Right now, millions of monarch butterflies are stretching their wings in Mexico and setting out on an epic journey north.

In just a few short months, descendants of these wandering insects will reach as far as Canada, a distance of some 2,400 miles (3,800 kilometers).

The monarch migration is one of the greatest of its kind, but due to habitat destruction and pesticide use, fewer and fewer butterflies make the trek each year. (Related: "Imperiled Monarch Butterflies Get $3.2 Million From U.S. Government.")

As recently as 1997, around one billion monarchs swarmed the wintering grounds in Mexico. But that number plummeted to just 33 million in 2013.

This year, there's some good news: Monarchs seem to be rebounding. The butterflies are covering about 10 acres (4 hectares) of forest, compared with 2.79 acres (1.13 hectares) in 2014, according to the nonprofit group WWF.

There are always threats. For instance, a sudden cold snap has led to storms that could kill the insects. These kinds of storms are precisely why we need more monarchs, says Tyler Flockhart, a conservation biologist at the University of Guelph in Canada.

“When the population is larger than expected, it gives us a bit of insurance,” says Flockhart. If a storm like this had hit when numbers were lower, then it could have had catastrophic effect on the migration, he says. (See National Geographic's amazing photos of monarchs.)

While good weather seems to be the cause of the surge in monarch numbers this year, efforts to provide the insects with habitat along their migration corridor have also picked up.

Watch: Monarch Butterflies Get Tiny Radio Trackers How monarchs make such a phenomenal 5,000-mile round-trip migration is what ecologist and National Geographic Emerging Explorer Martin Wikelski is trying to understand.

In Oklahoma, for instance, an unprecedented coalition of Native American tribes, industry, farmers, and conservation groups are working to distribute tens of thousands of milkweed and nectar plants, which monarchs require for food both as pupae and adults, according to Peter Raven, president emeritus of the Missouri Botanical Garden. (See "Monarch Butterfly's Reign Threatened by Milkweed Decline.")

"I think that it’s really that collaboration and the meaning that these insects have for nearly everyone in the country that is powerful," Raven says by email.

"It’s a specific model of what we need worldwide to create global sustainability—a concrete step."

Fortunately, the monarch butterfly is plentiful from North America to Africa, Europe, and Australia. Only its unique migration is endangered. (Read more about the world's great migrations.)

But many other species aren’t so lucky. So today, in honor of National Learn About Butterflies Day, let’s look at four other species that have quietly been marching toward extinction.

Schaus’ Swallowtail

Three years ago, when researchers went looking for Schaus’s swallowtail butterflies (Heraclides aristodemus ponceanus) in the Florida Keys, they turned up fewer than half a dozen individuals.

“Everyone thought that would be the end of it,” says Andy Warren, a moth and butterfly scientist and collections manager at the McGuire Center for Lepidoptera & Biodiversity at the Florida Museum of Natural History.

With wingspans up to 5 inches (about 13 centimeters), Warren calls swallowtails “big, showy things” that anyone can be trained to identify. However, these insects live in the tiniest of habitats—essentially just a few tropical hardwood sites in South Florida and the Keys.

But then, just as scientists were sounding the alarm that the Schaus’ swallowtail butterfly was about to go extinct, they emerged the next year in relative abundance.

Where did they come from? Warren says the species appears to be capable of spending multiple years as pupae—the stage of life between caterpillar and butterfly—likely as a way to wait for optimal conditions. This means that it’s extremely difficult to say exactly how many Schaus’ swallowtails are left at any given time. (See "How Your Backyard Can Save Butterflies.")

Even still, the Keys are a notoriously extreme environment, prone to rapid changes in vegetation, drought, fire, and hurricanes. The insects are currently considered endangered by both the state of Florida and the U.S. Fish and Wildlife Service.

Wolkberg Zulu Butterfly

At about the size of a U.S. quarter (26 millimeters), the Wolkberg Zulu butterfly (Alaena margaritacea) of South Africa can be difficult to spot. Making matters worse, the insects only take flight for three weeks out of the year, a span that just so happens to fall right in the middle of the rainy season.

Oh, and one other thing—since 1929, no one had ever seen a Wolkberg Zulu butterfly anywhere else in the world except for one small town near Kruger National Park.

In 2013, when a botanist came across a specimen that appeared to be a Wolkberg Zulu in an area outside its known range, the Lepidopterists’ Society of Africa dispatched an expedition to see if they could verify the sighting. On the first day, the experts found just one butterfly—and it was dead, frozen on a leaf.

However, the next day the scientists discovered several more butterflies floating to and fro and declared that, for the first time ever, the Wolkberg Zulu butterfly had a new home.

The International Union for the Conservation of Nature classifies the insects as vulnerable to extinction.

Island Marble Butterfly

The island marble butterfly (Euchloe ausonides insulanus) was thought to have gone extinct around 1908. But almost a hundred years later, the fuzzy green-and-white butterflies popped back up on San Juan Islands just south of Vancouver, Canada. The discovery marked the first time the species had ever been sighted on U.S. soil. (See "Why Do Butterflies Have Such Vibrant Colors and Patterns?")

No one knows quite where the marbles have been all this time, or how they have escaped notice, but scientists are working to make sure they don’t disappear again.

For starters, it seems the butterflies have taken a liking to nonnative mustard plants on the islands. As the National Park Service looks to restore prairie habitats—which would typically involve removing nonnative species—they'll need to determine whether the butterflies can also survive on native mustard plants.

Saint Francis Satyr

Some butterflies are threatened by pesticide use. Others are declining due to lack of host plants or too many parasites. But the Saint Francis satyr (Neonympha mitchellii francisci) suffers from too few beavers, which create ponds that in turn create meadow habitat for Saint Francis satyr larvae.

Unfortunately for the Saint Francis satyr, beavers have mostly disappeared from its native range in North Carolina. The butterfly now only exists within the confines of a military base called Fort Bragg, where military exercises like bombing create wildfires that make way for the meadows the butterflies require. (See "Butterflies Can Evolve New Colors Amazingly Fast.")

There are currently only thought to be something like a thousand Saint Francis satyrs left on earth, inhabiting a range of just 20 acres (about 8 hectares)—smaller than the average U.S. shopping mall.

Unknown Extinctions

And these are just the species we know about.

There are around 20,000 butterfly species known to science, with new ones being discovered all the time. There’s really no way to find out if most of them are in decline, says the Florida museum's Warren, and species are likely going extinct without us noticing.

“I’ve named a number of species that are known so far from just a single specimen,” he says.

“So, I guess they’re all sort of tied as the world’s rarest butterfly.”