Monarch butterflies are much tougher than they look. Every year the delicate insects endure one of Earth's most amazing migrations, embarking on a 2,500-mile odyssey across North America that spans four generations of butterflies.

This has been going on for at least a million years, with incredible clouds of monarchs flocking between wintering grounds in Mexico and summer habitats as far north as Canada. Lately, however, the phenomenon has begun to fade. Storms, habitat loss and milkweed decline are widely blamed for curbing the butterflies' abundance, and their 2013-'14 migration included the lowest population ever recorded.

Since monarchs envelop specific habitats in Mexico every winter, scientists can estimate their yearly population by measuring how much acreage the insects cover while resting in trees. At their low point in 2013, Mexico's wintering monarchs covered just 1.66 acres. Last year's total was slightly higher, reaching 2.8 acres.

This year, however, a butterfly boom has blanketed about 10 acres of those mountain forests with orange-and-black wings, according to the U.S. Fish and Wildlife Service (FWS) and the World Wildlife Fund (WWF). That's a one-year increase of 255 percent, and Mexican wildlife officials estimate it includes roughly 140 million individual butterflies — which would be a significant jump from 35 million just two years ago.

That doesn't mean monarchs are out of the woods yet. While 10 acres is great, it's still less than a quarter of the nearly 45 acres they covered in 1996, the species' most successful winter on record. Nonetheless, the 2015-'16 rebound at least offers hope that humans are capable of saving this wonder of nature from ourselves.

"The good news coming from Mexico makes me enormously enthusiastic," FWS Director Dan Ashe said in a press conference last week. "It indicated that we have the capacity to save the monarch butterfly of North America."

monarch butterflyAn adult monarch flits among summer wildflower blooms in Madison, Wisconsin. (Photo: Richard Hurd/Flickr)

Monarchs start reaching Mexico around October, then spend winter clustering on trees in pockets of high-altitude oyamel fir forests. In spring, millions fly back across the continent to start all over. (The species has since spread to other parts of the world, but a 2014 study found it likely evolved in Mexico, and has been migrating across North America for the past 1 million years or so. Its northernmost forays into Canada began more recently, after the ice sheets receded.)

Such large-scale migration would be impressive for any insect, but the process isn't just arduous for North America's eastern monarchs — it outlasts their life spans. That means they need to breed along the way so they can pass the baton to their offspring, who instinctively know how to finish the journey.

monarch butterfly migration mapEastern monarch populations take marathon migrations east of the Rocky Mountains, while western monarchs inhabit a smaller range and spend winters along the California coast. (Map: U.S. Fish and Wildlife Service)

And that's where milkweed comes in. Monarchs rely on this plant, both for food and as a place to lay their eggs. More than 70 species of milkweed exist, as MNN's Tom Oder wrote in 2014, but monarchs only use about 30 as hosts. And of those, just four species sustain 98 percent of eastern monarch populations.

While herbicides and habitat loss have been killing milkweed across the U.S. and Canada for years, illegal logging has also chewed into the rare Mexican forests on which monarchs' winter survival depends. Combined with the effects of severe storms — and with more extreme weather patterns associated with climate change — this has made an already-difficult journey even harder.

Yet efforts are under way to make life easier for monarchs again. Mexico set aside the 138,000-acre Monarch Butterfly Biosphere Reserve in 1980, for example, and it became a UNESCO World Heritage site in 2008. The government has also cracked down on illegal logging in monarch habitat, even recruiting local farmers for extra enforcement (although the problem hasn't gone away). And the U.S. is replacing swaths of lost milkweed as well as reining in use of herbicides that kill the plant. According to Ashe, the U.S. added 250,000 new acres of milkweed in 2015.

The monarch recovery is still fragile, notes WWF-Mexico director Omar Vidal. But while butterflies are prone to big yearly population swings, this winter's gathering in Mexico at least suggests our rescue efforts might be starting to work.

"Migratory monarchs need partnerships across North America that will address the use of herbicides on crops, illegal logging and our changing climate," WWF-Mexico says in a statement. "It's a big challenge, but together we can save them, and save this unique migratory phenomenon."

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.

Migratory monarch butterflies bounce back
The insects' winter migration has rebounded from a record low two years ago, but they still have a long way to go.