7 Things You Didn’t Know About the Annual Monarch Butterfly Migration
Boost your butterfly intelligence with these fascinating facts about how and why monarchs fly thousands of miles every autumn.
Most folks tend to view the butterfly as a solo artist, each pretty pollinator flitting from flower to flower individually. And for the most part, this is true. Except every autumn, monarch butterflies (Danaus plexippus) gather en masse for a long, incredible journey. If you’ve ever seen a bunch of butterflies winging it together and wondered what was up, read on for surprising facts about the amazing monarch migration.
The monarch butterfly, also known by several other names, including milkweed (for the plant on which they lay eggs) and tiger (thanks to their striped orange-and-black pattern), exists in various parts of the planet, but unlike others of their species, they cannot tolerate cold weather. That’s why the North American populations migrate south from late August through October. Monarchs found west of the Rocky Mountains make their way to California, while those east of the Rockies head all the way to Mexico.
A Group Effort
A group of butterflies is called a swarm or, more colorfully, a kaleidoscope. Scientists believe that monarchs migrate in large groups to keep each other warm at night, when they land by the thousands to roost in trees. There may also be a safety-in-numbers aspect at work, since a traveling or resting cluster might resemble one large creature, making would-be predators (typically birds, lizards, and toads) think twice.
Delicate as they may appear, monarchs manage to fly between 50 and 100 miles a day during their autumn migration from the northeastern United States and Canada to California or Northern Mexico, where they hibernate. The entire trip is between 2,000 to 3,000 miles, but monarchs move at a fairly quick clip of about 30 miles per hour.
Female monarchs lead the migration, with males following in their wake. Although the lovely ladies have smaller wings and smaller flight muscles than the fellas, their wings are thicker and sturdier, making them more efficient flyers, a University of Georgia study recently discovered. The journey south is accomplished by just one generation of monarchs—pretty impressive when you realize that the trip north, in the spring, takes three to four generations.
How Many Hectares?
The monarchs’ migration isn’t counted in numbers; it’s estimated by how many hectares the population occupies. (A hectare is a unit of area equal to 10,000 square meters.) In the 1996–97 season, monarchs occupied a whopping 18.19 hectares, according to the Monarch Butterfly Biosphere Reserve, while in the 2013–14 season, the number had dropped to an alarming .67 hectares. Numbers have rebounded but they’ve been up and down ever since, with monarchs occupying 2.10 hectares in the 2020–21 season.
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Estimating Times of Departure
In late summer and early fall, monarchs become sensitive to cooling nighttime temperatures and gradual reduction in daylight hours—a signal that it’s time to fly south. They also notice that milkweed begins to wither and yellow, another indication that it’s time to move on. During their winter hiatus, monarchs enter diapause, a hormonally controlled state of dormancy that is also sensitive to temperature changes. Once their internal alarm clock sounds, the butterflies awaken, knowing it’s time to head north again.
While monarch butterflies are not currently in danger of extinction, their migratory process is threatened: The monarch population wintering in Mexico last year decreased by 26 percent. The World Wildlife Fund cites the culprits of climate change and deforestation wreaking havoc with the monarchs’ southern hibernation territory and northern breeding ground. Herbicides and pesticides are also wiping out milkweed—the only plant on which monarchs lay their eggs. Go here to save the monarchs by helping to reseed 1 billion square feet of grasslands and wildflowers.