It’s not just birds that migrate this time of year. In September and October, monarch butterflies (Danaus plexippus) head for warmer climates, too. As far as scientists who study butterflies know, they are the only butterfly that makes a two-way migration. It’s an astounding feat of navigation and of endurance. The butterflies can travel over 3000 miles, riding breezes and stopping to rest and eat wherever they can find sustenance.
Monarchs in the U.S. don’t all go to the same place when they migrate. They are split into Eastern and Western monarch populations. Team East migrates to the Sierra Madre mountains in Mexico, and Team West migrates to California.
Eastern monarchs overwinter in Mexico from October to March. They come from Canada, New England, the East Coast and the Upper Midwest and Plains states, through the Midwest and down into the south, picking up more and more butterflies along the way. They then cross the Gulf of Mexico to their wintering grounds.
Monarchs at the butterfly sanctuary in Mexico. Photograph by Carlos Adampol Galindo from DF, México, CC BY-SA 2.0, via Wikimedia Commons.
To keep warm, they cluster together on trees in high-elevation oyamel fir forests where temperatures and humidity are optimal for their survival. There can be tens of thousands of monarchs on one tree. This overwintering site is protected by the Mexican government, and has been named the Monarch Butterfly Biosphere Reserve.
Team West monarchs have a shorter trip, heading to the coast of California, near Santa Cruz and San Diego. They gather in eucalyptus trees and in Monterey cypress and Monterey pine trees.
Monarch butterflies roosting at Amistad National Recreation Area near Del Rio, Texas, on their migration south from Canada to Mexico in the Fall. Photograph by the National Park Service.
Monarchs only fly during the day. They look for the warmth and shelter of pine, fir and cedar trees to roost in during cool autumn nights, though they will roost in other trees and shrubs if those trees are not available. In the morning they will bask in the sunlight to warm up their wings before taking off. Pushed by the wind, they can cover up to 80 miles in a day.
Researchers don’t know exactly what navigational tools monarchs use to find their way, but they believe it includes the position of the sun and the magnetic pull of the earth.
Given the monarchs’ life cycle (they live about nine months, longer than most butterfly species) most butterflies don’t live long enough to make the round trip. Monarchs overwintering in Mexico fly back to U.S. in spring, getting far enough to lay eggs and then die. Their offspring head further and further north. The complete journey occurs over three or four generations.
If you’re growing fall-blooming plants, especially natives, your fall garden may well be visited by passing monarchs. While milkweed is the only food source for the caterpillars, adult monarchs consume the pollen and nectar of many flowers, including asters, echinacea, liatris, coreopsis, gaillardia, goldenrod, and ironweed in addition to milkweeds.
At monarchwatch.org, there’s a chart that tells approximately when monarch migration peaks at different latitudes. And there’s a website for reporting monarch migration sightings: journeynorth.org. This is a citizen-scientist site that helps scientists track monarch populations. There are also national wildlife refuges you can visit to see this incredible natural phenomenon.
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