Original subtitles

Stretching out before me is the magnificent Yellowstone.

This is one of the most dynamic...

..unpredictable and exciting environments on Earth.

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 approaching plus 40 in summer

and at the heart of this change is the thaw.

This melt can last several months from March to July,

and is one of the most dramatic seasonal 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...

Whew! That's intense.

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

I'll be following some of the area's key wildlife.

We'll be getting to know families

through their highs and lows, starting right now.

And I'm looking at the bigger picture,

understanding the science of the thaw

during each season, across winter,

spring and summer.

That's brilliant.

Living here is tough.

The changing climate has brought freak storms,

flash floods and raging fires,

making life for the animals even more extreme.

With unprecedented access to this remarkable place,

over the next three nights we're bringing you Yellowstone

as you've never seen it before.

Welcome to Yellowstone: The Wildest Winter.

WOLF HOWLS

It's early March.

This is a world blanketed in thick snow and ice.

And after four long, hard months of winter,

the animals still have more weeks of brutal cold to endure.

We are in the north-west of the USA,

2,000 metres up in the Rockies.

This is an area known as the Greater Yellowstone Ecosystem.

It's the size of Scotland,

includes two national parks

and is bounded on three sides by mountain ranges.

In this programme,

we'll be following the lives of animals

that have to adapt to extreme change,

as winter turns into spring.

For many, this is the ultimate challenge...

..particularly for Yellowstone's biggest,

and, at this time of year, hungriest carnivores.

Patrick's heading high up into the mountains on their trail.

It's amazing to think that somewhere out there are grizzly bears.

But it's right at the tail end of winter and it's really cold,

so most of those bears are still going to be hibernating in dens

deep beneath the snow.

And, for most, that's a sensible strategy.

Right now the weather is bitterly cold

and down to minus 21 Celsius up here.

It's usually later in March

that the majority of grizzlies start coming out.

I'm in the Gallatin Mountains

in the north-west of Yellowstone.

Our grizzly expert, Casey Anderson,

is out here monitoring bear activity.

Hello, Casey. How are you doing, man? Good.

We're looking for any bears that are emerging from hibernation early,

and the most likely place to find them is at the higher elevations,

where grizzlies have their dens.

Good view, huh?

Absolutely stunning.

But it does look completely devoid of life.

How are we even going to start looking for these bears?

You're right, there's not much moving around out here.

Right now the only bear that probably will be out

is those first big males.

Casey believes some grizzlies are already leaving their dens

because of a recent spell of unseasonably mild temperatures.

So, just a few days ago we had really warm weather,

it felt like spring.

It's been so warm you've got to believe the bears are thinking

it's getting close to spring

and that can be really bad news, because it's not spring

and they come out here and winter can hit again

and that could really be devastating to a lot of animals,

including the bears.

Last week, right here, it was 15 degrees warmer than today.

Now it's back to more typical March conditions.

Casey, this weather's pretty...

HE COUGHS

Whew!

This is pretty intense. That's intense.

How do the bears cope with this?

Bears are tough, but this is difficult for them, too.

Suddenly, Casey sees an opening that could be a bear den.

You see, like, these natural rock formations. Oh, yeah.

It's like a little cave. Yeah.

It's perched high on a steep slope.

Not easy going.

The things I'll do for bears.

Would that be a good site for a bear to hibernate in?

The statistics are this -

70% of bears will dig in a north-faced slope.

At least 30% of this population does something else,

and something else is something like this.

Ready-made, in the sun,

they can get down in there and trap the heat.

It's perfect.

And it's not all one big sleep.

During hibernation, the pregnant females also give birth.

It's March now, and those little guys were born two months ago.

So, just imagine these little guys that big

and they're going to grow about that big in about another month.

And then in another month from now

they're going to pop their heads out, and this is going to be home.

It's just amazing, think about that, the first view.

It's a pretty good one.

We head off before we disturb any sleeping bears.

Emerging in these conditions would mean the odds are against them...

..and 50 miles further east that's exactly what's happened.

Scientists have tipped off one of our camera teams

about a grizzly bear that has taken the risk of emerging early.

It's clearly struggling.

The warmer weather of a week ago

has been followed by plunging temperatures and more snow.

This could make finding food impossible.

The grizzly finally makes it down from the mountains

to the valley floor.

There are plenty of bison here,

but even a starving bear won't tackle

these formidable beasts.

Bears rely heavily on their extraordinary sense of smell

to find food.

Their nose bristles with over a billion nerve cells.

It's their super sense...

..and it's led this bear to a pond.

A dip in the sub-zero water could zap his precious energy.

Grizzlies are superb at catching fish...

..but it's not fish he's after.

It's a bison.

Long dead, frozen all winter

and now released from its icy tomb.

This is probably the first meal the bear's eaten

in over four months.

It offers a huge protein hit...

..but he'll need to hang onto it.

The carcass's smell attracts scavengers.

These coyotes won't fight the bear -

he's 15 times their weight -

but they will harass him for scraps.

He tries to mask its scent by covering it with grass.

This will also slow down the bison's decay.

Having lost up to a third of his body weight during hibernation,

he's determined to keep these precious nutrients for himself.

Despite emerging from his den early,

this lone bear has hit the jackpot...

..but the stakes are high,

and the majority of grizzlies won't bother venturing out

until the consistent warm weather of spring.

But that still seems a long way off.

The snow shows no sign of letting up.

All the animals here face the same basic challenge -

finding enough food to survive until spring.

Our crews are out across the region

to capture the stories of how the wildlife is coping

in these last days of winter.

Yellowstone's elevated position and horseshoe of mountains

channel and trap the cold air driving in from the north,

and moisture coming in from the Pacific Ocean.

This leads to prolonged periods of extreme weather.

When the snow melts in spring,

it feeds into a thousand rivers and streams across Yellowstone.

These 2,500 miles of running water are hugely important,

not just for Yellowstone,

but they provide an essential resource

for millions of people and animals

right across the Western United States.

Scientists constantly monitor how much snow is building up here

during the winter.

To understand what's going on this year,

I am joining a team up in the Beartooth Mountains

in the north-east on their cool commute to work.

This is possibly the best office I have ever been to.

THEY LAUGH

This looks like a pretty good spot.

Oh, wow.

It's right at about 205 centimetres.

OK.

Lucas Zukiewicz is one of the hydrologists

who spends the winter carrying out health checks on the snow.

Our entire history of our snowpack,

starting mid-October,

is really in this snowpack here.

So we are going to dig a hole down into the ground,

and it is pretty much the coolest layer cake

that you're ever going to see.

LAUGHING: I love that.

I have a feeling that I might be here for months.

Right, OK. So...

Yeah, I've hit grass!

That is so cool.

I have to say, coming from a country where, you know,

if you get four centimetres of snow it's newsworthy,

to look at that and to kind of understand

that that is a record of your winter, that's quite cool, isn't it?

It's amazing. And the best part of this job is it's never the same.

Every year is different.

It's like hitting the reset button on it every year.

So it's time for us to analyse what the snowpack can tell us.

So, looking at it from bottom to top, here,

we are looking at a time series.

So we have our early season, October snowfall,

probably our Thanksgiving and Christmas snowfall

that we have here. You had a white Christmas?

We had a white Christmas this year, it was great.

Various measurements enable Lucas and his team

to work out how much snow is building up here,

and how dense it is.

The texture of it here is completely different.

It feels much more compact, exactly as when we were digging it out.

The density reflects how much water is contained in each layer

of the snowpack and that varies

depending on the temperature when the snow fell.

Maybe we see some pretty well bonded snow in here,

which is generally an indication of warm temperatures.

OK. Warm temperatures actually bond the snow pretty well together.

First of all, we're going to stick one right up here

on the surface of the snow.

Measuring the temperature is also key in helping to predict

when the bulk of the water will flood down into the valleys.

The next one at 170,

we are going to stick that right below

that little crust layer that we saw.

And these readings also provide clues

as to why some mammals hibernate deep in the snow.

Stick it right in the ground.

In the grass.

So, we can see here that our snow surface temperature -

what does that one say?

I would say minus eight.

But as we move further down the snowpack...

It's warmer! It's warmer, it's minus three degrees.

That's mad.

OK. And if we go even closer to the ground...

It's almost on zero.

Yep - so, the ground is generally close to zero degrees Celsius

through the winter, so the snowpack is also insulating the soil.

But it also makes sense,

when you look at something like a grizzly bear

hibernating in the winter beneath the snow -

I now understand why it chooses to hibernate there,

because it's warmer.

Snow caves are pretty warm.

That's brilliant.

This insulating effect

not only allows bears to hibernate in snow dens,

it keeps the grass alive.

Vital food for herbivores

and whilst it may look just like a white wall,

we're actually looking at an enormous reservoir.

When the weather warms up enough to heat the snowpack right through...

..billions of litres of water run off the mountains

into the rivers and lakes of Yellowstone.

Is the timing of the thaw important for you?

It is, because it affects when we see the bulk of the water

move into our river systems. Right.

This is our reservoir that we have in our mountains,

this is how we get our water, this is how it flows through the park.

So, do you think when the thaw comes,

it's going to be a normal year,

or an abnormal year?

We are slightly below average

with the snowpack that we have here in place, at this elevation,

but really it is going to be the next month or two

and the weather that we experience

that is going to dictate what we see this year in terms of our thaw,

and how that impacts the bigger ecosystem.

It will be interesting to see what happens in the coming weeks,

as the weather has already proved highly volatile this winter.

There was a jump in temperatures

and an unusually warm period at the end of February.

March has now seen a return to the colder,

more typical weather for this time of year.

In recent years,

Yellowstone has experienced milder winters more often

and this one appears to be following that trend.

Patrick has headed south to see how the temperature jumps this year

are affecting another iconic species.

Snow is a lot more complex than it looks,

and for one animal, a bird of prey,

the precise depth and type of snow

can literally mean the difference between life and death.

This is the great grey owl,

one of the largest owls in the world,

with a wingspan that stretches up to 1.5 metres.

It's a winter specialist.

It survives in some of the coldest forests on earth,

from Siberia to Scandinavia...

..to here, in north-west America.

Researchers believe several hundred great grey owls

live in the Jackson Hole Valley,

all within 150 square miles of forest.

Our cameraman, Jeff Hogan,

has had a lifelong love affair with this enigmatic bird.

I've been following and filming the great grey owls

for just about 30 years now.

It's still just as exciting and challenging

as it was the very first day.

This is a fantastic place for owls to be hanging out.

I mean, it is mixed aspens and conifer forest,

along with these meadows,

provide an incredible habitat for the prey that the owl feeds upon.

Great greys feed on rodents -

mainly gophers and voles - that burrow under the snow.

Their huge facial discs act like satellite dishes,

directing sound to their ears.

Their hearing is exceptional.

They can detect prey from over 100 metres away,

through snow almost half a metre deep.

But it is one thing knowing where to pounce,

another to break through to reach the meal.

Jeff is out looking for them.

He is concerned that, with the erratic weather conditions,

the owls face a big challenge.

Earlier this week we got a lot of snow, and then the skies cleared,

so you can look around here and see where the sun is beating down

on the surface of this snowpack, and it really softens it up.

It doesn't melt completely, but it makes it really soft and wet,

and then at night, when the temperatures plummet,

it will freeze hard and turn it to ice.

You can actually see this layer of crust.

You can really feel that.

Ouch.

This is hard.

And as the temperatures drop as night approaches,

this is going to get harder and harder,

and these owls have to bust through this crust,

and there may be another foot of snow to get to their prey.

Jeff hasn't seen any great greys,

which could mean they have been forced further afield to hunt.

So, I guess I'm going to have to keep on looking.

At this time of year, eating enough food is critical.

If the great grey owls can't hunt, they simply won't lay eggs,

or even nest this season.

Their population is already small and vulnerable.

Biologist Katherine Gura is keen to see how current snow conditions

might be impacting the owls' ability to feed

and that involves a simple experiment.

Katherine, you are stood out here with a ruler, a metre rule,

and these flasks of water.

What do these have to do with owls?

So, this is about the same weight as a great grey owl.

So, what we are doing is dropping these,

and we are seeing how far they penetrate the snow.

Because it in turn can show us how easy or hard it is

for great grey owls to penetrate the snow to get to their prey.

Right, Katherine, can I give you a hand? Absolutely.

OK, so what do I do?

So you want to drop that from one metre above the snow.

OK. So that's here.

One metre, that's here. Yeah.

OK, so it hasn't really gone that far. It's gone about...

17 centimetres.

With their prey often underneath twice as much snow,

this is why the owls are having trouble feeding.

Our concern is that it is changing the timing of their behaviour.

So if there's not enough prey,

I mean, that's probably going to spell disaster.

Are you worried about that? I definitely am.

I've been working with these owls for years.

I love watching them throughout their nesting season,

and it really concerns me

if they are not going to be able to have enough prey

to even attempt to nest, or to eventually support the young.

This year, with the tricky snow conditions,

the owls appear to be suffering.

To find out how they are really getting on,

or indeed if they are around at all,

Katherine needs to conduct her survey at night.

The night-time call of the great grey owl

is one of the early signs of spring,

and, for scientists, this is the first time

they will be able to work out exactly where they are,

what they're up to, and how they are doing.

If the great greys are going to be successful in raising chicks,

they should be out there right now,

and we hopefully should be able to hear them,

but the thing is, this year, nothing is certain.

Jeff has also joined us.

He is hoping his luck finding owls will change

under the cover of darkness.

The survey area covers several square miles.

Now, my top speed in snow shoes is half a mile per hour.

This could be a long night.

All right, this looks like a good spot to stop and survey.

OK, so what are we going to do?

So I will do the male territorial call,

and then we want to be as quiet as possible,

and we'll listen to see if one responds.

Right, so, fingers crossed we get an owl. OK.

I'll do my best.

SHE MIMICS OWL HOOT

OWL RESPONDS

Yep, that's it.

Right here. Yeah, this is great.

OWL HOOTS

It's responding.

So, is that probably a male?

So, that's a male territorial call.

So they will do that, either to attract a mate,

and also to tell other males that this is his zone, his territory.

Should I give it a go?

You should do it.

HE HOOTS

No, that's too deep!

See if he likes it.

HE HOOTS

Mine's too manly.

You've scared it off.

He doesn't want to get beat up!

So, given the conditions that we're having this year,

this is a really good sign,

that we can hear the great grey owls. Mm-hm.

I was definitely worried, with kind of the hard crust that we have had,

that they might not be on territory yet,

but it is a good sign that they are here and defending it.

OWL HOOTS

So, there are males out there

and they're defending their territory.

Given this year's unpredictable weather, it's a promising sign.

Jeff and Katherine will continue monitoring,

and we'll be back to see how the owls are getting on

later in the season.

All across the region,

the challenge for much of the wildlife

is simply to hang on until spring.

It may appear that animals like bison

spend the winter stoically struggling through the snow,

but there is a respite for some...

..and it's thanks to a feature that is synonymous with Yellowstone.

It's famed not just for its landscapes and its wildlife,

but also for its geology.

Yellowstone has more geysers than anywhere else.

In fact, 60% of all the geysers anywhere in the world are here,

and the reason for that is a giant magma chamber,

literally a pool, a sort of lake of molten rock,

sitting just below the Earth's crust,

literally right under my feet here.

Scientists believe there's enough magma down below

to fill the Grand Canyon more than 11 times,

and in West Yellowstone,

there's plenty of evidence of all this geothermal activity,

particularly with the dramatic geysers.

These hot springs intermittently send fountains of water and steam

into the air.

It looks a fairly inhospitable landscape,

but around 200 bison spend the winter living and feeding here.

These bison are part of what is known as the central herd,

and they don't need to work really so hard to get at the vegetation

beneath the snow.

The earth here is warmed by that magma chamber

bubbling away beneath the crust,

and that means there's far less snow,

and the vegetation is much easier to get at.

But scientists have discovered that these easy pickings

come at a high price.

The hot water pushing up from deep underground

dissolves the chemical compound silica from the volcanic rock,

and deposits it on the grass.

Now, silica coats the vegetation around geysers and hot springs,

and it's a little bit like eating something

that has been covered in a very fine dusting of ground glass.

It's enormously abrasive and very, very tough on the teeth.

The other issue is the high levels of fluoride in the water.

This chemical is a by-product of historic volcanic eruptions.

In low doses, such as in toothpaste, it's good for your teeth,

but here it's so concentrated it actually softens,

rather than strengthens, the enamel.

Now, the combination of silica and an excess of fluoride in their diet,

means that the bison down here lose their teeth

significantly earlier than the bison up in the north of the park.

In fact, life expectancy amongst this herd

is at least five years less.

So, although these bison have an easier life

than their snowbound cousins,

it's definitely a much shorter one.

Finally, by mid-March,

other parts of Yellowstone also get a reprieve from deep snow.

It's melting faster than it's falling.

Warmer air has come up from the south

and pushed out the cold front in the north.

Daytime temperatures rise above freezing, unlocking the rivers...

..and the animals make the most of it.

Underwater plants are more nutritious

than those beneath the snow,

and are a vital food source for this moose,

which needs to pile on the pounds fast.

This muskrat is enjoying the vegetation, too.

His double layer of waterproof fur keeps him warm and dry,

even whilst eating on an icy platform...

..and, when he dives down, his lips seal shut

behind his big rodent teeth,

so that he can gather food without swallowing water.

Even a winter casualty is good news.

This elk carcass will sustain many scavengers over the coming days...

..and it may be here

thanks to Yellowstone's most charismatic predator.

HOWLING

SHE GASPS: Oh, my God, look at the wolves!

This is the most...

This is the most incredible view!

What becomes so evident when you see a pack of wolves

running across the snow like that

is just how perfectly adapted they are as winter predators.

There are some things that...

in life that floor you,

and seeing a pack of wolves...

I am completely, completely overwhelmed by the sight!

To see what effect this year's thaw is having on these animals,

I'm joining wolf biologist Doug Smith.

We're on our way up a ridge on Blacktail Butte

in the Northern Range,

home to a pack of 11 individuals

that lay claim to a territory of 250 square miles.

Doug and his team constantly monitor the wolves in Yellowstone.

He's concerned that climate change is having an impact on them.

They just ooze charisma, don't they?

Even 2km away.

It's true.

There's something enchanting and mysterious about them

that has gone through the ages.

Usually wolves thrive in the winter.

They are snow specialists.

Their feet fan out,

enabling them to run across deep snow at over 25mph.

This gives them an advantage over their prey, elk and bison...

..which struggle with their cloven hooves.

Wolves generally have even more of an edge in March,

when Yellowstone's herbivores are weak and hungry...

..but this year is different.

By Yellowstone's extreme standards,

2016 has been one of the mildest winters

ever recorded.

Temperatures have fluctuated wildly,

but, overall, they've hovered three degrees

above the historic average.

A warmer winter is great for herbivores

as there is less snow,

and they are less likely to die of cold...

..but for their predators, Yellowstone's wolves,

it can make life a lot more difficult.

Because this winter has been so mild,

do you think that that is going to have a big impact on these wolves?

It certainly could.

I mean, hard winters wear your prey down,

and that makes them easier to kill for wolves.

A mild winter, their prey is going to do a little bit better.

This winter was warmer and, with below-average snowfall,

and it is early to tell what the effect of that is,

but we think that wolf food consumption rates, kill rates,

are down because of that.

So these milder winters favour the prey,

and so does that mean fewer pups?

Which leads to smaller packs.

This changing winter weather could be a huge impact.

Wolf kill rates are down this year,

but our camera teams across the region

have caught some hunts on film.

Perhaps there will be clues

as to the problems wolves are facing right now.

Talk me through what's going on here.

I mean, that's classic wolf hunting behaviour.

What wolves are looking for are behavioural cues

that indicate to them that there is a weak bison.

Right. But what's interesting is, the bison are staying in these areas

where there is either no snow or little snow,

because they have got firm footing and they feel confident. Yeah.

And so what the wolves are doing is trying to bump them into a place

that's more favourable to them, ie, deeper snow.

This winter there's less deep snow... Right.

..so the wolves are having a harder time.

Plus, these brown places have forage for the bison to eat

so they're going to be healthier. Yeah.

That's also bad for the wolves.

The next hunt involves the wolves' main winter prey, the elk.

Now, here is an elk encounter... Yeah.

..and this is even more dramatic.

Look at that! This female elk is very healthy,

and she is actually attacking the wolf,

but she won't leave the dry ground. Yeah.

Classic example of the importance of snow depth and footing.

Look at the elk. Healthy elk will always outrun a healthy wolf.

So, talking to the old-timers who lived here in Yellowstone

during the 1960s and 1970s, every winter was a deep snow winter.

It was the rare winter that was not much snow.

Now, it's the rare winter where you have deep snow.

It is more common to have shallow snow,

and that's switching this balance between predators and prey.

With so few successful hunts,

scientists have noticed that some of the wolves

are turning to a summer feeding strategy.

Instead of working as a pack, they're hunting alone,

digging into burrows to catch small mammals.

It's much less rewarding, much harder work,

and, for this time of the year, definitely unusual.

The repercussions could be huge.

The female wolves are pregnant right now,

and poor feeding may lead to weaker pups.

We won't know until early summer.

It's the end of March.

The signs of spring are getting stronger every day.

I guess I weigh more than a coyote.

With temperatures warming,

many more grizzlies are coming out of their dens.

Our bear expert, Casey Anderson, is out searching for them

up in the Gallatin Mountains.

Sometimes it's not that hard

to work out where they've passed through.

Hey, look.

You can see, right here, it's all kind of worn out,

you can see these little claw marks.

Well, sometimes when the bears come out of the den,

they can be a little mischievous.

If you look right here,

one of the first stops the bear had made was this snowmobile seat.

Just ripping, clawing, biting, just playing, really.

It's part of living in grizzly country.

It is never certain when bears will emerge in Yellowstone,

but this year has been more unpredictable than ever.

It's been, really, a strange year so far.

The spring temperature

has definitely gotten a few bears up earlier,

we know that one bear has gone down,

pulled that bison carcass out of the pond.

That was a risk worth taking.

But it was a risk.

Other early grizzlies may not have been so lucky.

The adaptations animals make in Yellowstone

to chime with its extreme seasonal changes

are finely balanced,

and in tune not just with temperature

and degree of snow cover,

but also with length of day.

Right up here, there's a long-tail weasel.

It's running around right up here.

So, these long-tail weasels, as soon as spring actually is here,

they will turn brown,

and they'll be camouflaged for the spring, summer and fall.

When winter comes again, they turn white, just like this.

The fact that he's still completely white

shows that it is very much winter.

The changing colour is triggered by hours of daylight,

not by temperature.

He won't turn brown until next month.

And just like the fox and the coyotes,

he's looking for all the rodents that live underneath the snow cover.

He's just tunnelling down, going and grabbing these things,

and popping up and eating them.

But whilst he is still white, if the snow melts too quickly,

he'll lose his camouflage advantage,

and could be at risk from predators himself.

And one mammal who would happily eat him is a fox.

You can see this, like, little story unfolding here.

This is a fox doing what they're so good at in winter time.

A bear's greatest sense is their sense of smell,

whereas a fox's greatest sense is their sense of hearing.

And they literally hear a little rodent way down in the snow,

they'll stop, and you'll see them,

they'll do this thing where they pitch their head side to side,

and what they are doing is really triangulating

and zoning in to the exact location of that rodent,

and then they just do this giant leap up in the air

and get this momentum with gravity, and just, poof!

Right down into the snow,

and grab that little guy.

He doesn't even know he's coming.

He thinks he's down there, all protected.

Little does he know, up there is a fox.

A missile coming straight down and getting him

and look at that, that's quite a ways down.

A little vole is a pretty good snack for a fox.

Whereas for a bear, it is nothing,

there's no way it could survive on voles alone through the winter,

so that's why they choose to hibernate,

there's nothing out here to eat if you're a big old...

bumbling-around bear.

PATRICK: Down in the south, there is also still plenty of snow

in the forest habitat of the great grey owl.

Cameraman Jeff Hogan has been out night and day tracking them

to find out whether they're breeding successfully.

We have an owl.

Jeff's been hearing and seeing the male owls consistently,

which is a reassuring sign.

He'd feared they might have left the area due to a lack of food.

We're going to set up here.

This year, the issue has been icy snow.

It's prevented the great grey owls snow-plunging deep enough

to catch their prey.

The owls each need at least one rodent a day,

and if they don't eat enough now, they just won't breed.

He didn't catch anything.

There's something right here underneath him,

because he's back.

He's going to make another attempt.

Oh, he jumped.

Oh, he's on the ground now.

I don't think he got anything.

He just flew up with nothing.

I've been here since dawn and he hasn't caught anything yet today.

It appears the owls are still struggling.

Then, at last, a sign of hope.

A female arrives.

This could get exciting. We've got two great greys right here.

Following one bird is pretty exciting, but to have two birds...

..it's amazing, it really is amazing.

Let's see what goes on.

Oh, here, he's flying in.

Oh!

Mating. Mating!

Oh, my.

I've never seen this before.

This is great news.

It's incredible, absolutely incredible.

For the end of March,

courtship, mating.

The next step is nesting.

Eventually, the pair choose to nest on an old, broken spruce trunk.

How many eggs are laid will depend on how much food the owls consumed

earlier in the season.

On average, they lay four,

but this year, it could be less.

We will be following them to see what happens.

It's now April and the temperatures are warming up.

This is Jackson Lake in the Tetons,

and normally it would be frozen well into May,

but already the ice is breaking up.

The arrival of the melt is a tipping point

for the wildlife of Yellowstone,

and this year, after an extremely mild winter,

it's three weeks early.

At long last, though, it's official - spring is here.

As the days get longer, the sun is higher in the sky,

and its increased energy melts the snow and raises daily temperatures.

Although there's still snow on the ground,

the warming temperatures are actually changing the shape

of the crystals within the snow pack.

Now, they've turned into tight little balls,

and there's a lot of water in between them,

so the packs become a lot slushier.

It's literally at melting point.

All this snow is just water in waiting.

Millions of tonnes will gradually melt,

flooding into the lakes and rivers of north-west America.

It's a process that will continue until July.

It's the valleys that lose their snow first,

and then, finally, the mountain peaks release their huge reservoir.

Soon, these warmer temperatures will lure the last male grizzlies

out of hibernation...

..and, with the snow retreating,

plants, too, are bursting into life.

A nutritious feast for hungry bears.

Patrick's back in the Gallatin Mountains with Casey Anderson

to find out why a top predator

sometimes chooses the vegetarian option.

Look right here.

This is biscuit root.

This is a grizzly favourite.

Bears are omnivores.

They don't just survive on meat.

Their diet also includes insects, fungi, and lots of vegetation.

Many bears time their emergence

to coincide with biscuit root coming into flower.

So, that is what they're looking for, right there.

This bit of root at the bottom.

There's tonnes of calories in it.

So how much biscuit root do they have to get through?

I've watched them eat hundreds if not thousands of these roots.

What does it taste like? You tell me.

I was afraid you were going to say that, but I'm up to the challenge.

Bottoms up.

It's like carrots. Yeah.

Yeah, like a kind of starchy carrot.

Yeah. To be honest with you, when I'm hiking around out here,

sometimes I look in my backpack and see what I have for lunch,

and I look on the ground, and I'm like,

"I'm going to go grizzly-style here and start digging away."

As they put on weight after the winter,

and more grizzlies come out,

they begin to communicate with each other.

Each male can have a home range of over several hundred square miles,

and they need to work out who else is out there on their patch.

So, this is an old backcountry camp in here,

people come up and camp from time to time,

there's a little tent over here,

and this old outhouse,

that tree right next to it is the rub tree

where these big males will rub their back

and urinate all over the place, right by the outhouse.

So it's a bear toilet?

It really is.

Look, see these big claws right here. Wow.

And look right here, Patrick,

where the sap has stuck all the hair...

Oh, yeah. ..stuck to the tree. All down to here.

So, all this here,

this is...

bear hair. Yeah.

So, what they do, they come over,

and they just usually come up and put their back on the tree,

and they'll rub their scent all over it,

and they reach up and grab like this, and they bite the tree.

Then they drop down, particularly the males,

and they do this thing that I call the cowboy walk.

So, they just stick it and they twist their feet like this.

PATRICK CHUCKLES

They have real rigid legs, and they're just really twisting.

So each bear that comes to this tree will step in that same exact spot.

And they do it over and over again.

It leaves these big divots in the ground.

So, if you're a smaller bear

and you really have to reach for that next foot divot,

you know there's a bigger bear in town.

This is a well-known rub tree.

We're setting up a camera

to see how many of the big males are out there.

See this right here? Right.

It's going to take a recording,

and it's going to text me to my cellphone.

No way. Yeah, real-time.

So, as soon as a bear's there, I'm getting a text.

So, hang on. You're going to get a text from a bear?

Exactly, that's exactly what's going to happen.

A few days later, Casey has called me in to look at the results.

Right, what have we got? Let's take a look and see what it's caught.

Uh-oh!

THEY LAUGH

That's just brilliant.

That is an elk.

A cow elk. So, this is something that always happens.

You set up your camera trap, you think you've got the right angle,

and then one of the animals comes along and wants to change the angle.

Oh, man! It's giving it a good go. She's kicking it.

I think it shows their playfulness.

They come over to something new, curious, and it's like...

"I think I'm going to play a little soccer with it."

That's just great.

Let's have a look at a couple of the other clips.

Whoa, first clip!

Exactly what we wanted. Wow.

That's a big male.

I guess it's what we would expect,

this time of the year,

that these big guys will be the first ones coming out of the den.

The females and the cubs are still up high.

Casey, Casey, this is a huge bear.

Remember when we set this camera trap,

we were anticipating that we would get the whole bear, but... Mm-hm.

Just missing the head. He's bigger than that.

He stands at 2.5 metres tall.

He's giving it a real bear hug!

That's a bear hug, for sure.

Wow. And there we go,

doing that kind of cowboy walk you were talking about, right?

Really trying to grind in his scent,

leave those marks.

That cowboy walk.

That means one thing - "I'm in town and I'm the boss."

But what are they picking up,

and how do you think that affects their behaviour?

Well, as they emerge, it's the beginning of the mating season,

so it's important for males to know if there is a bigger male around.

It's like... HE SNIFFS

"Oh, this big dude's here,

"maybe I don't want to be here," and they'll go the other way.

Equally for the females, they want to know, you know,

who is going to be the father of their next offspring.

Yeah. If there's any big tough guys around.

Although the unusual weather a few weeks ago

had brought some of them out of hibernation early,

these males appear fighting fit.

Spring is in the air, and the bears are getting frisky.

But they're not the only males getting boisterous.

These elk are shedding their antlers.

New ones will regrow over the next few months,

ready for the autumn rutting season.

But perhaps the biggest male show-offs are down in the south,

and Kate has a front-row seat for their fabulous performance.

WITTERING

It's mating season for the sage grouse.

They really like to make a proper song and dance about it.

GROUSE WITTERS

That sound... It's really difficult to describe.

It's like a sort of...

popping and a drumming.

It's that, combined with these wonderful, very showy

puffed-up white chests,

and those fanned-out black tail feathers,

that will attract the females.

Ooh, there's one.

And they will literally do a fly past -

they'll sort of fly over and look down

and say, "Oh, yeah, I don't mind the look of that one,

"I'll give that one a go."

Greater Yellowstone is a stronghold for the sage grouse,

a species under threat.

Once numbering tens of millions,

there are now just a few hundred thousand of these birds left.

Many of them rely on Yellowstone,

where there is still plenty of their habitat - sage brush.

It is now around 11 degrees,

perfect for breeding.

But success will depend on how the thaw progresses.

Chicks won't do well if the temperatures soar too quickly.

So far during the thaw, some of our animals have done well,

and some haven't.

The jumping around in temperature

at the end of winter confused some bears

and made life difficult for the owls.

One of the mildest winters on record and a lack of snow

prevented the wolves hunting successfully,

yet the elk and bison emerged from the season in high numbers,

and with plenty of fighting spirit.

Soon this landscape will be transformed

from brown and barren to lush and green,

but the arrival of spring doesn't necessarily mean

that life will get easier for our animal families.

All those newborn youngsters

will mean that there's lots of competition for food,

and the predators will be ever present,

on the lookout for the young and the vulnerable.

Tomorrow night,

will the grizzly bear cubs find enough food to eat

when they venture out into the big, wide world?

We join a beaver family,

but will they deal with the deluge?

The water level is so much higher.

After a difficult winter for the wolves,

will their luck turn?

And there is a nest of great grey owl chicks,

but will they all survive?

Oh, he is struggling.