansion is increasing, fragmenting the landscape. This condition has substantially modified the climate of the region. The conversion of large areas of forest and pasture to agricultural use in the United States and Canada increased the populations of milkweeds (Asclepias spp.) on which the Monarch butterfly depends; however, over the last decade, increased application of herbicides on agricultural fields with transgenic crops of maize, cotton and soy bean in the United States has drastically reduced the populations of milkweed.
There are two simultaneous migrations, the eastern and western.
The eastern migration includes butterflies that reproduce to the east of the Rocky Mountains from southern Canada and a large part of the United States. These butterflies travel to hibernation sites in the center of Mexico in the States of Michoacan and Estado de México. The same individuals that arrived to Mexico (Methuselah generation) begin the return journey to the United States at the start of spring. This migration comprises more than 90% of the population of North American Monarch butterflies.
The western migration includes the butterflies that reproduce to the west of the Rocky Mountains from southern Canada and a portion of the United States. These butterflies travel to various hibernation sites distributed along the coast of California. The same individuals that arrived to California begin the return journey towards the north at the start of spring. This migration comprises less than 10% of the population of North American Monarch butterflies.
There is evidence that some butterflies can change routes and arrive from the west to the hibernation sites in the center of Mexico. The migratory generation lives for between 8 and 9 months, while the other generations live for approximately one month.
In addition to migratory populations, there are Monarch butterfly populations that do not migrate. These have been found in the south of Florida and throughout Mexico. The Monarch butterfly has also been introduced to Australia, Hawaii and other countries.
The great majority of Monarch butterflies reproduce in the United States and Canada. Two or three generations are completed there from April to August. In Canada, the Monarch butterflies live mainly in the provinces of Nova Scotia, Prince Edward Island, New Brunswick, Ontario and Saskatchewan and, to a lesser degree in Alberta, Quebec, Manitoba and British Columbia.
In the United States, the Monarch butterfly has been recorded in all of the states apart from Alaska. The least frequented states are those of the center of the country: Montana, Wyoming, Colorado and New Mexico.
Migration begins from the middle to the end of August. The butterflies enter Mexico during September and October, arriving in central Mexico in early November. The main states traversed by the Monarch are Coahuila, Nuevo León, Tamaulipas, Zacatecas, Querétaro, Aguascalientes, Guanajuato, San Luis Potosí, Hidalgo, Estado de México and Michoacán. They have been recorded to a lesser degree in Baja California, Sonora, Chihuahua, Durango, Jalisco, Puebla, Morelos, Distrito Federal, Veracruz and Tlaxcala.
During the migration in Mexico, the Monarch butterflies use several federal protected areas; National Parks (NP), Biosphere Reserves (BR) and Flora and Fauna Protection Areas (FFPA), and state protected areas such as Special Zones Subject to Conservation (SZSC), Natural Monuments (NM), State Reserves (SR), Conservation Reserves (CR), Areas of Sustainable Use (ASU) as follows:
- Maderas del Carmen FFPA
- Sierra de Picachos SZSC
- Serranía de Zapalinamé SZSC
- Sierra Cerro de la Silla SZSC
- San Luis Potosí
- El Potosí NP
- Sierra de Álvarez FFPA
- Sierra de Abra Tanchipa BR
- Real de Guadalcázar SR
- Nuevo León
- Sierra Gorda de Guanajuato BR
- Pinal del Zamorano CR
- Sierra de los Agustinos ASU
- Estado de México
- Iztaccihuatl-Popocatepetl NP
- Monarch butterfly BR
- Laguna Madre y Delta del Río Bravo FFPA
- El Cielo AEPRB
- Altas Cumbres SZSC
- Bernal de Horcasitas NM
- Sierra Gorda de Querétaro BR
- El Cimatario NP
- Los Mármoles NP
- Barranca de Metztitlán BR
More than 390 hibernation sites in California are distributed along the coast of the state from San Francisco to Ensenada covering a strip of more than 900 kilometers. Historically, forests of Monterey Pine (Pinus radiata) covered this region; however, these have been replaced by urban developments and small remnants of forests of eucalyptus introduced around the year 1850.
Despite the large number of sites, only about 30 sites are occupied in all years. A total of 70% of the sites are found between the counties of Santa Cruz and Santa Barbara.
The most important sites for hibernation in California include Monarch Grove Sanctuary, George Washington Park, Point Lobos State Reserve, Palo Colorado Canyon in Big Sur, Andrew Molera State Park, Sycamore Canyon in Pfieffer Beach, private site in Big Sur, Prewitt Creek and Plaskett Creek in Pacific Valley, in the county of Monterey.