The Toughest Spring Yellowstone: Wildest Winter to Blazing Summer

WIND GUSTS

Stretching out before me is the magnificent Yellowstone.

This is one of the most dynamic...

..unpredictable...

..and exciting environments on Earth.

HELICOPTER HUMS

Deep in the Rocky Mountains,

this vast wilderness is home

to North America's most iconic wildlife.

But every year,

Yellowstone's animals are pushed to their absolute limits.

Temperatures can swing from minus 40 in winter

to almost plus 40 during the summer.

And at the heart of this change is the thaw.

The melt can last several months from March to July.

And it's one of the most dramatic, natural events on Earth.

How do the animals cope

with such extremes of temperature?

We're here to find out.

I'm joined by biologist Patrick Aryee...

Ooh! That's intense. >

..and a team of wildlife cameramen and expert scientists.

Last night, we brought you winter.

Wildlife struggled in freezing temperatures.

BISON GROWLS

Tonight, it's spring.

Life is about to change, but not always for the better.

The thaw will melt the snow in the mountains

and cause over one million tonnes of meltwater

to crash through Yellowstone.

Only the strongest animals will survive.

Welcome to Yellowstone - The Toughest Spring.

It's the end of April and seven degrees Celsius.

The thaw is well under way.

BIRDS CRY

Spring has come to Yellowstone.

Mammals, big and small, are emerging from their dens,

birds are courting and nesting,

the days are getting longer and warmer.

Now, you may think that life is going to get easier.

Yellowstone is bursting into life.

Hummingbirds return after spending their winter in the warm south.

Moose feast on the first green shoots.

BIRD WARBLES

Bison migrate back to their calving grounds.

We're in the north-west of the USA,

2,000 metres high up in the Rockies.

Known as the Greater Yellowstone Ecosystem,

it's an area the size of Scotland,

includes two National Parks

and is bounded on three sides by mountain ranges.

Biologist, Patrick Aryee, is at the southern end of Greater Yellowstone,

catching up with one species

that faces its greatest challenge in spring.

Behind me are the Teton Mountains

and this is the Snake River,

home to North America's largest rodent, the beaver.

There are 15 of them living on this stretch of river

and they've built this enormous dam.

I can't believe that this dam is not a man-made structure

and that it's been made by beavers.

I mean, it's incredible.

Look at the size... of these...branches, and even...

Look, here we've got these huge rocks as well.

It acts as a way to slow down the waterways, like speed bumps, almost,

but I've got to say that they are nature's most incredible engineers.

But at this time of year,

their dam is in real danger.

Soon enough, all that snow is going to melt

and all that meltwater is going to start flowing down

into these river systems.

And as that water starts to flow and cascade along this river,

it could damage and even destroy,

not only the dam, but also the lodge.

And the lodge is where the beavers have been holed up

for the last five months.

Now, we've got a camera set up so we can actually see

exactly what's going on inside there right now.

If I come over here...

That is incredible.

Oh, they're just starting to wake up.

BEAVERS GROAN

In this lodge, we've got... There's two yearlings in there,

two two-year-olds and then one adult.

There should be another one, but I can't see it.

This is our beaver family

and we're going to be getting to know them very well.

BEAVERS SQUEAK

And we'll be following them over the coming months to see how they cope

when the spring floods hit.

Just seen one dive.

Beavers are most comfortable in water.

Their lumbering bodies are quite ungainly on land.

He's huge!

This stretch of river is home to 15 beavers.

In April, the river bank is usually covered in a blanket of snow.

So the beavers stay snuggled in their den.

But this year, the snow has already disappeared.

According to scientists, the thaw is three weeks early.

That means the beavers can start venturing out

on the search for new green shoots.

This is brilliant, I'm just following this beaver...

and hopefully, I'll get to see him when he comes up onto this next dam.

This beaver hasn't eaten fresh food for five months.

Over winter, the beavers' food supply is locked in snow and ice.

They feed on twigs, leaves and bark of aspen and willow trees.

But when there's so much snow on the ground, they can't get to it.

Yet, beavers don't hibernate,

so they must keep eating throughout the winter to survive.

Their solution is ingenious.

They collect a mound of twigs and vegetation,

big enough to last for five months

and store it by the underwater entrance to their home.

Even when the river freezes over,

they have a larder just a moment's swim away.

But by the end of winter,

the rotten twigs have lost most of their protein.

This beaver is surviving on his own body fat.

The early thaw, however, has given him a lifeline.

This year, he can forage for fresh food

three weeks earlier than normal.

It's just what his hungry family needs.

OK, that adult is now back inside the lodge.

And it's got a mouth full of vegetation.

BEAVERS SQUEAK

These beavers have lost a third of their body weight over winter.

To gain that weight back,

they'll need to eat a kilogram of twigs and leaves every single day.

The younger beavers squeak to demand food

but when competition is this fierce...

it's every beaver for itself.

YOUNG BEAVERS SQUEAK

In spring, twigs are rich in sugars and proteins,

just what they need to kick-start their recovery.

But to reach the fresh food, the beavers must first get to work.

Not only are beavers phenomenal engineers, building all those dams,

but they're also professional lumberjacks

and they can take down whole trees, just like this one,

and they do that using their super-sharp teeth.

These bottom teeth are just like chisels

and what they'll do is get their head in sideways

and chew away at the bark.

With smaller trees, they'll go right the way through

and you'll often see beavers running away

just before the trees fall down.

But with bigger ones, like this one,

they'll only get so far and then they'll move away

and let the wind do the rest of the job.

Now this is a really big tree.

But the part the beavers are really interested in is up here,

the top third,

and I can see where they've gnawed away at this end

and hauled all their branches back to the river

to construct their lodge and the dams.

And making sure their dams are reinforced

before the spring floods hit is crucial.

There are five dams on this stretch of river.

This entire waterway, this system has been created by these beavers.

They've made these...series of dams,

which kind of create this lock system.

The beavers have turned, what was a small stream,

into a series of deep ponds.

These pools allow them to stay in the water while foraging.

On land, they're easy prey for predators, like wolves and bears.

But crucially, this dam system also slows down the floodwater.

At two metres wide and 40 metres long, the main dam is the strongest.

But if this one breaches, the others will go too.

BEAVER MOANS

Right now, the beavers are safe in their lodge.

But I'll follow them closely

and find out how they get on as spring takes hold.

For many animals, spring is the most important time of year.

It's now May.

BIRD WARBLES

As soon as the snow melts,

fresh vegetation appears.

Scientists call it the "spring green-up"

and it drives all life in Yellowstone.

One animal relies on it more than almost any other...

The Rocky Mountain elk.

There are over 20,000 elk in Yellowstone.

As the green-up moves from the lowlands to high mountain pastures,

the elk move with it.

This magnificent herd of elk that are straddling right across

the sagebrush here and up the hill,

are on their way to the calving grounds,

a little bit further north from here.

They'll be giving birth over the next few weeks.

They'll hide their youngsters in the undergrowth

while they go off to feed.

This is a crucial move in order to protect them from predation

by the largest carnivore here... the grizzly bear.

Now, there are some grizzlies

that have become particularly adept at finding those youngsters.

And that source of protein

is really great for adult bears and their cubs,

needing desperately to put on weight after a long winter hibernation.

It's as the snow line retreats from the valleys to the foothills

that the female grizzlies and their cubs start emerging.

They usually stay close to their dens until the end of May,

but this year, the warm weather has enticed the bears away

earlier than usual.

But the elk calves won't be born for another month.

So life for these cubs will now be harder.

They will have to scratch a living digging up roots and grasses.

Grizzly expert, Casey Anderson,

is already seeing signs that there are plenty of bears around.

They've been using trees as scratching posts.

Yeah, this thing has been getting hammered, look at all this.

They're leaving their scent behind.

Cos there's just all kinds of mud and hair stuck on here, so...

Yeah, it's going to be cool to see who is back in town.

Patrick is joining Casey in the Gallatin Mountains

to find out whether the early bears

are getting enough to eat.

Casey's tracked the mother and her two-year-old cubs to a wooded area

halfway down the mountain.

They've been feeding all morning and are likely to be somewhere close by.

From here on, we've got to go slow, even lower our voice.

The wind's really going in the wrong direction.

The wind's carrying our scent right towards where the grizzly bears are

and they have a sense of smell

-that's about seven times stronger than a bloodhound.

-All right.

'Casey thinks the bears will stay in this area for a while

'to make the most of the early green shoots.

'So for us, it's a waiting game.'

It's amazing cos they just come out of nowhere.

In that deep sagebrush,

they love to just get down out of this wind

and lay there, flat to the ground, cuddle up, keep each other warm.

So we just got to... to keep scanning.

You'll see like a little ear or they'll stretch their paw up.

Yeah, look for those little subtle things popping out.

What makes you think that this is a good spot for bears?

Look right down here in this... green patch.

You'll see where the bears have been excavating there

and that's just classic in the springtime,

they'll find these first really green spots,

they've got a lot of moisture, get a lot of sun.

So there has been a lot of activity down in that meadow there, so...

there's a really good chance that... whoever did that,

whatever bears did that are going to come back to that spot

and do it again.

But plants contain less than half the protein of meat.

So they'll have to eat huge amounts to keep up their calorie count.

-CASEY WHISTLES

-Hey, hey...

Where?

Right-right... Right down in the trees.

Oh, yeah! Yeah, yeah, yeah, yeah!

Three bears.

HE LAUGHS

One... Two...

Three. And there's like...

Like a... Looks like a big female.

One light cub, one dark cub.

-It's pretty awesome.

-Yeah.

Those look like two-year-old cubs.

Being in this area just draws in females with cubs.

Spot like that, well, you know,

we've got dandelions, grasses and sedge.

Even biscuitroot will grow in that area

and that's all on the menu

for a grizzly bear this time of year, so...

They come right to that spot and they'll keep coming back,

over and over and over again, till everything's gone.

So, basically, this area here

is kind of like an all-you-can-eat veggie buffet?

Yeah, it's a salad bar.

A dandelion plant only contains about three calories,

and grass...even less.

The early thaw is at least helping.

But they must find two fields every day with rich sources of food,

like this one, if they're going to survive.

It's not just grizzly bears

that must exploit the best pockets of food.

The Rocky Mountain elk are still moving north following the green-up.

And another of Yellowstone's residents is ahead of them...

the bison.

This year, the bison and elk are lucky.

Not only have they had a milder winter,

but the early thaw means there's plenty of grass around.

And now in spring, it's full of protein.

I've come to the Northern Range of Yellowstone.

Park biologists have reported

that a large herd of bison

has come here to feast

because this year

the snow has already melted.

The park is home to about 5,000 bison.

BISON GROWLS

This herd has made it through to spring fit and strong,

which is crucial for the survival of their newborn calves.

Calving season is just beginning here in Yellowstone National Park.

This is the very first one I've seen. It's suckling, I think.

A quarter of the baby bison in Yellowstone

die in their first two months,

but this mild weather will give them a much greater chance of survival.

There's a tiny one, looks like it's probably only a day or two old.

Beautiful, you can see that lovely chestnutty colour.

They're extraordinary, bison calves.

Within half an hour of being born, they're up on their feet

and within an hour or two, they can run to keep up with their mothers.

BISON GROWLS

Sticking close to their mothers is vital.

Because this spring, Yellowstone's top predator is particularly hungry.

There are 100 wolves in Yellowstone National Park.

They rely on bison and elk to survive the winter.

But whilst the mild winter has been good for the bison,

it's been bad for the wolves.

They've struggled to hunt.

Back in the winter, biologist Doug Smith

observed the problem first-hand.

Doug and his team monitor these wolves every single day.

They've noted that whilst mild winters help the prey,

they put predators at a disadvantage.

When you're in ankle-deep snow, or just elbow-deep snow,

a healthy elk, bison, deer will always outrun a healthy wolf.

Wolves and elk run at the same speed,

but elk are built for endurance -

with sure footing, they can keep going for longer.

So this winter, the wolves have been going hungry.

They're having a particularly tough time here in the Northern Range,

where five packs compete for food and territory.

Doug has studied these wolves for over 20 years

and in the last decade,

their numbers have fallen dramatically.

One pack, known by park biologists as the Lamar Canyon Pack,

is extremely weak.

If they don't gain strength quickly,

they risk losing their territory to other wolves.

Our wildlife camera team is down in the Lamar Valley following them.

From a mile off, they spot some unusual behaviour.

The wolves are taking on a whole herd of bison.

They're hoping to snatch a newborn calf.

This is a sign that the pack is desperate.

Doug is helping us to analyse what's going on.

These bison are doing classic defence.

They're grouped together, they're big.

They're not moving other than to attack the wolves.

This is an age-old dance between power and agility.

Wolves may be quick, but bison are strong.

Getting this close risks a kick to the skull.

Wolves would like to spread this group apart,

break it up and then that would free up one of these calves

that they could grab.

A female charges, threatening to lash them with her hooves.

You get caught by a foot, you get caught by a horn - injury, death.

It's fascinating footage to watch.

Uh, they're being aggressive, yet very careful.

Wolves definitely feel safer pursuing a fleeing prey

cos they can kind of come in from behind, grab and pull down.

To attack a standing herd means these wolves are desperate.

As they get hungry and it goes more days without food,

they're going to take more risks.

They finally close in.

This is the break in the herd they've been hoping for.

ADULT BISON GRUNTS, WOLVES WHINE

The mother can't do all this by herself, the wolves outnumber her.

They may get this calf, I can't believe this, you rarely see this.

ADULT BISON GRUNTS

But the calf makes it back to the safety of the herd...

..leaving nine very hungry wolves.

The Lamar Canyon Pack has failed.

Doug is concerned that these warmer winters and early springs

are part of a worrying trend.

We can see climate change affecting the park now, um...

Our wolf research has only been going a little over 20 years,

but you talk to people who were in Yellowstone 40 or 50 years ago,

and winter severity is just not like it used to be,

and this is a big factor in how this system works.

This winter and last winter were two of our...shortest winters we've had.

This, in a way, in a big picture way, favours the bison.

So we lost our snow at low elevation very early this year

and so any time the snow goes away, it gives the bison better footing...

Uh, they can deal with the wolves,

and secondly, it exposes the vegetation to sunlight.

The condition of bison will start to improve earlier, uh,

and so that hurts the wolves a little bit.

And so sitting aside and watching life evolve with that going on,

uh, has got me scared stiff.

It's the rate of climate change that Doug and his fellow scientists

find so shocking.

Temperatures here have been rising fast.

And the last decade has been the warmest since records began.

With Yellowstone's winters becoming shorter

and the spring starting earlier,

one animal in particular is extremely vulnerable.

The great grey owl.

These owls live in some of the coldest forests on Earth,

from Siberia to Scandinavia.

Yellowstone is just about cold enough for the owls to survive,

but if it gets any warmer,

scientists believe they won't be able to hold on.

Back in winter, cameraman Jeff Hogan

followed a pair of great grey owls in the Teton Mountains.

Great grey owls rely on small rodents like voles and gophers,

which live beneath the snow.

But owl biologists believe that the changing climate

is affecting their hunting.

These owls have to bust through this crust

and then maybe another foot of snow to get at their prey.

But erratic winter weather caused the crust to melt and then refreeze,

making it impenetrable for the owls.

And if they can't hunt, great grey owls won't nest or lay eggs.

Now it's spring,

Jeff has been searching for owl chicks in the Beartooth Mountains.

The forest here is 300 years old

and ideal habitat for great grey owls.

-There they are, three of 'em.

-Oh, wow!

OWL SCREECHES

Oh, my goodness.

The smaller one, she's the runt.

Despite the difficulties of winter, we found a nest with three chicks.

This is extremely unusual.

When food is scarce, the smallest chick can be eaten by its siblings,

but this runt is still alive.

How much of a success is it

that we've got three great grey owl chicks here?

In these parts here, this neck of the woods,

-it's about a 20% success rate for the third chick.

-Mm.

So most, you know,

four out of five perish by this... by this time.

So we've got some great parents here?

Yeah, the reason is our winters have been quite mild,

so the rodents, the prey, has really benefited

and their populations have really grown,

which of course then benefits the owls.

And that could be making the difference

on whether that third chick survives or not.

The early thaw this year has turned the owls' fortunes around.

The milder weather has led to a boom in rodent numbers.

To reveal how this has been helping the owls,

Jeff has been searching for their pellets.

Yeah, I've got a pellet here I collected this morning.

And, look, you'll see the size of this...

-It's kind of...

-PATRICK LAUGHS

It's kind of disgusting.

OK, well, if I just break this apart...

Well, you can see, it is literally just...

..hair and bone.

Yeah.

-GASPING:

-Look at that!

That is an entire skull.

-Look at that, you can even see the teeth!

-Mm-hm.

Oh! Here we go, look, look, look, look!

-There you go.

-Oh, that is brilliant.

Yeah.

A second skull,

which means they would have had two meals in quick succession.

So far, the owls are doing well,

but spring in Yellowstone is unpredictable.

After I leave, the weather takes a turn for the worse.

A storm blows in.

It could mean death for the chicks.

This rain makes...makes it a little more difficult for me...

to work in, but...

..it makes it even more difficult for the adult owls to hunt,

being that it's just so noisy,

and that masks any sounds that the rodents may be making.

The runt is at the bottom of the pecking order.

As soon as food becomes scarce, she'll be the first to lose out.

If the weather doesn't change, the runt may starve.

THUNDER RUMBLES

May is always the wettest month of the year in Yellowstone.

But this year, it's far wetter than average.

The reason can be found in the Pacific.

The warming ocean creates much more moisture in the air.

This is channelled directly up to Yellowstone.

And when it hits the wall of mountains,

the fury of the monsoon is unleashed.

THUNDER RUMBLES

We experience the most violent storm that's been seen here

for many years.

Rain falling over three mountain ranges

drains straight into the Yellowstone basin.

Overnight, water levels rise by a foot.

For two weeks,

rain pours down and thunderstorms reverberate around the mountains.

Then, at last, in early June...

..the weather breaks.

But there's water everywhere.

Melting snow, as well as rainwater,

is now flowing into the river systems.

Scientists have told me to come to Yellowstone Falls

to see the full effect.

Back in March, Yellowstone Falls was just a mere trickle,

and now it's a dramatic torrent of tumbling water.

Every year the thaw is different

and it's the rivers that hold the clues

to tell us how this year's thaw is progressing.

Thousands of animals rely on this river to survive.

Greater Yellowstone sits on the Continental Divide.

This water flows west to the Pacific Ocean and east to the Atlantic.

Without this huge reservoir of water,

agriculture and industry, as well as wildlife in the American West,

would die.

So understanding how MUCH water is draining into the river systems

is vital.

Teams of scientists

have monitored the streamflow in Yellowstone's rivers

for the last 86 years.

They've noticed that recently the pattern of the spring runoff

has been changing.

This piece of equipment measures streamflow.

Hydrologist Cheryl Miller is keeping a close eye on the rising water

in the Yellowstone River.

The data she gathers will reveal how big the spring runoff will be.

It's all done with this little orange boat

that uses sonar to measure the depth and speed of the water.

Combine those and you get the flow, and that's the key figure.

The rain has already raised the water levels,

but it's the thousands of tonnes of snow still in the mountains

that will make the real difference.

-OK, I've got it.

-All right.

So, if you do get a very sharp rise in temperature,

if that snow pattern melts very quickly...

And, I mean, you know, I've been in that snow pattern,

the snow was above my head.

Presumably, if that DOES melt very quickly,

the results downstream must be potentially quite dramatic,

quite-quite destructive.

Indeed. High energy floods can make great changes along the river.

What's happening now is the days are warm, but that more importantly,

the nights are warmer,

and so that snow pack never refreezes at night.

It continues to slowly melt during the night

and so that's what allows the snowmelt and the runoff

-to accelerate during this time of year.

-Yeah.

Just looking at the river,

does it appear that the flow is faster now than it was

when you last collected data?

It is, it has come up in the last three or four days that it's...

It may not be a lot faster, but the volume is definitely bigger,

there's more streamflow than there was just a few days ago.

Cheryl's data reveals this year the melting snow

is coming off the mountains more quickly than usual.

The faster it runs off, the less water is absorbed into the ground.

These rapid spring melts are more likely to cause flash floods.

Patrick's finding out how that's affecting the beavers.

Here in the Snake River Valley,

all the meltwater from the Tetons is running down the mountains

and it's headed directly for our beaver dam.

'I'm meeting up with cameraman Charlie Hamilton James.'

-Hey, Charlie.

-Hey, Patrick.

-How's it going, man?

-Yeah, it's good. Lovely weather, isn't it?

'He's spent the last week

'watching the beavers struggle with the rising river.'

The water level is so much higher

compared to the last time I was here.

Yeah, actually, really only in the last few days.

-There was a lot of rain.

-All of that water has resulted in this?

Yeah, and what it's done,

-it's actually starting to flood the dam.

-Mm.

In some places, the dam has already breached.

The beavers are working nonstop to make sure all the leaks are plugged.

They built this dam five years ago and every year they fortify it.

This one's been strengthened with a particularly high number of stones.

So what makes this spot prime habitat?

Well, actually, weirdly, beavers have made this spot.

This actually started life as a stream

and it's the beavers that came along and stuck these dams in it

and blocked its path and created these long, slow ponds,

and when you create a pond, you create another pond,

you create another pond... You create a wetland ecosystem.

They're basically engineers, but they're also gardeners.

I like that, gardeners.

So they're creating this whole environment that suits THEM.

But it's not just them that benefit from this, right?

-You've got amphibians, other mammals...

-Oh, absolutely.

..birds that rely on this.

I mean, just sitting here now, there's birds everywhere.

You know, swallows flying round,

drinking and catching flies off the surface.

There's kingfishers eating fish.

There's lots of minnows here,

lots of small fish perfect for kingfishers.

Otters come through here, ospreys, eagles...

So, you know, they're ecosystem builders, is what they are, beavers

and more so, really, than any other animal.

If the main dam is knocked out,

it's not just the beavers that will lose their home.

The top is just about holding.

But Charlie and I are heading underwater

to find out how solid the base is.

Just where my feet are now, it actually starts here.

These... The foundations have been laid here.

So, you know, it's like ten foot wide.

You can't see it all cos, you know, most of it's underwater.

We've got these drysuits on.

Yeah, and really, Patrick, in the spirit of full immersion...

..we should go in and actually have a look.

-Is this where I find out I've got the dud suit?

-Yeah.

Actually, this is so much better than I thought it would be.

This is awesome!

It's much clearer than I thought, you can really see it.

It's only when you get underwater that you see the skill

that's gone into building this dam.

The beavers choose logs up to two metres long...

..and place them at an angle of 30 degrees to hold back the water.

This is precision engineering.

This is amazing, isn't it?

It's so impressive.

This is, you know, it's-it's kind of a work of art.

But the beavers' hard work may not be enough.

If the river rises any further, the water could break through.

And this watery world might just disappear.

It's getting cold now, innit?

THEY CHATTER

It's now mid-June and temperatures reach 28 degrees...

Seven degrees above average

for this time of year.

The snow recedes halfway up the mountains...

revealing the highest feeding grounds in Greater Yellowstone.

And close behind are all the animals

taking advantage of the final stage of this year's spring green-up.

At 2,500 metres,

this vast wilderness provides food for thousands of animals.

Elk that overwintered down in the valleys,

finally arrive back in these high pastures,

where they'll raise their calves.

These meadows are a feasting ground, full to the brim with wildlife...

the hunters as well as the hunted.

Opportunistic black bears take advantage of a low bird's nest

and defenceless chicks.

At this time of year, any food is fair game.

The grizzly bears are also joining the feeding frenzy.

I'm catching up with Casey,

who's following the fortunes of one young bear family.

These newborn cubs are only four months old.

With nowhere to hide from predators,

this meadow is a dangerous place for baby bears.

But their mother has no choice.

She's following the melting snow to make sure that both she and her cubs

get enough protein from the new growth.

-CASEY CHUCKLES

-Aw, they're so cute!

So small.

So, you know this bear?

Yeah, you can count on her like clockwork.

Everything that a little cub needs to learn to survive its lifetime,

they're learning in these first few years with its mother.

Like where to go, what time of the year, how to dig the roots.

They're learning all of that right now.

This one here, you know, the year after year that I've watched her,

she usually keeps almost all of her cubs.

She's a great mother.

In the ten years that Casey's been following this female,

he's also seen her teach her offspring a more gruesome skill.

She's a known predator.

During this time of year,

she'll actually go around looking for elk calves that were just born,

that are stashed out here in the sagebrush

and these little cubs will be in tow, and they'll be watching.

So it's a very learned behaviour

and some bears, right in the same area, don't hunt elk calves at all,

cos their mother didn't teach 'em.

They didn't get the chance to learn this.

These little guys are going to learn that

and I'll tell you, at this time of year,

-that protein for these little bear cubs is essential...

-Mm-hm.

..and that may just be the thing

that gives them the advantage to be strong bears.

But of all the survival skills they'll need to learn,

the most important is how to spot danger.

Now, we've got Mum in the middle of this sagebrush

and these little cubs bobbing their head up and down.

Surely, you know, this is, this is a dangerous time for them as well.

How difficult is it, you know, to be a newborn grizzly bear cub?

Very difficult. Stats say about 50% of bear cubs in their first year,

they don't make it.

And there are so many things out there that are trying to kill them.

A male grizzly's one of them.

You know, a male grizzly knows that

she won't mate as long as she has cubs.

If he comes in there and he can eliminate those cubs,

then she will come into oestrus and she will mate.

So... Big male bears - big danger for these guys.

Male grizzlies are known to roam this area,

so their mother must stay on high alert.

And what's that scar on her head?

It looks like she's got some sort of scar.

Yeah, it's new for her,

so I'm assuming now that she's got these little cubs,

that scar is probably from protecting them against a big male.

And when it comes to that defence, these females, they don't care.

They're going to go against the biggest male grizzly

and it looks like she probably protected her cubs,

but came away with a pretty big scar.

Oh, she's nursing right now.

She's laying on her back.

-Oh, yeah...

-Yeah, they're just going to get in there,

they're going to relax.

Nothing better for a little cub than that right there,

but Mum... she still has to pay attention.

And you can see, she'll just keep picking her head up and watching,

looking for danger.

There's a good chance these two little dudes will grow up to be adult bears.

And she's... She's a rock star mum, for sure.

Bears spend the heat of the day

bedded down in the shade of the forest.

But before this mother takes her cubs back to the woods,

she has one more trick to teach them -

how to keep cool in the hot sun.

These little patches of snow

that are left from the snowdrifts in the winter

are everything for these bears,

cos it's really their AC unit, cos the hotter it gets...

They really seek out these snowfields

just to go and lay their bellies on and cool down.

CASEY LAUGHS

That's the best thing, they're so playful, so full of energy, just...

They'll roll around like that for hours.

You know, they're using that snow like a slip and slide,

just like kids playing on toboggans and sleds.

They're starting to learn how to use their claws and their paws

and get their balance.

If you look around out here, I mean, this is a pretty rugged terrain,

so they'd better get their feet underneath them pretty quick.

I really believe that they like to have fun, just like any kid.

But these snowy playgrounds will soon disappear

as temperatures are now climbing every day.

In late June, we hit 30 degrees.

The snowpack is collapsing.

The great Yellowstone thaw reaches its peak.

Thousands of tonnes of meltwater cascade through the canyons...

..bulldozing everything in their path

and crashes on into the valleys.

Over 250 cubic metres of water

is passing through the river per second.

This meltwater is vital for the regeneration of the land.

But it comes at a cost.

Despite the floodwater,

Yellowstone's residents have to continue

their daily quest to find food.

Biologists have just told us that a herd of bison

is attempting a river crossing not far from Yellowstone Falls.

BISON GROWLS

Our camera team arrives

to find a two-month-old bison calf already struggling.

It's been swept away from its mother.

BISON BLEATS

Without her body to shield it,

the calf is pounded by the full force of the current.

The other females have their own young to protect.

This one is on its own.

And it's losing energy fast.

BISON GRUNTS

But the exhausted calf makes it safely

to the new pastures on the other side.

This one is lucky.

And further south, the Snake River is also running high.

There's so much floodwater that one of the beavers' dams has burst.

Yeah, this is up, isn't it?

Look at that. I mean, it's just... It's just blown out, hasn't it?

Wow. Yeah, that is completely busted.

-Yeah.

-You know, that water's flow...

And it's a lot faster as well.

Do you know what? You're not going to stop this.

The river's got to come down here, hasn't it?

It'll either bust through the dam or it'll go over the top of the dam, or it'll go around the dam.

-Yeah.

-And this dam just obviously wasn't quite strong enough, it's just bust through it.

And, you know, when this dam busts, it can take out the next dam.

-You know? It's a kind of domino effect.

-Right.

Oh, yeah. I mean, it's just...

That is strong. So there's no way this can...

you know, a pile of sticks is going to stop this.

Are they even going to bother to fix that bit?

No, I don't think they will because...

Well, actually, you know, it'd be impossible to fix it.

-Yeah.

-They're going to... I reckon they'll wait and in a month maybe

or more, until the water goes right back down to pre-flood level,

then they'll build it back up.

You know, it doesn't look like much when you look upstream,

but right here,

all this water is pushing up against my body and there's so much force,

and that's what the dam is having to contend with,

that's what it's trying to hold back.

If this dam has bust...

..what's happened to the beavers?

The beavers are busier than ever.

The water is flowing right over the top of their dam.

But at the moment, it's still holding.

They won't be able to fix it until the water levels fall.

Inside the lodge, all the old bedding is pushed into the water.

They leave it to soak for a while...

and when it's clean, they drag it back in...

giving them fresh new bedding for the summer.

With temperatures now reaching 30 degrees,

the owl chicks are struggling.

One has already left the nest.

These are birds that thrive in colder temperatures,

but their nest is now exposed to the full force of the sun.

Great grey owls struggle to lose heat,

so the remaining two chicks must leave the nest

as quickly as they can.

The second chick is getting up on the lip,

on the highest point of the nest and just flaps like crazy.

He's looking over the edge

and this is what they do before they jump.

But the chicks are vulnerable.

Their wings aren't strong enough to fly.

When they jump, they'll plummet ten metres to the forest floor.

Oh, he's going to go, he's going to go! Jump, jump!

Go, go, go, go! Oh!

He's leaning way over on the other edge.

He could go any second.

It's the riskiest moment of this owl's life.

It's not the most graceful maiden flight,

but it's success nonetheless.

He's hanging upside down!

Got it all messed up.

Oh, he is struggling.

He's hanging by one talon.

There he goes!

It's one thing to launch, it's another thing to land.

The chick lands safely,

but it must head straight back up into the tree tops,

away from predators on the forest floor.

With Yellowstone's climate getting warmer each year,

these owls may not be able to live here for much longer.

Summer temperatures are becoming too hot.

She doesn't know what to do or where to go.

The runt is now in danger.

She's being struck directly by the intense heat of the sun.

In these conditions,

great grey owls struggle to regulate their body temperature.

Panting helps her to cool down.

But her downy feathers, which are so efficient in cold weather,

may prove fatal.

CHICK CHIRPS

Our little girl's up again.

Finally, but...

She's hot too. She... She wants out of there. I can tell.

This is a leap of faith she has GOT to make.

She's back on the launch pad.

This is where I think she's going to go.

She's...

She launched! She launched!

CHICK CHIRPS

So this forest is now home to three more great grey owls.

I'm impressed. I'm proud of this little girl.

You know, maybe one day she's going to just come back here

and raise a great grey owl family of her own.

As the longest day of the year approaches, spring becomes summer.

The animals we've followed

have made it through Yellowstone's most challenging season.

This year, the thaw started three weeks earlier than usual.

An area of 30,000 square miles has been transformed...

..and the mountains have released over a million tonnes of water.

And finally, from top to bottom, Yellowstone is free of snow.

Now our animal families face the new challenge of summer,

when temperatures can soar and vegetation wither.

And that leads to a new threat -

wildfires.

Tomorrow night, we bring you summer,

when soaring temperatures force the animals of Yellowstone

to change their behaviour.

Bears enter cowboy country as they search for food.

The wolves display unusual hunting strategies

to keep their 11 pups alive.

And the beavers flee from their lodge

in a dramatic twist to life in this changing environment.