Heading South

October 1, 2014

Monarch butterflies have the right idea.



Instead of sleeping, shivering, or hiding their way through a snowy winter, monarch butterflies take to the skies every fall for their annual migration to warmer climates.

From as far north as the northeastern United States and Canada, where monarch caterpillars feed on milkweed plants, these insects travel hundreds of miles to mountainous spots in Mexico and southern California where they spend the winter hibernating.

Reports of hibernating butterflies in southern California date back to the 1860s, but scientists believe the butterflies were in the habit of migrating long before humans noticed. Even after we realized monarch butterflies were leaving home come fall, nobody figured out where else they went until 1975, when scientists tracked them down in the mountains of central Mexico.

These butterflies require a very specific winter habitat that only a few areas offer. The Oyamel fir forests take up a very small area of the mountaintops of central Mexico. The roosting sites are about 2 miles above sea level and the fog and clouds that surround them provide essential moisture for the butterflies, who cluster together on protective trees.

How do they know where to go? Nobody knows for sure. Only about every fifth generation makes the migration, so they don’t learn from their parents. Monarch butterflies probably use the sun combined with their own circadian rhythm to establish which direction they need to fly. They might also use an internal magnetic compass. The earth is one giant magnet, and birds and insects can sense the magnetic field and follow it south. Neither of these hypotheses explain how they manage to find the same small area year after year. Scientists are still studying.

As our instruments become more advanced, we’re better able to study migrating butterflies. This fall, satellites over St. Louis, Missouri, captured a picture of a large, butterfly-shaped cloud heading south.




After some confusion, scientists realized they were seeing a swarm of migrating butterflies. This was an especially welcome sight, since butterfly populations have been decreasing in recent years, due to changing weather patterns. The site of thousands of flying butterflies is a hopeful sign!