page_title: Monarch butterfly migration
section_title: Population and migratory study methods
hierarchical_section_title: Monarch butterfly migration / Population and migratory study methods
caption_attribution_description: English: Western monarch populations 1997-2013 from Xerces data
context_page_description: Monarch butterfly migration is the phenomenon, mainly across North America, where the subspecies Danaus plexippus plexippus migrates each summer and autumn to and from overwintering sites on the West Coast of California or mountainous sites in Central Mexico. Other subspecies perform minor migrations or none at all. This massive movement of butterflies has been called "one of the most spectacular natural phenomena in the world".
  The monarchs begin their southern migration from September to October. Eastern and northeastern populations, up to 500,000 monarch butterflies, migrate at this time. Originating in southern Canada and the United States, they travel to overwintering sites in central Mexico. The butterflies arrive at their roosting sites in November. They remain in their roosts during the winter months and then begin their northern migration in March. No individual butterfly completes the entire round trip. Female monarchs lay eggs for a subsequent generation during the northward migration. Four generations are involved in the annual cycle.
context_section_description: Initially, direct observation was the primary method used to assess monarch migration. More sophisticated methods have been developed since 1975. 
  Population counts "dramatically" vary year to year. The cause of the variations are attributed to natural occurrences,  different  methods used to count migrating butterflies, and man-made changes to the habitat. The  validity of the population census at overwintering sites in North America is questioned. The discrepancy between migrating populations and populations present at the overwintering sites suggests a significant adult mortality during the migration. The Commission for Environmental Cooperation has determined that population variations require a long-term and large scale monitoring effort Population estimates of adults, or of eggs and larva, and milkweed abundance, should correlate with the censuses at the overwintering sites. Data are currently unavailable at this time to determine these censuses but a current study by The Monarch Larva Monitoring Project is designed to determine whether or not population censuses in Mexico match the population censuses in the Midwestern United States and Canada.
caption: Western monarch populations 1997–2013 from Xerces data
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