Yellowstone

For six months Yellowstone endures an unrelenting winter.

An ice world of hunters.

And hunted.

But now footsteps on the slopes of a mountain ...

herald a change in the season.

The sun regains its strength.

Water flows again.

The race begins to recover winter's losses.

To play.

Learn.

And breed.

But the summer ahead will be fleeting ...

and far from easy ...

in this unpredictable wilderness.

It's April.

The land lies exhausted by winter.

Everything is waiting for the return of the sun's warmth.

The end of winter comes late ...

to the mountainous northwest of the United States.

And in particular, it lingers in Yellowstone.

The heart of Yellowstone is a high, cold plateau ...

8,000 feet up, surrounded by the spires of the Rocky Mountains.

After five months sleeping in those mountains,

a powerful presence returns.

This female grizzly bear ...

has awoken early to find food for her two-year-old cubs.

But she may have brought her family here too early.

The Tetons,

on the extreme southern edge of the Yellowstone plateau.

It's where the spring thaw normally begins.

But this year it's come late.

Deep snow is still a novelty for her cubs.

But she is much more focused.

Led by her nose, she risks her 300-pound bulk ...

on the thin ice of a bend in the Snake River.

It doesn't look promising ...

but her cubs follow with only a little hesitation.

With paws that can knock down an elk,

she feels delicately in the water.

A fish frozen beneath the ice by this cold winter.

One fish won't feed the family ...

but she knows there are almost certainly more.

The cubs must learn to find them.

A little success for one of the cubs.

This mother bear's experience ...

gives her family a good chance of survival until spring arrives.

They will return again and again as the thaw releases more fish.

On the warmer plains and valleys that surround the plateau ...

grazers are on the move.

They left the high country to avoid the worst of the winter,

and now begin the long journey towards the heart of Yellowstone ...

in time for summer.

These elk are led by experienced adults ...

who have made this trip many times before.

They are followed on their migration by the most ancient of American grazers.

Pronghorn.

Though predators may follow, none can outpace them.

Over distance, these are the fastest animals in the world,

taking a long migration in their stride.

Migrating elk and pronghorn must follow the advancing thaw.

But higher up in the valleys one animal is well ahead of them.

Bison are adapted for cold.

These herds need to be in Yellowstone early ...

to have time to bring up their young in the short summer ahead.

Calves born during the journey will be tested.

Clues to why the herds are drawn to the Yellowstone plateau ...

lie in its deep and unique history.

This plateau is the cratered bowl of a huge sleeping volcano ...

and molten rock below the surface still pushes the land up ...

to the cold of high altitude.

But it has not been the only force to shape the land.

Millennia ago the volcano was covered by an ice sheet ...

thousands of feet deep.

The few glaciers that border Yellowstone are an echo of a time ...

when ice scoured and smoothed the entire plateau.

When these great ice sheets retreated,

they left pulverised rocks in their wake,

the foundation for fertile soil ...

and some of the richest natural grasslands in America.

But in May this year,

in the heart of Yellowstone, that grass is still beneath thick snow.

Something this ground squirrel was clearly not expecting.

But as the sun's strength returns,

spring bulbs start to push through the late snow.

At last a sign that winter is losing its grip.

Bluebirds, too delicate to survive deep winter here,

now return to take advantage of the first hatch of stoneflies ...

from a thawing river.

As six months of snow and ice begins to melt,

Yellowstone starts to come back to life.

Cascading snowmelt swells the streams and rivers.

It's a hazard that the migrating herds must cross.

Young are barely up to the task.

The Yellowstone River has increased its flow tenfold.

In less than two weeks ...

the thaw has brought a remarkable change to the heart of Yellowstone.

As the migrating herds arrive in the plateau's river valleys,

the spring is here.

This is why they have made the journey.

After the famine of winter,

food is everywhere.

Birds have flown in from as far away as the Arctic and subtropics.

Canada geese join flocks of white pelicans to breed here.

A snow-covered wasteland is transformed ...

to an American Serengeti.

But it's a landscape like no other.

Geysers made Yellowstone famous.

Yellowstone sits above the molten core of a volcano.

Underground water becomes super-heated and erupts into the air.

Strangely, geysers and hot springs can creep around the landscape.

The curious shapes of Grotto Geyser are actually trees,

entombed and petrified by minerals dissolved in the water.

Hot water scorches and drowns the roots of trees encroaching on the valleys.

It can keep the forests in check and the rich pastures open.

Those pastures now produce the new grass needed to make rich milk ...

and feed a bison baby boom.

The few calves born on the migration ...

are now joined by hundreds of new youngsters.

The predators have been waiting for this.

Coyotes have a voracious reputation here,

but on its own this one is certainly no match for a healthy bison and her calf.

Pronghorn give birth to twins,

each carefully cleaned to hide their smell from predators.

While they are with her they are relatively safe ...

and for now the coyote is content with the afterbirth.

But the mother pronghorn must leave her calves alone while she grazes ...

and with legs as wobbly as stilts,

their best defence is to lie low.

She has young of her own to worry about, a demanding litter of nine,

whose arrival has been carefully timed to coincide with an abundance of food.

But they are not top dog here.

Yellowstone's top predator, the wolf,

dominated during the winter but is far less visible in summer.

They prefer to stay close to well-hidden dens ...

to protect their pups from rival packs.

This pack seem prepared to allow their young a brief foray ...

to experience the world beyond their den.

Even in Yellowstone ...

it is extremely rare to see wolf pups like this.

Before long they are summoned back into the forest by the pack.

It will be winter before they are regularly seen again.

As May turns to June,

Yellowstone sees its nursery season.

By early June summer is finally here.

In the flower meadows ...

plants turn sunlight into the sugar of nectar,

and that's what Yellowstone's smallest migrating bird has been waiting for.

The Calliope hummingbird has made the journey all the way from Mexico.

He'll make this flower patch his own ...

and he'll fight to defend it.

He'll even take on a far larger Rufous hummingbird.

But his high-octane lifestyle critically depends on the summer sun,

and in Yellowstone that's never guaranteed.

In the mountains surrounding the plateau, summer has not yet arrived.

At this altitude it is still bitterly cold.

These mountains barricade Yellowstone in a ring of ice.

And if the sun is swallowed by cloud,

the cold can drive down from the mountains ...

bringing with it a punishing echo of winter.

It can snow in any month in Yellowstone.

The warm nursery meadows of a few days before ...

are now buried by a June blizzard.

A curtain of snow dulls senses.

This mother grizzly bear must now be especially careful.

A male grizzly will often kill cubs if he finds them.

And though she would rather sit out the storm, she must move.

Panicky cubs could easily be lost in a blizzard.

As soon as she feels it is safe,

she settles to suckle and reassure them.

Bison can shrug off this sudden change in the weather.

They are, after all, built to survive a Yellowstone winter.

But this tiny hummingbird is more vulnerable.

Flowers damaged by snow will die ...

and the nectar the hummingbird needs goes with them.

Hummingbirds must be resourceful to survive.

Bark wells dug by a sapsucker, a type of woodpecker,