♪♪ CHURCH: On the southern border of the United States lies a remote desert kingdom, home to staggering landscapes and a remarkable world of wild creatures.

This is Big Bend -- where the legendary Rio Grande river has cut deep canyons between Texas and Mexico -- but the wildlife knows no borders in this truly special place... ♪♪ Here, the most talked-about frontier in the world is a wonderland of serene beauty... Where mountain lion and black bear roam.

Where bats stalk the desert floor at night.

A land of blazing skies and tortured rock, and home to one of America's great national parks -- the road ends at Big Bend.

[ Theme music playing ] ♪♪ ♪♪ ♪♪ ♪♪ CHURCH: The exact details are unknown, but some 30 years ago, a female black bear set out on a remarkable journey from northern Mexico.

She was heading for Texas.

♪♪ Human borders meant nothing to this bear; she was on a mission.

She was looking for a new home, a place free of competition... where she could bring up her cubs in peace.

Far on the northern horizon, there was such a place.

♪♪ Mountains reaching for the sky with flanks of green hinting at unseen treasures.

A wonderland for a pioneering bear.

Bears had been wiped out of Texas many years before.

Some had since strayed over the border, but she was the very first to stay and raise a family... and what a place she picked.

Surrounded by blistering desert, Big Bend hides a mountain kingdom of sparkling streams and hidden canyons, bursting with forests and life.

♪♪ Here, she'd be in good company -- for Big Bend is home to a national park with more kinds of wild creatures than almost any other park in the United States.

♪♪ In Big Bend, she had found America's wildest frontier.

♪♪ [ Birds squawking ] It is fall in Big Bend, and this acorn woodpecker is busy.

For just a few weeks, these oak trees will be laden with acorns -- perfect food for the long winter ahead.

If only he can find a safe place to store them.

♪♪ The tallest dead tree in the valley is as good as he'll get.

High above thieves in the forest below, he and his family can concentrate on the work in hand.

♪♪ It's a laborious business, each hole must be drilled just right: too big, and the acorns fall out; too small, they just won't fit in.

♪♪ ♪♪ As the weeks go by the larder grows, a bonanza for the long months ahead.

But their stash is just too tempting.

♪♪ While he's off hunting, a young female moves in with less than noble intentions.

♪♪ Maintaining this winter stash takes up a huge amount of time and energy, well worth the effort if they can keep it safe and secure.

♪♪ But life can be very cruel to a Big Bend woodpecker.

♪♪ ♪♪ There are many reasons Big Bend is good for bears, and this is one of them.

Many months have passed, and fall is a distant memory -- but there are still acorns to be found... and bears love acorns.

This bear is skin and bone -- he's had a tough winter.

Nothing is going to stop him.

♪♪ ♪♪ For six days in a row, he keeps returning to this fountain of food -- much to the disgust of the acorn woodpecker.

But there's nothing he can do except watch months of hard work being dismantled in minutes.

[ Woodpecker squawking ] Most adult bears would think twice before climbing so high in a rotten tree, but the fear of falling seems to be the lesser of two evils.

If there's more food higher up, this bear will do whatever it takes.

♪♪ ♪♪ [ Bear roars ] ♪♪ ♪♪ ♪♪ Acorns are full of protein and fat -- lifesavers for a desperate Big Bend bear.

♪♪ [ Exhales, munching ] Now he just has to get down.

[ Water lapping softly ] The name Big Bend was born of a river -- the legendary Rio Grande.

♪♪ For over 1,000 miles, the Rio Grande carves a giant arc around the wild lands of southern Texas.

♪♪ To the 18th century settlers, the remote country cradled by this giant curve in the river became known as the Big Bend.

♪♪ Today the river and its bend marks the border between Texas and Mexico.

♪♪ The Rio Grande is one of the youngest river systems in North America, its story freshly written in the stone it has carved along its three million year journey.

To a thirsty desert, water brings the gift of life.

♪♪ Big Bend is hummingbird heaven.

Some 15 different kinds have been spotted here, out of barely 30 in all of the United States.

Many are just passing through -- tiny birds on huge journeys.

But some are born of Big Bend.

These are lucifer hummingbirds.

For the lucifers, it's breeding season -- and they must build up their energy reserves for the challenges ahead.

♪♪ The male lucifers have dazzling purple throat feathers, which need to be in peak condition to impress the less-colorful females.

But first the field must be cleared -- even other hummingbird species are not tolerated.

There's only room for one male lucifer in this town.

♪♪ ♪♪ After all his territorial battles, you'd think the female would give in easy.

But she herself now needs to be chased, for him to prove he's worthy her attention.

Winning over this lady truly is an exhausting business.

However, she still seems unsure.

But he has one final trick up his sleeve... some hummingbird flamenco.

♪♪ ♪♪ Who could resist such perfection and poise?

♪♪ All the hard work is paying off.

It's "take me home, darling."

♪♪ The human story in Big Bend goes back around 10,000 years.

Stone Age pioneers hunted and gathered across the region but left only the faintest traces.

In the last 1,000 years, several Native American tribes came and went -- the Chiso and Apache.

The very last were the Comanche, who journeyed across these lands just 150 years ago.

♪♪ The greatest challenge to Big Bend's wild world came, of course, from more recent settlers as they pushed into the remotest corners of wild America.

Until 1848, Big Bend was part of Mexico.

[ Music playing, indistinct chatter ] Since then, waves of Mexican and American settlers have moved all across the region -- to ranch, or mine precious quicksilver.

That mix of cultures and people live here to this day.

MAN: ♪ It never took money to feel like livin' ♪ ♪ Never can try for me... ♪ [ Fire crackling ] ♪♪ CHURCH: For the wild inhabitants of Big Bend, it mattered little what tribe or creed were the human settlers of the region.

What mattered to them was how many humans came and how they treated their wild neighbors.

♪♪ ♪♪ ♪♪ In the decades after the last Native Americans were pushed out, Big Bend was the end of the road... A place to escape the law.

If in real trouble, you could head south to Mexico across the Rio Grande.

Once you made it to the river, it was rarely deep enough to slow you down.

♪♪ Today in Santa Elena Canyon, a unique resident of Big Bend crosses from the United States to Mexico dozens of times a day.

With deserts in every direction, this is one of the last places you'd expect to find beavers.

♪♪ At sunrise, one of the most talked about international borders in the world is also a peaceful place to find breakfast.

The second-largest rodents on Earth, beavers spend their nights foraging for food along the Rio Grande.

Early morning means a final snack before they head for bed.

Young willow trees and cane make up the majority of their diet.

Each beaver must eat between two and three pounds of vegetation a day to keep going.

The sound of chewing beavers has echoed through these canyons for thousands of years.

[ Munching ] Another exercise beavers devote their time to is fur control.

To keep their coat in top condition, beavers spend long hours grooming their fur and oiling it from special glands under their tail.

♪♪ ♪♪ ♪♪ After a big feed, nothing seems to make a beaver feel quite so good.

♪♪ ♪♪ From the mid-1600s, America's beavers were heavily hunted by the new European settlers.

They were as good as cash in the Wild West.

But hidden in this remote backwater, Big Bend's beavers managed to survive these centuries of wildlife extermination.

The Texans behind the national park here dreamed of connecting Big Bend with the neighboring protected lands across the river in Mexico...

Choosing to let these natural treasures unite the two countries in a place where the wildlife knows no borders.

♪♪ ♪♪ Big Bend sits in the Chihuahuan Desert, the largest in North America.

Come spring, small corners explode in a blizzard of blue.

These are Big Bend's famous bluebonnets.

Growing up to three feet tall, the flowers dwarf their famous cousins across the rest of Texas.

♪♪ Another desert plant that defines Big Bend is the Ocotillo.

For much of the year, Ocotillos look like a burnt clump of dead branches.

But come spring -- and the right conditions -- these gray stems deliver a bounty of blood orange flowers.

♪♪ ♪♪ Shared by the United States and Mexico, the Chihuahuan Desert is one of the most biologically diverse on the planet.

And while the big critters get all the glory, it's the little creatures that make this region truly special.

Thousands of different kinds are the backbone on which all of the wildlife of Big Bend depends.

Big Bend is one of the hottest corners of Texas.

Even in spring, temperatures can rise to 100 degrees.

Not that some of Big Bend's residents seem to mind too much.

[ Rattling ] Rattlesnakes, like this western diamondback, and most other reptiles, do need external heat to warm their muscles and get moving.

But some spring days can be too hot even for these cold-blooded creatures.

♪♪ But sunbathing is a particular specialty of this Big Bend resident.

Texas horned lizards need a lot of sunlight to produce healthy levels of vitamin D. Hard to spot with their remarkable camouflage, it takes a lot to get one of these characters moving... even when dinner delivers itself.

♪♪ ♪♪ ♪♪ Night brings a whole new world to Big Bend.

♪♪ Temperatures can drop 60 degrees from daytime highs.

The darkness and chill evening breezes bring out a whole new cast of characters.

This is their time.

♪♪ [ High-pitched croak ] [ High-pitched croak ] ♪♪ ♪♪ More than 20 different kinds of bats roam the night sky here.

♪♪ And this is one of the most remarkable -- the pallid bat...

So called because of their light coloring, pallids are almost unique among bats... ...because of their hunting strategy.

♪♪ Unlike almost any other bat, pallid bats stalk their prey on the ground.

♪♪ But they wouldn't take the risk if it wasn't worth the reward.

♪♪ One theory about their unusual hunting strategy, is that pallids are much better able to hear their prey from ground level.

♪♪ ♪♪ But as many hunts fail as are successful -- prey can give off toxic chemicals or simply fight back.

♪♪ ♪♪ But their unusual survival strategies don't stop there.

Unlike almost any other insect-eating bat, pallids also drink nectar from cactus and other desert flowers.

Because they have to burrow so far to get to the nectar, the pallids get covered in pollen.

And they'll deliver great quantities of this precious cargo to the next flower they visit on their lifelong journeys through the desert night.

Depending on your disposition, Big Bend has been called both a geologist's paradise and nightmare, so complicated are its landscapes and their stories.

♪♪ Some 140 million years ago, this was one vast, shallow, salty sea, dumping layers of sand across the entire region.

♪♪ Huge pressures within the Earth's crust then forced up the mountains, and later, magma burst through, forming these volcanic islands in the sky.

♪♪ And all of these stories in stone have been constantly scraped and sculpted by sun, wind and water.

♪♪ [ Insects chirping ] In spring, visitors might just catch a flash of crimson in Big Bend's skies.

These are vermilion flycatchers -- and while common all over Mexico, are in the northern limits of their range in Big Bend.

This male's dazzling colors advertise his success as a hunter.

In peak condition, he's positively radiant compared to his female partner.

♪♪ The couple have three newly hatched chicks and for the next few weeks the youngsters will demand all of their parent's time and energy.

Both mom and dad feed the new arrivals, but when they need to be kept warm or quiet, it's usually mom who steps in to protect them.

♪♪ Not far away, in a nest barely the size of a golf ball, this Anna's hummingbird has two healthy chicks almost ready to leave the nest.

♪♪ This is unique footage -- the first time these birds have ever been seen nesting in Big Bend.

♪♪ Though the chicks hatched barely two weeks ago, they've already grown most of their adult feathers.

They're keen to leave their tiny home, but their first attempts at freedom can be a little clumsy... and dangerous.

If this chick falls to the ground its life is over.

There's little the panicked mother can do.

♪♪ ♪♪ After five agonizing minutes, the youngster is back where it should be -- hopefully with a lesson learned.

Not far away, the flycatcher dad is still hard at work hunting for his offspring.

♪♪ Barely two weeks after hatching, the chicks leave their nest for good.

But they haven't yet learned proper flying and hunting skills, so the two parents are still flat out keeping up with their demands.

♪♪ And then one day, the parents don't return.

Barely three weeks out of the egg, they're all on their own in a hot and hostile desert.

Summer has arrived in Big Bend.

When the sun climbs high in the sky, almost nothing stirs... ♪♪ Only mad dogs and Englishmen go out in the midday sun -- so an old song goes.

Neither are common in Big Bend.

Their place is taken by one of the region's most colorful characters.

A heat specialist, this male great earless lizard seems to relish the midday sun.

Earless lizards don't live very long, so this mature male must hook up with a mate as quickly as possible.

Unfortunately, there are plenty of other males about.

Maybe showing off his splendid colors will send them packing.

♪♪ Males are territorial and try and guard their patch from all comers.

It's an exhausting business.

♪♪ He's spotted a female... so he tries some moves that wouldn't be out of place in a city gymnasium.

♪♪ How many push-ups can you do... when its 100 degrees?

♪♪ His masculine maneuvers pay off -- the female is on her way.

Mating is brief.

She'll go off and lay four or five eggs in the sand, while he gets back to the important business of looking good for the next lucky lady.

While summer brings extreme heat to Big Bend, it also brings rain.

[ Thunderclaps in distance ] Clouds roll in from the Gulf of Mexico, laden with water sucked off the surface of the Caribbean Sea.

[ Rain pattering ] In the very hottest months of the year, Big Bend gets most of its annual ration of rain.

At first it almost seems the desert creatures are stunned by its arrival.

♪♪ But there's no time to waste, for it won't last long.

♪♪ ♪♪ The water holes are magnets for some of Big Bend's most elusive creatures, usually almost impossible to find in its vast desert landscapes.

♪♪ ♪♪ [ Splashing ] [ Bear grunts ] Some 700,000 years ago, the ancestors of an iconic desert animal reached North America across what is today the Bering Straits.

They had evolved in North Africa and their journey into America took them as far south as Texas and northern Mexico.

♪♪ These are desert bighorn sheep.

Countless numbers of this ram's ancestors once roamed America until centuries of settlement and hunting almost wiped them out.

Just a few thousand individuals survived... and none in Texas.

But then began one of America's great wildlife reintroduction stories.

It started with just seven animals released in 1973.

Today more than 1,000 run wild in some of the most spectacular landscapes in Texas.

♪♪ Desert Bighorns are fantastically well adapted to Big Bend's parched landscapes.

Supreme climbers, they rarely stray more than a few hundred yards from a cliff or steep slope -- safe zones where no predator can match their speed or agility.

Male bighorns usually keep to themselves in small bachelor groups -- until breeding season.

Then all the rules change.

Hormones are loose on the desert wind.

Before all-out war, the bachelors start honing their combat skills.

Who will do final battle for the females?

And it's not all noble jousting -- they have plenty in the dirty tricks department... some of it below the belt.

♪♪ ♪♪ This provocative behavior helps to narrow the field: Who is truly willing to take on who?

Weeks of this minor warfare finally sort out who is up for the ultimate fight.

♪♪ ♪♪ These massive collisions would shatter most animals' skulls, but everything about the bighorn is built for battle.

♪♪ ♪♪ The winner has won the right to mate with the females -- this year's youngsters will carry his genes.

Finally, it's time for some rest and recuperation.

He's played his part in the return of bighorn sheep to Big Bend country.

Summer is fast becoming a memory.

But the summer rains have left their legacy.

One third of the year's rain fell between July and September.

In the mountains, it's a gift that keeps on giving.

♪♪ This is not New England in the fall, but a canyon in the middle of the Chihuahuan Desert.

♪♪ Stands of Douglas Fir, quaking aspen, and bigtooth maple have survived in the Chisos Mountains since the last Ice Age.

And outposts of oak -- and oak means acorns.

♪♪ ♪♪ ♪♪ For Big Bend's pioneering bears, fall is a time of peace and plenty.

[ Growls ] Long afternoon naps in the leaf litter have some downsides.

But few itches will survive a good stretch and a favorite tree trunk.

This female's pregnant and building up precious layers of fat for the lean months ahead -- and with luck, some new arrivals in spring.

♪♪ Big Bend's bears don't hibernate, but do go into long periods of low activity, or torpor, to conserve energy.

For this mother-to-be, her fat reserves will be her lifeline.

♪♪ [ Wind blowing ] Winter in Big bend is a brief affair.

If the north winds blow and the snow queen does visit, warm southerly breezes soon send her packing.

♪♪ ♪♪ ♪♪ By late March, temperatures are climbing.

And under the cover of darkness, a voracious predator is closing in on Big Bend.

♪♪ Flying through the night he has come from the south, where he spent the winter waiting... and preparing for his journey north.

♪♪ For the next five months, he will bring a swift and silent death to countless creatures of the Big Bend night.

He is an elf owl -- the very smallest owl in the world... barely five inches high and weighing less than a tennis ball.

But for elf owls, size truly doesn't matter.

They'll tackle prey almost as big and powerful as themselves.

♪♪ ♪♪ ♪♪ This male elf owl now has a partner.

She's moved into their nest -- an old woodpecker hole in a giant agave stalk.

Sitting on eggs, for the next few weeks she'll need him to do most of the hunting.

And when she's hungry she lets him know all about it.

[ Twittering ] ♪♪ ♪♪ From dusk to dawn, the male scours the surrounding desert for fresh supplies.

♪♪ ♪♪ After all his efforts, he seems tempted to have a quick snack on the job.

But Mom is not impressed.

Sitting on three ravenous chicks, she's not tolerating that kind of behavior.

Every scrap of food is precious now.

And with another three or four weeks before the young leave the nest, this extraordinary little father is facing many more hunting missions through the long desert nights.

For the acorn woodpeckers, spring has been uneventful -- so far.

But trouble is on the horizon.

♪♪ And this time it's a family affair.

♪♪ ♪♪ ♪♪ The woodpeckers can only look on in anger.

♪♪ ♪♪ But there is some consolation: this mother bear is both healthy and heavy, and not desperate enough to risk climbing to the very top of the tree.

♪♪ The woodpecker's stash up there is safe -- for now.

♪♪ But there are two other thieves in training.

♪♪ Still too young for the taller trees, it won't be long before these student climbers want to push their limits -- and they'll be light and strong enough to go wherever they want.

♪♪ ♪♪ [ Cub squawking ] But that is some months away yet.

For now, there is so much to see on the ground.

Over the next year they'll depend on their mother to feed and protect them... ...and show them around the new world they have been born into.

♪♪ These cubs are truly special.

Bears born in Big Bend, who will call this place home... ♪♪ ...a mountain and desert kingdom that for so long was missing their kind.

♪♪ They carry on the legacy of that pioneering female who came across the desert barely 30 years before.

If nothing blocks their way, these cubs may come and go from their ancestral lands in Mexico.

♪♪ Big Bend's bears were the first to return to Texas... and with luck, they will not be the last.

♪♪ ♪♪ ♪♪ ♪♪ ♪♪ To learn more about what you've seen on this "Nature" program, visit pbs.org.

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