Episode 4 Earth's Seasonal Secrets

Every year, spectacular seasons transform our planet.

They're the driving force of all life on Earth...

..bringing opportunities and huge challenges.

Spring bursts with new life.

But it's a race to grab fleeting chances.

Summer is all about glorious abundance.

But the heat can push animals to their limits.

Autumn is nature's great gold rush.

But competition is fierce.

Winter creates a frozen wonderland.

And only the most resourceful will survive.

In every corner of the planet,

animals rise to overcome the seemingly impossible...

..and use extraordinary tricks...

..to thrive against the odds...

..as the seasons create the greatest shows on Earth.

Spring.

The sun's getting higher, the air's getting warmer...

..and nature bursts back to life.

But the clock is ticking.

The good times are fleeting.

Life in spring is one big race.

It's a rush to grow up and find your way in the world.

But often, the first challenge is simply...

..getting out of bed.

In the mountains of Alaska,

there's one animal that's waking up after six months fast asleep.

In early spring, all across North America, 60,000 grizzly bears come

lumbering out of their dens.

They've slept their way right through the worst of winter.

This mother gave birth ten weeks ago.

Her cubs have been fattening up on her milk ever since.

They're now 20 times bigger than when they were born.

But there's nothing to eat up here.

She has to get them down to the warmth of the valleys.

This is the first time these cubs have seen the outside world.

But there's no time for playing about.

Their mother hasn't eaten a thing since autumn.

Every thousand feet they descend, the air gets one degree warmer.

Every step takes them closer to spring.

For some, it's a perilous journey.

This family chose to make their den 8,000 feet up.

It kept them safe from attack by other bears through the winter.

But now they have to get down again.

These cubs are two years old,

and they've made this journey before.

They don't seem too worried by the steep climb down.

Their mother needs them to keep up, but not everyone's paying attention.

In just a couple of months, this young bear will leave his mother for good

and lead his own life...

..and he's already showing signs of wilful independence.

But this is probably not the best place to strike out on your own.

He's in danger of getting lost.

Bears are good climbers...

..but this is a risky manoeuvre on a vertical icy slope.

He got himself into this mess

and he's going to have to get himself out.

His mother can't wait for stragglers.

BEAR PANTS

This is probably the first time he's ever left her side...

..and as he's still reliant on her,

he needs to find her.

For young grizzly bears, spring is full of life lessons.

This time next year, they'll be finding their own way down.

But, for now, Mother knows best.

Meanwhile, in the lowlands, there's a magical transformation happening.

In the valleys, the grizzly bears have left the snow behind.

It's taken them days to get here,

and they've barely eaten on the way.

But it's ten degrees warmer down here.

Spring has landed.

And this is what they've come for -

the fresh greens.

At this time of year, it's all new growth, rich in sugar,

and that's just what they need after their long winter fast.

They've got through spring's first challenge -

to wake up and find a feast.

While thousands of grizzly bears are emerging across North America,

further south, in Mexico,

there's a tiny creature whose spring awakening is on a truly epic scale.

In the forests of the Sierra Madre Mountains,

the trees are festooned with monarch butterflies.

They've spent the winter asleep, huddled together for warmth.

There are millions of them.

These trees have been the perfect roost.

The branches shelter them like a blanket,

keeping out the freezing nights.

On a sunny spring morning, the butterflies begin to stir.

When the sun warms their bodies above 13 degrees Centigrade...

..they start to fly.

More than 100 million take to the air.

It's the biggest gathering of butterflies in the world.

They're getting ready for a long and arduous journey.

They stream from the colony and make their way to water to drink.

Then they bask in the spring sun

to get their muscles warmed up for flight.

Soon they'll be off.

They're all going in search of one very special plant...

..and it only flowers in spring...

..milkweed.

The butterflies won't lay their eggs on any other plant,

so they must time their wake-up perfectly

for when the milkweed blooms.

Generations of butterflies fly 3,000 miles to Canada...

..following the flowering of this one particular plant.

It's the greatest insect migration on Earth,

and it only starts when the sun wakes them up in spring.

Life in spring is all about timing.

As soon as the light returns,

animals have to be ready to seize the moment.

And, for some, that means attracting a mate.

For a few weeks, birds suddenly become sensitive to the longer days.

The extra sunlight flicks a switch deep inside their brains...

..and this kick-starts their sex hormones.

All around the world, song birds sing at dawn to keep out rivals

and attract mates.

But it's not just singing that'll get you noticed.

As males rush to impress females, it leads to some outrageous flirting.

In the North American grasslands, a sage grouse woos the females by

inflating his magnificent chest sacs.

An Asian Temminck's tragopan starts by hiding from the female.

But then he plays peekaboo,

trying to dazzle her with his exotic good looks.

And in India, a peacock shakes his flamboyant tail,

hoping to impress a passing hen by shimmying his feathers.

Success is never guaranteed.

On the lakes of North America,

one bird's courtship display is unlike any other.

Clark's grebes.

They've just flown in from the Pacific coast,

and here in Oregon is where they'll breed.

At this time of year, these sheltered freshwater lakes are

the perfect place to raise a family.

These two lovebirds paired up last season.

But they've spent the winter apart so they need to get reacquainted.

If they stick together as parents,

they'll give their chicks the best start in life.

They need to prove their mutual devotion...

..so they dance.

They start by copying each other's movements

as they preen their feathers.

Then, to show he really means it, the male will find a fish...

..and offer it to her as a gift.

With eyes only for each other,

they reach the climax of their romantic display.

Only grebes can do this. No other bird this heavy can run on water.

But this display might help them get their breeding cycles in sync.

At this time of year,

the pressure on males to impress a female is relentless.

One ploy is to provide her with the perfect nest to lay her eggs.

But if you live in Antarctica, the most desolate place on Earth...

..nest-building materials are hard to come by.

At Cape Crozier,

Adelie penguins are in a hurry.

They've spent the whole winter at sea but,

as the days get longer, more than 150,000 come rushing back to land...

..to breed.

The season is so short here,

they have to be in the right place to build

a nest as soon as the ice thaws.

The males are first to arrive,

racing each other to stake a claim to last year's patch.

But with over 60 miles of ice to cross,

just getting to the nest site is a challenge in itself.

They're heading for this rocky slope,

the first to feel the warmth of the spring sunshine.

When the females arrive in a few days' time,

the males must impress them with a good-looking nest.

But with temperatures rarely above freezing,

no trees or shrubs can grow here.

One thing there's plenty of...

..is pebbles.

A neat little pebble nest will stop the eggs rolling around on the ice.

But finding just the right stones is a time-consuming business.

To get ahead, some are prepared to cheat.

The victim senses something's wrong...

..but can't quite put his finger on what.

The thief's nest is coming along nicely.

But even a villain can become a victim.

PENGUIN SQUAWKS

Although he's a lot quicker to spot when he's being robbed.

PENGUIN SQUAWKS

Well, he would, because it takes one to know one.

PENGUINS SQUAWK

Finally, they're finished,

and just in the nick of time -

the females are back.

The first to arrive will grab the best-looking nests.

But what also impresses them is a male with a good voice.

PENGUINS SQUAWK AND CHATTER

A well-held note is the sign of a well-fed male.

And that's just what the female wants.

It shows he has the resources to stay and incubate the egg while

she's off feeding out at sea.

Choices made and thousands of couples start breeding.

Within six weeks, the slopes are crowded with spring chicks.

CHICK CHEEPS

This is the season when many animals race to pair up and breed.

All this activity leads to one thing - an abundance of babies.

With more hours of daylight and plenty of food,

spring is the perfect time for young animals to fatten up and grow.

But the season moves fast.

And it won't be long before

they have to start fending for themselves.

So it's also the time to learn the basics of survival.

This Alaskan grizzly bear mother

might look like she's just playing...

..but this rough and tumble

is teaching her four-month-old cub a vital lesson...

..how to hold his own in fights

with other males when he's a grown-up bear.

This otter mum in Yellowstone is taking her ten-week-old cubs for

a fishing lesson.

They're already expert swimmers, but catching a cut-throat trout is

beyond their skills.

So it's a case of watch...

OTTER SQUEALS ..and learn.

And, for now,

they'll have to make do with caviar.

The Pantanal in Brazil, the largest wetlands in the world.

It's the end of the dry season.

For a few weeks, this is their spring.

Light showers of rain bring lush grasses and swarms of insects

before the Pantanal is flooded by summer storms.

This is when animals have their young...

..born into a world where there's plenty to eat.

In a nearby forest, the trees are full of ring-tailed coatis.

They're related to raccoons and they stick together in big family groups.

While their pups are young,

they're kept out of danger in the safety of the woods.

But now they're ten weeks old...

..and it's time to make their first trip out into the spring grasslands

to start looking for their own food.

COATI SQUEAKS

There may be 30 in the family band so there should be plenty of adults

keeping watch.

In this wide-open space, it's easy to get lost.

So for every young coati, the first rule to remember is never lose sight

of your mother's tail.

They're surrounded by unfamiliar faces, but who's friendly...

..and who should you avoid?

Southern lapwings are good to have around.

They follow the coatis, picking up little bugs the family leave behind.

In return, they keep an eye out for predators.

Armadillos only eat insects...

..and so do giant anteaters.

Nothing to worry about here.

But this savanna hawk is watching with a less than friendly interest.

LAPWINGS TRILL

The mother makes a dash for the forest to get her pups to safety.

But with so many babies, it's hard to check if everyone's made it home.

COATI SQUEAKS

Sure enough, this pup's got left behind.

COATI SQUEAKS

Mistakes like this can be fatal.

COATI SQUEAKS

He knows he must somehow get back to the safety of the forest.

Spring is full of perils.

And this coati pup just got a little wiser.

Round here, it pays to stick with your family.

As the spring sun warms the air,

the urge to breed isn't just confined to animals.

Plants are doing it, too,

and they have some clever tactics.

In Japan, the mass blossoming of cherry trees is one of the planet's

most spectacular spring events.

The trees need to attract insects to spread their pollen,

but they're only fertile for a few days,

so they flower all at the same time in one glorious show.

Their extra-sweet nectar keeps the insects coming back for more.

North American water lilies spend all winter asleep under the ice but,

in spring, it takes them just four days to rise through the water

and bloom.

As the sun thaws the frozen North, the Arctic poppy flowers.

Not many plants can survive above the Arctic Circle,

but this one has a special trick.

As the sun passes overhead,

it follows its progress, capturing extra warmth...

..and pollinating insects linger

a little longer inside its cosy petals.

A British woodland in early spring.

The trees are still bare.

But down on the floor, there's a whole host of plants just waiting to

grab their moment in the sun.

Early flowers race to bloom

before the trees grow their leaves and steal the daylight.

With tough competition, you need a trick or two to get ahead.

Yellow and white flowers reflect more light,

making them stand out to insects down here in the shade.

In this race for the light, one plant puts on a show on a scale seen

nowhere else on Earth.

With their rich soils,

Britain's ancient woodlands are home to half the world's population...

..of bluebells.

These vibrant carpets of flowers give bees a vital boost of nectar at

this early time of year.

But spring flowers need more than just sunshine.

They need rain.

In South Africa,

one of the driest places on Earth is about to be transformed.

THUNDER RUMBLES

The Namaqua Desert,

a 600-mile strip of land along South Africa's west coast.

For most of the year, this is an arid place,

just sand and wind-blasted rock.

But for a few weeks after the winter rains,

spring puts on a dazzling display.

These Namaqua daisies don't waste precious energy producing scent.

They want to attract one special pollinating insect...

..and that insect is only drawn to colour.

And here he is...

..a monkey beetle.

He's spent the winter underground, but he's come out

just in time for the flower show.

As he hops from flower to flower...

..he's spreading the daises' pollen...

..but he's only interested in finding a female.

There's one.

She's a golden beauty.

But he's got competition.

The female is so busy feeding,

she doesn't even notice the two males brawling over her.

But the battling beetles are good news for the flower.

They're now covered in pollen,

which they'll spread to the next flower they visit.

The female has lost interest.

But the males fight on.

He's won the brawl...

..but the female's gone.

With so many beetles drawn to these flowers,

he shouldn't be single for long.

A result - the beetle finds a mate...

..and the flowers are pollinated.

Now next year's spring display will be just as spectacular.

When winter releases its hold,

spring can bring great opportunities for those prepared to take advantage of them.

In the search for food or a safe place to breed,

many animals will make extraordinary journeys.

In the Arctic, the ocean fills with fish.

Little auks fly nearly 2,000 miles

to Svalbard just to raise their chicks on

the cliffs around these teeming waters.

Off the coast of Alaska,

millions of Pacific herring

swim up from the depths to spawn in the warming shallows.

As the Arctic tundra unlocks in Greenland,

snow geese fly all the way from Mexico to raise their young

on the fresh new grass.

These long migrations can be gruelling tests of endurance.

In Africa, one small bird is about to set off on a 3,000-mile journey

across one of the most inhospitable places on Earth.

SWALLOW CHEEPS

Barn swallows have spent the winter in Nigeria...

..but the rising temperatures are their cue to leave.

Two million birds are on the move.

There's plenty of food here,

but it's not the best place to bring up chicks.

There's too much competition...

..so they're heading for spring in Europe.

These birds are tiny.

They weigh not much more than a couple of pound coins,

but they're about to undertake one of nature's toughest migrations.

Ahead of them, there's a deathtrap.

The Sahara Desert.

It's much too big to fly around.

The swallows have no choice but to fly across it.

1,000 miles of almost nothing but sand.

It will take them four days to get across.

Somehow, they will have to find water on the way

otherwise they will die of thirst.

As they cross Libya...

..in an ocean of sand, there's a tiny speck of blue.

Umm al-Maa,

a lake of ancient ground water seeping to the surface.

But this oasis is not what it seems.

Over thousands of years, the lake has been evaporating.

It's nearly five times saltier than the sea.

If the swallows tried to drink this, it would kill them.

FLIES BUZZ

But there's an unexpected solution.

At this time of year, thousands of brine flies emerge from the lake.

Their bodies filter out the salt...

..so, effectively, they're tiny packages of fresh water.

The swallows gorge on them and get all the water they need.

And they're not the only ones.

Wagtails also break their northern journey for the fly bonanza.

This is the swallows' only stopover,

but it's enough to get them across the desert.

By the time they reach Europe,

spring has arrived in the Northern Hemisphere.

They now have four months to feed up and raise their family before they

fly all the way back to Africa.

For other animals on the move,

the challenge is not so much about distance, but navigation.

In the Arctic, spring unlocks

the land, but the vast areas of frozen sea

take longer to crack.

So animals here must find their way

through a constantly shifting landscape.

As the sun gets more intense and the sea ice melts, pools begin to form.

The darker water absorbs warmth and the pace of the ice melt increases.

The great sheet of ice starts to split.

These huge cracks are known as leads,

and they're just what one animal is looking for.

Narwhal.

These strange whales are some of the most secretive animals in the ocean.

Over winter, they've been feeding at

the edge of the sea ice but, in spring,

as the ice begins to shrink, predators start to move in.

Killer whales.

They have a taste for narwhal.

To avoid them, the narwhal begin a 600-mile journey north to find

safer waters close to land.

And these giant leads are the only pathway through.

It's a hazardous journey.

These cracks could close over at any time.

The narwhal will have to navigate

carefully through a constantly shifting maze of ice.

They've barely got going and they've hit a dead end.

They use their heads and tusks to try to batter their way through.

The ice is too thick.

Rather than go back, their only option is to look for another lead.

But that means going under.

They need to surface every 15 minutes to breathe...

..or they'll drown,

so it's a constant search for the next airhole.

They've found another lead.

But the gap has suddenly narrowed.

There's barely room for one-way traffic.

And they hit another snag.

Another pod of narwhal is trying to find their way through,

and they're coming from the opposite direction.

It's a stand-off.

There's nothing to be gained from a fight.

The newcomers give way and everyone continues in the same direction.

These tussles will soon be over.

The Arctic sea ice will melt...

..and the narwhal will spend their summer breeding in the bays.

Some animals spend their whole life travelling.

In the North American tundra,

caribou make the longest migration of any land mammal on Earth.

When calves are born in the spring, they have to be ready to run.

This far north, by the end of spring,

the sun is warming the tundra almost 24 hours a day.

The grass is fresh and nutritious,

and the caribou barely stop eating.

More than two million animals

move across the tundra on a constant search

for the best grazing.

They might travel over 20 miles a day without even stopping to sleep.

But while they're benefiting from spring's good times,

it's about to bring out their worst enemy.

At this time of year,

the sun is strong enough to melt the permafrost,

the frozen soil beneath the grass.

The tundra is covered in pools of water, and they're the perfect place

for mosquitoes to breed.

Great clouds of them emerge and head straight for the caribou.

The mosquitoes are voracious.

They can drain half a pint of blood in a day.

They drive the caribou crazy.

The caribou do anything they can to avoid being bitten.

The ones on the outside of the herd are the most exposed...

..so everyone tries to shuffle into the middle for maximum protection.

When they can't stand it any more, they bolt to higher ground.

Up here, there are still patches of snow, and it's too cold for

the mosquitoes to follow.

The caribou finally get some relief.

The constant search for fresh grass

drives the caribou's nomadic way of life.

Every year, they cover distances of 3,000 miles.

Round the world, spring is one big race to grab the good times.

The increasing power of the sun makes plants grow...

..and brings out the insects...

..and that provides food for everything else.

As the sun's strength increases, in forests, a feast is about to appear.

After 1,000 hours of winter chilling, buds start to burst.

Inside each one, the leaves are neatly pleated,

spring-loaded to expand as quickly

as possible and start capturing the sun.

But from the moment they unfurl, they're under attack.

Caterpillars are insatiable,

but they're a vital link in the spring food chain.

Bluetits feed each of their chicks 15,000 caterpillars by the time

they leave the nest.

Even in rivers, sunlight brings out the insects.

In southern France,

a damselfly is just beginning her new life as an adult.

But when there's a hungry marsh frog in the water...

..it's rather brief.

In some places, the emergence of insects is truly sensational.

Beneath the surface of an English river,

mayflies are beginning to stir.

They've spent the last two years living underwater as larvae.

But on one day in spring,

as the sun gets more intense,

they emerge as adults in spectacular numbers.

Their only mission is to find a mate and lay their eggs.

By hatching all together, they increase their chances of success.

They may only live for 24 hours, so they have to be quick.

They appear like this on rivers across Europe every spring.

It's a brief free-for-all, and everyone joins in.

But millions escape and live just long enough to lay their eggs.

The show's over until next spring.

But it's in the open ocean

that spring puts on the most spectacular feast of all.

It all starts very small.

In seas with plenty of nutrients,

the increasing sunlight sparks the growth of microscopic plankton.

This assorted collection of tiny life forms drifts around with

the currents but, in spring,

their population explodes.

They multiply 1,000 times in just a few days and create great

oceanic blooms that stretch for hundreds of miles.

They're so big, they can be seen from Space.

These explosions of plankton are so rich,

they can support some of the biggest animals on Earth.

WHALE WHISTLES

Humpback whales - a mother and her two-week-old calf.

They've just left Hawaii, where her baby was born.

They're heading north to the plankton-rich seas off Alaska.

The warm waters of Hawaii were the perfect nursery,

but there wasn't enough food to sustain them.

She's had nothing to eat for six months,

and she's still providing her calf

with more than 100 litres of milk a day.

They have a 3,000-mile journey ahead of them.

Together, they have to cross the North Pacific, the world's biggest ocean.

It's a tough journey for an adult.

For a tiny calf, it's a marathon.

His mother slows her pace to let him keep up and supports him from below

when he gets tired.

But if she times it right,

they'll arrive to a feast of such epic proportions,

it will keep them fed for months.

In the seas off Alaska, the water is filling with life.

Vast numbers of animals are turning up to graze on the plankton.

Billions of krill arrive - tiny shrimp-like animals.

A single swarm can contain two million tonnes of them.

They're one of the most abundant animals on the planet.

And that, in turn, feeds a lot of fish.

Huge shoals of herring come up from the depths to feed on the krill.

And they are the feast that everyone has been waiting for.

Sea birds have travelled thousands of miles to make the most of it.

And the herring bonanza draws in

one of the mightiest predators of them all.

For a humpback whale and her calf, this is what their journey has all been for.

She might eat up to a tonne of herring a day.

It's a feast that will carry on all through summer.

All across the world, spring brings unique opportunities.

But you've got to be ready to make the most of them.

Those who time it right will reap the rewards.

In Britain, barn swallows will spend the long summer days feeding up

before they head back to Africa.

In Alaska, young grizzly bears

will spend a few more months with their mothers,

learning all they need to know for a life on their own.

In Antarctica, Adelie penguin parents will fatten up their chicks

with the bounty from the sea.

Spring may have brought challenges

but, for many animals, it's given them the perfect start in life.