THURMAN: It's your first day in the world, your first step into a vast and daunting landscape... ...full of possibilities... wonder... and peril.

[ Wind whistling ] You've just been born in the Rockies, an untamed wilderness in the heart of North America.

And you have much to learn.

Each new challenge will test your resolve.

Every new opportunity will hone your skills.

♪♪ Above all, you'll need to stick close to Mother.

Only her care and vigilance can protect you... and prepare you for a life in the wilds of the Rockies.

♪♪ ♪♪ ♪♪ [ Wind whistling ] THURMAN: It's late February in the high deserts of central New Mexico.

Evening temperatures can drop well below freezing, locking these Sandhill cranes in a layer of ice.

Those that have freed themselves must walk gingerly.

[ Ice cracking ] They're huddled together in this shallow wetland for a simple reason... security.

Coyotes are patrolling the shoreline... ...looking for any openings.

♪♪ [ Warbles ] But there are far too many eyes, and the cranes are already awake.

♪♪ About 20,000 cranes over-winter here, mostly family groups made up of couples and their young.

It may not seem like the most romantic setting, huddled together with a few hundred of your closest relatives.

But make no mistake.

This is a love story.

In the still of the morning, couples call to each other... [ Cranes calling ] ...keeping track of their lifelong mates.

Some have been together for more than a decade.

♪♪ But the end of winter marks the beginning of a new journey.

Cranes signal with their heads that it's time to take flight.

♪♪ One by one, they launch awkwardly into the sky.

Countless generations before them have undertaken this same journey -- an unbroken migration that has lasted thousands of years.

They are headed north, across the sandstone cliffs of New Mexico and into the heart of the Rocky Mountains.

The Rocky Mountains stretch some 2,000 miles, forming the backbone of North America.

From central New Mexico, the Rockies continue northward across six western states to the northern edge of Canada, encompassing at least 100 different mountain ranges and dozens of unique habitats, from shifting sand dunes to deep river valleys.

Each providing a home for countless animal families on the verge of a new beginning.

♪♪ ♪♪ And just as they have done for millions of years, the Rockies begin to shed the months of frozen dormancy... ...and prepares to welcome the next generation into their midst.

♪♪ Weeks after leaving New Mexico, Sandhill cranes arrive to their natal range in Yellowstone Valley.

[ Cranes calling ] When it comes to settling down, cranes prize personal space.

This is a call to anyone in earshot -- "This spot is ours."

Having flown here with his parents, the yearling tags along.

But now it's time for Mom and Dad to renew their vows... with a dance.

♪♪ The youngster watches his parents intently... ...learning the very moves he will one day employ to woo his mate.

♪♪ ♪♪ ♪♪ At last, all this extravagance culminates in one final move.

And now all they have to do is wait.

♪♪ Nestled in the heart of the Rocky Mountains, Yellowstone is the world's first national park... ...and the largest nearly intact temperate ecosystem on Earth.

Spring is the time when new families are taking their first tentative steps into the world.

♪♪ For many young, it's their first big adventure aboveground.

It's a time to play... explore... and torment your siblings.

And in some cases, test your mother's patience.

Play is an essential part of childhood... ♪♪ ...and one that, sadly, will not last forever.

For this playful grizzly mom and her 3-year-old cub, this will be their last spring together.

♪♪ For some, this time of year marks the end of a relationship, while for others, it's the beginning of a new one.

♪♪ In the open grasslands of Yellowstone, a mother bison is giving birth.

[ Bison grunting softly ] [ Birds chirping ] Mom frees her calf from the membrane that entangles it.

For her, this is love at first sight.

The same hormones that surge through a human mother's brain now surge through hers.

For the next several months, this calf will be the singular focus of her life -- the one thing she will nurture and protect.

Yellowstone is slowly awakening, puling itself out from under winter's grip.

It's a treacherous time, as the ground is melting away beneath them.

And for a newborn still finding its footing, it's easy to get knocked off course.

This is a deadly situation.

Many bison, much older than this calf, have met their ends here.

But there is little his mother can do.

He's on his own now.

Although he's only days old, he must find the strength... [ Bison grunting softly ] ...the will to survive, to fight with every muscle in his body.

To fight or to die trying.

The relieved mother hurries to greet him... ...licking him dry, as she did the day he was first born.

He was fortunate.

But in the months ahead, the Rockies will continue to test our calf's resolve.

♪♪ Some 55 to 80 million years ago, an intense period of plate tectonic activity reshaped the western half of North America and gave birth to the Rocky Mountains.

[ Wind whistling ] This massive wall of mountains has an enormous influence on weather patterns across North America.

Air currents moving eastward from the Pacific are disrupted by the Rockies... ♪♪ ...generating massive cyclones across the Great Plains of North America.

♪♪ ♪♪ And just as the Rockies have transformed the land, they have also shaped the animals that call it home.

Mountain goats survive where few others can, even in the depths of winter.

Nannies and their week-old kids pick their way carefully through the ever-softening snow.

Mom has memorized every nook and crag of this mountainside.

And she knows an avalanche risk when she sees one.

Her kid was born into a vertical world, with all the tools he needs to become an accomplished mountaineer.

Now all he needs is experience, and he's enrolled in the ultimate master class.

Their destination is a high alpine meadow they hope will have verdant new shoots.

[ Thunder rumbling ] At 10,000 feet, weather can turn quickly.

But mom is patient and reassuring.

And no one is better equipped to weather the storm.

♪♪ ♪♪ ♪♪ As the days grow longer, spring unfolds in a spectacular display of wildflowers.

Bear grass covers the alpine meadows of Glacier National Park.

And balsamroot blankets the low, rolling hills of Western Montana.

♪♪ Rocky Mountain bighorn sheep and mountain goats eagerly gobble up the new blossoms.

♪♪ ♪♪ Bighorn births are perfectly timed for this bounty of spring.

[ Birds chirping ] And in secluded spots all along the mountainside, mothers are tending to their newborn lambs.

This imprinting isolation can last for several days.

It's a chance for mothers and young to bond with each other, learning their individual scents and sounds before rejoining the rest of the herd.

For one new family, that task is doubly hard.

She has birthed twins, an exceptionally rare occurrence for Bighorn sheep... ...and one that obviously makes the task of mothering twice as demanding.

With each passing hour, the twins grow stronger.

[ Lamb bleats ] They are playful and energetic... ...but still wobbly on their feet.

Predation from mountain lions, coyotes, and golden eagles are an ever-present threat, and the sooner mom and the twins can rejoin the herd, the safer they will be.

♪♪ To survive in the Rockies, young must grow up fast... ...many within the span of a single season.

But in these early stages of life, offspring are wholly dependent on their mother's care.

For some, the job is more demanding than others.

With 11 hungry pups to feed, this fox mom has her hands full.

Raising this many pups is not only a physically demanding job, but one that requires an extraordinary amount of patience... which this mom seems to have in abundance.

It's a good lesson for our mother of twins, and one she'll need to take to heart, as her lambs grow more adventurous.

[ Birds chirping, calling ] For one crane family, the raising of their young has only just begun.

After a month of waiting, the eggs are beginning to hatch.

One of the colts has already emerged, but his sibling is not quite ready.

The thick crane egg has been slowly weakening, as the tiny colt inside absorbs the shells' calcium into her young bones.

♪♪ This process can take hours.

And now that her first colt is out and ready to feed, mom is anxious for colt number two.

She nudges the egg to break the rest of the shell.

♪♪ ♪♪ And with a gentle assist, her second colt, still weak from its cramped quarters, is finally free.

♪♪ ♪♪ [ Duck quacking ] Dad won't take any chances with visitors close by.

♪♪ But within a few hours, both colts are shakily on their feet.

♪♪ Their first meal will be the pulverized eggs they lived in.

♪♪ This will help them grow strong from the start.

♪♪ ♪♪ The first hatched is quick to challenge his sibling.

Like many first-time parents, they learn through experience.

By feeding the colts separately, they avoid in-fighting and ensure they both get equal portions of food.

Foraging through the muddy mess for small worms and insects is a routine that comes more easily to some... than others.

By the end of summer, these tiny colts will be as tall as their parents.

The very act of having young heightens a mother's senses.

And it is, by far, the most consuming relationship in an animal's life.

A mother grizzly emerges from the dense forest with three tiny cubs in tow.

Named after the black volcanic glass that forms Yellowstone's Obsidian Cliff, she leads her cubs on their first big outing.

♪♪ Obsidian's cubs were born in the depths of winter.

And for months, their mother fed them, without eating.

These cubs are her entire world, and she is theirs.

♪♪ ♪♪ Despite their fiercely protective mother, these cubs are vulnerable and sticking as close to mom as possible, trying hard to get her attention.

♪♪ But Obsidian will need more than grasses to sustain them.

It's time to introduce her cubs to their new home... ...a fantastical world of bubbling pools and steaming mountains.

They live atop one of the largest active volcanic systems on Earth.

♪♪ Beneath the Yellowstone Caldera is a reservoir of hot magma, 30 miles deep.

And it lies just under their feet.

♪♪ With each passing day, Obsidian takes her cubs farther and farther into her territory.

But these journeys are not without their risks.

A single encounter with a male grizzly could prove fatal for the cubs.

♪♪ Obsidian will need to remain vigilant if she hopes to keep her family safe.

♪♪ ♪♪ ♪♪ ♪♪ Up in the high elevation, these snow-free days won't last long.

So, while mothers are busy feeding, kids are getting to know their extended family while playing an endless game of King of the Hill... ...learning skills that will prepare them for life in a vertical world.

Soon, mothers hurry to collect their kids as the herd continues to move.

[ Kid bleats ] But amidst the excitement, one kid has lost track of his mom.

[ Bleats ] He calls to her, but no reply.

And he can't see or smell her anywhere.

All the goats are rapidly shedding their winter coats, so mom looks different from day to day.

♪♪ The only way to find out is to get close enough to smell.

♪♪ One by one, he's rejected.

Nobody seems to want him around.

The sun is beginning to set, and the kid is growing increasingly anxious.

♪♪ ♪♪ In a last-ditch effort, he chases after a lone nanny walking away.

When he reaches her, he sniffs cautiously, having made so many wrong guesses already.

This time... he guessed right.

♪♪ As twilight descends over the mountain, the two bed down together on a nearby cliff.

The kid is overjoyed to have his mother back.

[ Kid squeals ] On this mountain, their bond is everything.

♪♪ A fog has enveloped the craggy peaks of western Montana.

Bighorn sheep are just beginning their day.

♪♪ And for these young lambs, the fun is about to begin.

Playgroups have formed.

♪♪ ♪♪ ♪♪ Many lambs are still timid, uneasy with their footing on the steep testing ground packed so tightly with the rest of the young.

Mothers, too, are rightfully concerned with their world so suddenly expanded, they must try to keep their young ones safe.

♪♪ ♪♪ These young rams have no such concerns.

♪♪ And they seem intent on provoking this year's new lambs into action.

♪♪ If they can brave these games, it will serve them well in the years ahead.

Bighorn sheep form alliances that last lifetimes.

The twins have settled into this new friend group, almost as important as their mother's care.

Mom is quick to comfort her small lamb but equally quick to nudge him back into play.

Building confidence is crucial to their survival.

♪♪ For parents, growing up seems to happen in the blink of an eye.

For young, every day brings a new adventure.

♪♪ It's all about building relationships... ♪♪ ...testing your boundaries... ...and finding out... exactly what you're capable of.

♪♪ ♪♪ ♪♪ ♪♪ ♪♪ [ Birds chirping ] Raising these young colts is keeping Mom and Dad busy.

Colts have the appetite of a teenager and can grow an inch a day.

They will need every morsel if they are going to migrate south this autumn.

But this family has a bigger problem on their hands.

[ Bell ringing, lawnmower running ] Their little wetland is wedged in the middle of a sea of townhouses.

Long before this suburb of Bozeman, Montana, was here, this area was part of a vast network of wetlands that may have been home to generations of cranes.

[ Train whistle blowing in distance ] This couple has stubbornly clung to this tiny patch of wetland, but that's become a dangerous choice.

Only about 1 in 10 colts survive to adulthood, and those odds are considerably worse in urban settings.

Even as protective as these parents are, in this urban maze, they will need a new set of skills to survive.

[ Dog barking ] [ Panting ] Dad paces rapidly back and forth, tossing grass and sticks into the air.

[ Crane squawks ] He's sending a message -- "Stay back!"

[ Children playing ] Safety and security are choices typically made with their wings.

But until these colts are old enough to fly, this family is grounded.

But the colts are growing fast.

And their parents must decide just how much of the suburbs they can take.

Once the family leaves their home, there's no looking back.

If they are to find safety, they will need to make the journey on foot.

[ Motorcycle engine revs in distance ] [ Truck horn honks, vehicles passing ] ♪♪ ♪♪ The parents take their first cautious steps into the road.

♪♪ And the tiny colts slide down the hot asphalt to follow behind.

♪♪ This 4-lane road is the most exposed these colts have been in their young lives.

♪♪ ♪♪ Much like walking a tight rope, you have to keep looking straight ahead.

Whatever you do, don't look down... on the road.

[ Horn blaring ] ♪♪ [ Horns blaring ] ♪♪ ♪♪ [ Horn blares ] ♪♪ They have survived their first big step.

Growing up and raising a family is tough enough, but we've made that task doubly hard.

Expanding suburbs and industrial development continue to fragment habitat across the Rockies.

For some, roads and fences become barriers to migration... ...often splitting apart families and making animals vulnerable to predators.

♪♪ And each year, the wild becomes just a little less so.

Yet the Rockies still remain our last best chance for preserving many of North America's most iconic species.

♪♪ ♪♪ By early summer, snow that has been locked away in these high mountain peaks begins to melt away.

Rivers swell as spring run-off reaches its peak.

It's a powerful force.

The Lamar River is raging.

And this presents a new set of challenges for our bison herd.

The cold, quick-moving water bottles up the herd at the shoreline.

As the leaders wade out, testing the current strength... ...calves must stay close, trust in the herd, and remain calm.

♪♪ ♪♪ The current is beginning to pick up and causing many newcomers second thoughts.

♪♪ Midway, in the deepest section, panic sets in.

The calf tries to turn back.

Quickly, he's swept downriver, farther and farther from his mother.

♪♪ He struggles to swim upstream... ...as his mom can only watch from the shoreline.

♪♪ His legs slow from icy-cold exhaustion... ♪♪ ♪♪ ♪♪ ♪♪ ...the lucky calf is reunited with Mom.

Maybe a little wiser about the power of the wild world around him.

♪♪ ♪♪ Not all youngsters in Yellowstone Valley are so lucky.

[ Thunder rumbling ] Sometime in the twilight hours, tragedy struck.

[ Bats screeching ] ♪♪ ♪♪ [ Birds chirping ] Now, in the light of day, Obsidian seems restless, nervous.

♪♪ She huffs to call her cubs.

♪♪ But now her family of three... is down to two.

♪♪ One of her cubs is missing.

We may never know just what happened in those twilight hours, but the cubs certainly do.

♪♪ Losing a youngster has made Obsidian even more vigilant.

She retreats into the forest to nurse her two remaining cubs.

No family is immune to the harsh reality of growing up in the Rockies.

Life here is tenuous.

And while the loss of life for one means survival for another, nothing is ever so simple.

Our bison herd rushes to the place where one of their own has fallen -- a female that died during labor.

We gather together to mourn the loss of our loved ones.

Is it so difficult to imagine that bison may do the same?

♪♪ Tails are raised in distress by the sight of a wolf waiting in the wings.

Eventually, the herd begins to disperse... ...leaving one lone 3-year-old.

Perhaps a son, still grieving.

♪♪ While his sense of smell clearly tells him she's gone, he's determined to defend her a while longer.

♪♪ There is far more that connects us than separates us, especially in matters of love and loss.

♪♪ ♪♪ In the small clearing, Obsidian is still nursing her cubs.

♪♪ A shadow on the hillside above her draws her to her feet, perhaps a passing bison.

But Mom is not taking any chances now.

She keeps a watchful eye while her cubs play.

Obsidian faces the same struggle of every parent -- how much freedom is too much?

♪♪ ♪♪ As the summer sun bakes the plains, tensions are building.

It's mating season.

And for our young calf, everything has suddenly changed.

For months, he's been tied to his mother's side, but now her attention is focused elsewhere.

♪♪ And the large bulls he's watched all summer are suddenly too close for comfort.

They're agitated and chasing his mother around.

He tries to keep up with her, but that's a dangerous choice.

♪♪ This time of year, bulls are competing for the right to breed.

♪♪ And they display their fitness in the most dramatic fashion -- kicking clouds of dust into the air as they urinate and then roll their 2,000-pound bodies against the bare earth.

This bull is sending a message.

It's a chemical billboard.

It tells the other bulls he is at the peak of his game.

♪♪ ♪♪ One of the large bulls is sticking close to the calf's mother, tending her.

♪♪ But he's treading a difficult path.

He's surrounded... and the competition is stiff.

♪♪ If he wants to keep her to himself, he's going to have to prove that he can.

♪♪ ♪♪ It's a deadly serious game.

♪♪ The success of future generations depends upon the outcome of these gladiatorial battles.

♪♪ And as the two bulls exhaust themselves, someone seizes an opening.

♪♪ ♪♪ ♪♪ Amidst the chaos, the calves seek comfort in each other.

And as with every competition, there is a victor.

♪♪ These are the calves' last days of childhood.

And in just a few years, they'll compete for real.

♪♪ If they hope to be so fortunate, they must endure all that the Rockies have in store for them in the months ahead.

♪♪ As colors shift into autumn, Obsidian's calm demeanor has returned.

[ Birds chirping ] She calls for her two remaining cubs.

[ Obsidian growls ] Now, at the end of their season, her cubs are chubby and confident as they follow after Mom.

Soon, Obsidian and her cubs will retreat to the warmth and safety of their winter den.

While some take refuge underground, others must prepare to head south to outrun winter's icy grip.

But for many others, they will have to endure winter's fury head on.

There will be battles to fight... ...and alliances to forge.

Some must rely on the strength of their tribe.

While others must go it alone.

For all these brave souls, the experience of growing up in the Rockies has only just begun.

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

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