The Monarch Butterflies ANNUAL migration OF over 3,000 miles across North America iS one of the most mysterious and greatest natural events on the planet.

            Story by Jason Bittel, National Geographic October 2017

Each fall, millions of monarch butterflies leave their summer breeding grounds in the northeastern U.S. and Canada and travel over of 3,000 miles to reach overwintering grounds in southwestern Mexico.

But unlike birds or other animals that also embark on epic migrations, these individual butterflies will never return. 

No one knows how they know how to reach their destination!

As the days get shorter and the temperatures drop off, monarchs begin to abandon breeding and feeding territories in search of a safe place to spend the winter.

For monarchs, that overwintering ground is found high up on just a few mountains in central Mexico. Once there, the monarchs huddle together by the millions on the branches of oyamel fir trees.

These trees, also known as sacred firs, create a microclimate that protects the insects.

“The tree canopy and ecosystem provide a blanket effect for the monarchs, so the temperatures don’t go too high or too low,” says López.

After waiting out the winter, these individuals head part of the way back north to warmer climes such as Texas, where they mate and lay eggs on milkweed plants. In just a few days, the eggs hatch into brilliantly striped caterpillars of black, gold, and white. These monarch larvae consume vast amounts of milkweed before forming a chrysalis and transforming into adult butterflies.

Butterflies congregate at a wintering roost in Sierra Chincua, Mexico. The cool mountain climate slows their metabolism and saves energy.

At this point in the cycle, the new butterflies take to the skies again and fly another few hundred miles north before finding another patch of milkweed and repeating the process.

It might take the monarchs as many as four to five generations to complete the journey all the way back up to Canada, says Sarina Jepsen, who directs the endangered species program for the Xerces Society, an international nonprofit focused on invertebrate conservation.

Then, when fall rolls around again, the monarchs make use of air currents to wing all the way back to Mexico—a seemingly impossible feat for such a delicate-looking insect.

"Interestingly, the waves of monarchs heading north will complete their entire life cycles in just five to seven weeks each," says Jaramillo-López. 

"But when fall rolls around again, a special 'super generation' of monarchs that can live up to eight months will make use of air currents to wing all the way back to Mexico—a seemingly impossible feat for such a delicate-looking insect."

"This makes the migrating monarchs so unique as they are the same species but for some reason live much longer."

Why Do Monarchs Migrate, and How Do They Know Where to Go?

This part is still very much up for debate, says Jepsen. But there are a couple of theories.

One explanation might be that when monarchs march north, they are essentially following the bloom of their primary food source—milkweed plants. And then when winter comes and those plants die back, the animals retreat to a place with conditions that protect them from the weather.

Another idea involves their immune systems. “Migration also allows monarchs to escape habitats where parasites have accumulated at the end of the summer,” says Sonia Altizer, an ecologist at the University of Georgia. 

Altizer has found that adult butterflies infected with a protozoan parasite called Ophryocystis elektroscirrha, or OE, can’t fly as well in lab tests and travel shorter distances in the wild. (Read m

“We think that migration weeds out the most heavily infected monarchs, removing them from the populations,” says Altizer.

As for how the monarchs know where to go, a study published in 2016 suggests that the critters navigate based on their relative position to the sun. But scientists are still ironing out exactly how this works.

Why Are Monarch Butterflies Important?

While monarchs may seem small and insignificant, the creatures play a crucial role in the ecosystems they inhabit.

As adults, monarch butterflies visit countless numbers of wildflowers each year as they seek out nutrient-rich nectar. In doing so, the monarchs transfer pollen from one plant to another and assist in those species’ reproduction. 

And even though monarch caterpillars and adults are poisonous to most predators, thanks to toxins they acquire from milkweed, some animals are still able to stomach them. Orioles and grosbeaks in particular make a feast of monarchs over the winter, and ants, wasps, flies, and spiders have been known to prey on the caterpillars when they get the chance.