Original subtitles

NARRATOR: 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.

"YELLOWSTONE"

"SUMMER"

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

8000 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 superheated 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

quickly fill with sugar-rich sap as soon as the warm weather returns.

A little pilfering provides this hummingbird with the energy it needs

until the new flower buds open.

Summer moves quickly.

By late June the blizzards of a few weeks before seem a distant memory.

At last the summer is well-enough established

for this hummingbird to take the plunge and raise a brood.

The living is easy.

But it won't last long.

At this altitude the air is so dry

that rain evaporates before it even hits the ground.

The water that flows off the plateau is no longer being replaced.

Already there is drought in forests beyond the meadows.

Yellowstone has vast ancient lava fields.

This porous volcanic rock holds onto very little water.

It is dominated by lodgepole pine

a tree that can cope with the arid conditions.

The forest may be parched

but there is still water here in lakes formed by ancient glaciers.

A visiting male otter woos a female on the lake's shore.

But she seems a little distracted.

Her two pups are nearby.

They're about 10 weeks old

and the time is right to lead them from the holt.

Males are not above stealing food from cubs

so she makes it very clear that he is not going to be joining them.

This female grew up on the lake

so she knows where to take the cubs for a very special trip.

Yellowstone cutthroat trout

amassing in the mouth of a stream that feeds the lake.

They're preparing for their annual spawning.

But the trout must wait.

The stream is still running fast

too turbulent to lay their eggs.

It's a nervous time for them and for good reason.

This osprey has a brood to feed and will be back.

Now the otters are here too.

She chooses her moment to lead the pups upstream.

Then leaves them to watch

and gives them a lesson in hunting.

A large trout like this full of eggs is a trophy catch.

She chews its tail first to make sure the fish can't get away.

While the pups are still demanding milk

the mother otter will often keep the whole fish for herself.

Her pups are happy enough with caviar.

As the flow becomes slow enough for the trout to lay eggs

there is a brief window of opportunity to spawn

before the river gets too shallow to swim upstream.

The female digs a trough with sinuous flicks of her body

and the attending male fertilises the eggs as they are laid.

Millions of new lives.

It's now early July

and what looks like more snow

is actually a blizzard of seeds and insects.

There is more life in the park now than at any other time of the year.

It's a turning point in the summer.

Through the coming month the days will get hotter and drier.

The plants will grow less vigorously

and the pace of summer will slow.

The meadows are not so welcoming now.

The grass becomes less nourishing

as it puts its energy into seeds and roots.

Bison lose the last of their winter coats.

In the heat tempers fray.

To escape soaring ground temperatures

cowbirds choose a bison's back as a cool place to feed

and catch the breeze.

Biting insects now become a draining nuisance.

A grizzly bear escapes the heat and bugs

in the cool waters of a lake.

As the ground dries

bears find little food to eat out on the Yellowstone plateau.

There is less to graze and digging roots is hard work

so they start to head to the cooler mountains beyond.

In the mountains

glacier lilies that finished flowering a month before on the plateau

are only just coming into bloom.

Up here where spring has only just arrived

bears can find food again.

As the thaw reaches up into the mountains

a part of Yellowstone that few ever see is briefly revealed.

From its first day above the snow

this pika is obsessed by food.

It's in a frantic race to restock its winter larder

before the snow closes in again.

They're a part of a community of small mammals

which live life on the edge up here.

Carefully chosen plants are harvested and dried in haystacks

before being stored below ground.

This larder must be over 50 times the pika's bodyweight

to get it through winter.

The summer is so short up here

that few animals can permanently live any higher.

But higher is exactly where the bears are heading

above the tree line

towards the barren rocks of the peaks beyond.

It seems an unlikely journey for an animal led by its stomach.

Why they do this will only become apparent

as the summer draws to its close.

Meanwhile 2000 feet below them on the Yellowstone plateau

the grasslands are facing drought.

Bison calves are more independent of their mothers.

The adults' focus is now set on a ruthless competition to mate.

A male bids to enter a herd with females in season.

He uses dust to accentuate all his size and power.

Another male already established in the herd

answers the display.

(GRUNTING)

And waits.

The newcomer responds by scent marking and moving even closer.

This challenge cannot be tolerated.

The resident male has won.

Having seen off his rival

next summer's calves will likely be his.

Victory in the dust of high summer.

But now there is a larger challenge to the whole of Yellowstone.

These clouds don't signal rain.

They are vast smoke plumes.

Yellowstone is burning.

Throughout the summer

the dry lodgepole pines have become like a tinder box

and lightning has struck the match.

The worst wild fires will burn for weeks.

In 1988 a third of Yellowstone burnt in a single summer.

Devastation.

Animals that depend on these forests will starve this winter.

But Yellowstone itself has a longer perspective.

Ashes fertilise the soil and fire opens it up to sunlight.

As the forests regenerate

new life finds opportunity.

In August fledgling hummingbirds gorge in fields of fireweed

that have risen from the forest's ashes.

For them the summer is already nearly over.

They must chase the sun south before winter returns.

But just as the first of Yellowstone's new generation are preparing to leave

its last visitors are arriving.

Night after night

army cutworm moths have been escaping the heat of a distant prairie.

They have been flying in on favourable winds

hiding away in rock crevices in the mountains.

This is why the grizzly bears climb to the barren peaks.

Two miles up they begin to search for the moths.

It's the end of summer

and the last snow of winter holds on only here.

It offers a few refreshing mouthfuls for travel-weary cubs.

Many of these bears learnt about this site from their mothers

as a part of local bear tradition.

The family sets to work finding moths digging into the loose rock.

Though each moth is very small

a bear that makes a successful strike

can harvest up to 40000 in a single day.

That's 20000 calories and vital weight gained for winter.

Having spent all summer avoiding males

the mother now finds one much too close for comfort.

Perhaps he's seeking a mate but he is unwelcome.

Leaving her cubs she confronts the male head on.

As she approaches to see him off again

she realises she has been injured.

A damaged jaw would be a disaster for her and her family.

Clearly in pain

she leads the anxious cubs to high ground away from the male.

She nurses to calm and reassure them.

The injury does not seem serious

but she decides to lead her family back down from the mountain.

This will be the last time these cubs feed here as a family.

By next year they will be independent

ready to face their world alone.

Sun has briefly brought warmth to Yellowstone

long enough to fuel the raising of new generations.

But just as summer reveals the full extent of Yellowstone's diversity

a curtain of winter ice is set to descend again.

The days are getting shorter

the sun losing its power.

Now there are just a few short weeks to prepare for winter.

One final flourish of life

in Yellowstone's briefest but most glorious season.

Autumn.