Sixty thousand years ago,
humans left Africa for the first time.
Since then, we haven't stopped,
traveling to every corner of the Earth.
But our journey is just one of many.
At any given moment,
billions of animals are on the move.
And the health of our planet depends on it.
Migration is the most vital survival strategy.
Whether to feed...
to breed...
...or to find a new home.
And it's not without risk.
This series traces extraordinary journeys
in a world that is changing faster than ever.
And only now are we beginning to understand
that all life on Earth
depends on the freedom to move.
Every year, this land in Botswana is transformed.
These scars are the only sign that it was covered by water
just three months ago.
Then, rich and green.
Now, baked and barren.
This change drives animals to move en masse.
Cape buffalo are marching across the Kalahari Desert
towards the last remaining water.
Groups have come together and formed a mega herd.
The largest in Africa.
All are making the same annual pilgrimage
to one remote marsh.
And others are relying on them to do so.
These lions are buffalo specialists.
And the mega herd's arrival
is the most important event of their year.
For weeks, they've been waiting for the buffalo army.
Eyes fixed on every shape crossing the horizon.
Finally.
The approaching horde is a wall of horns and muscle.
But the lions must find a weakness.
And this could be it.
A lone bull.
He may be separate from the herd,
but he is four times heavier than a lion
and one of Africa's most dangerous animals.
The pride must approach with care.
One by one, they casually walk towards him
as if uninterested in a hunt.
In fact, they're falling into position.
The females take it in turns to attack...
tiring the bull.
Now, the heavier males join the fight.
The pride has honed their technique so well
that in just three minutes,
they have defeated their most formidable opponent.
The migration on which the lions depend
is one of millions driven by the seasons.
And they, in turn,
are a consequence of another journey.
Our planet's year-long loop around the sun.
As it travels,
it does so on a tilt of 23 and a half degrees.
And it is this that creates our seasons.
To understand how, keep your eye on the North Pole.
For half the year, it's angled away from the sun,
bringing darkness and winter.
For the other half, it swings towards the sun,
bringing longer days.
It's this annual cycle
that drives the greatest movements of life on Earth.
And in the Northern Hemisphere, this brings the year's biggest change.
Spring.
In late March, the days lengthen faster than at any other time.
Sunlight increases, quickly heating the land.
Triggering rainstorms.
The Northern Hemisphere suddenly becomes warmer
and wetter.
Perfect for plants.
The land turns green.
And one animal exploits the boom in plant growth like no other.
A week ago,
this desert locust in Ethiopia
was a shy, solitary creature that moved very little.
Not anymore.
When available food skyrockets,
locusts switch to swarm mode.
They start marching
and multiply ten times faster.
As their numbers explode,
they strip the land bare.
So to find more and more food,
they must keep moving.
The swarm grows.
And grows.
Devouring every plant in its path.
But this is just the start.
On foot, they can only move so far.
To go further,
they must change form.
They grow wings.
Now, with an even larger appetite,
the entire army takes to the sky.
They eat...
and fly.
Eat...
and fly.
They come together to form a super swarm,
200 billion strong.
The biggest seen in 70 years.
It consumes 400,000 tons of food
every single day.
So if the whole swarm is to feed,
it must move further and faster.
It travels 100 kilometers a day
across farmland and towns.
It sweeps across country after country...
and most astonishing of all,
it even crosses the Red Sea.
It's the first time a swarm clears the Himalayas and reaches Tibet.
Everywhere it goes,
it brings chaos.
It will keep moving until the changing seasons bring drought.
For when their food finally disappears...
so will they.
In April, day length in the North continues to rise.
[subdued expectant music playing
The increase in sunlight warms the oceans,
creating vast algal blooms
and a sudden abundance of food.
For one animal, this is the moment to travel.
Midnight on a remote island off British Columbia.
Underground...
something stirs.
This ancient murrelet chick has only just hatched,
yet it's about to face the toughest journey it will ever make.
She hasn't had her first meal
and urgently needs food.
Mum makes contact.
Across the island, hungry chicks emerge from their burrows
following their parents' calls.
But if they're expecting to eat now,
they're going to be disappointed.
This is the only seabird
that is fed exclusively at sea.
The chicks just need to get there.
They can't yet fly,
so their only way to the ocean
is on foot.
While waiting till nightfall makes sense to avoid predators,
it's a little harder to see where you're going.
And when you're barely bigger than a golf ball,
anything can be an obstacle.
Their way may be blocked by the lowest of logs...
and the smallest of streams.
For some, the journey is over before it has even begun.
But not for this chick.
Finally, the beach.
And it's the last land it will touch for two years.
To find Mother,
it calls into the darkness.
Together again.
But the chick has still not eaten.
To reach the nearest food,
they paddle nonstop
for a remarkable 70 kilometers.
Here, Mother can finally dive
to collect the chick's first meal of krill.
But this is just a pit stop.
The pair paddle a further thousand kilometers.
It's a month-long migration to the rich northern seas
where they can feast.
It's probably the longest journey made by any newly hatched bird.
And because the oceans are so productive in summer,
the murrelets can spend virtually their entire lives at sea.
1600 kilometers northwest of Hawaii
lies a remote speck of land,
Laysan.
Home to one of the world's largest seabirds.
The Laysan albatross.
This chick, six weeks old,
will become a supreme long-distance traveler...
...flying millions of kilometers in its 60-year life.
Now, however, at four months old,
it's still flightless
and going nowhere.
Unlike the murrelet,
it depends on food being brought to it
by its parents.
But they could be fishing up to 3,000 kilometers away.
So it may be waiting a while.
- - Through pouring rain...
...blasting winds...
and baking heat.
Every bird crossing the horizon brings hope.
An albatross.
But the wrong kind.
A black-footed.
Though she has almost certainly brought food.
Starving, the Laysan chick tries its luck.
Clearly, for this mother, it's family first.
But what looks like a lifeline...
...could be a death sentence.
Across the island, chicks are dying.
There is now so much plastic in our oceans
that it reaches even the most remote islands on Earth.
Carried here by the currents.
The parents mistake small floating pieces for food
and bring it to their young,
accidentally killing them.
It's almost certain that every chick on the island
has plastic in its stomach.
Those that are fed too much starve to death.
Laysan albatross are in the greatest danger.
They consume more plastic than any other seabird.
And it's getting worse each year.
At last, after nearly three weeks at sea...
the chick's father returns.
Having spent so long apart,
they need to renew their bond.
This could be the last meal the chick is ever given.
And almost certainly contains plastic.
Its father returns to a life traveling the Pacific.
And the chick is left to its fate.
But it's not over for him yet.
Albatrosses normally throw up hard-to-digest parts,
like squid beaks and bones.
Ejecting lighters and bottle caps, however, is more of a challenge.
But this chick...
is a survivor.
And with that,
he is one step closer to takeoff.
In a few weeks, on the right wind,
he will finally be able to make his big debut
as one of the greatest ocean travelers.
The course of our tilted planet through space
affects one place more than any other.
Summer in the North.
The Arctic now faces the sun
and is bathed in round-the-clock daylight,
melting an ocean covered in ice.
To survive in this shifting world,
life must move with it.
This young polar bear's home changes so much each year
that he can't keep to a limited territory.
Instead, he must roam for thousands of kilometers
in search of seals.
But every summer, as the ice begins to melt,
he must spend less time walking...
and more time swimming.
Now, for almost eight hours a day.
He can smell prey from 30 kilometers away.
So he knows that this bearded seal is nearby.
The seal is unaware of the danger.
And the bear needs to keep it that way.
The success rate of hunting from the water is very low.
Maybe third time will be lucky.
This young bear has yet to master the art of stalking from the sea.
But it's important that he does.
He may have to rely on it more and more each year.
Our climate is warming.
And nowhere is this having a greater effect
than in the Arctic.
This year, the temperature climbs
to a staggering 38 degrees Celsius...
melting sea ice faster than ever.
And one Arctic animal is affected more than most.
To feed,
walrus need a combination of a shallow seabed,
abundant clams,
and ice floes on which to rest.
But there are now fewer places to do so.
Making each summer more difficult than the last.
Especially so for a new mother.
Three days ago, she had to leave the herd to give birth on her own.
And since then, she hasn't left her calf's side.
Nursing mothers need nearly twice as much food.
But they can't leave their calves alone while they themselves feed.
So she must go in search of others
to help her care for her young.
A newborn walrus is able to swim from day one.
But he's not yet strong enough to travel far.
So, for his first journey,
he hitches a ride.
With fewer ice floes each year,
they must swim further to find help.
And until then, his mother can't feed.
Even when they do find others,
the ice must be low enough for the calf to climb out.
Too high.
This looks just right.
But with suitable ice in short supply,
space is very limited.
- It's not the welcome they needed.
Even when there's enough ice,
without anyone to help, she is unable to feed
and must stay with her calf.
Their search must go on.
The calf will depend on her for the next three years.
Wherever she goes,
they will go together.
In the next decade, the summer sea ice could disappear altogether.
If that happens,
even for the Arctic's most seasoned travelers,
there may be nowhere left to go.
In late June,
the North feels the full impact of the Earth's tilt
and receives more daylight than at any other time.
And, in one corner of the Pacific,
this creates the greatest opportunity of the year.
For six months,
Alaska's Bering Sea appears lifeless and empty.
Then, one day in June...
all that changes.
Humpback whales arrive
after a month-long voyage...
for the biggest event in their calendar.
Thousands of them.
Across the Pacific,
other travelers are also on their way.
Sooty shearwaters race to get there in time...
flying 1,000 kilometers a day nonstop.
The occasion is so important,
they come from the other side of the world.
At 65,000 kilometers,
it's the longest animal migration on Earth.
A million of them join the whales.
All have come to feed.
It's one of the greatest gatherings of life on Earth.
Here, five months ago,
there were just seven hours of light a day.
Now, there are 17.
The change triggers an explosion in the numbers of plankton and krill.
For the whales, it may be the only time they feed in the entire year.
Over the summer,
each scoops up 900 tons of food.
They all have just 90 days to eat as much as they can.
Soon, the days will shorten
and bring an end to the great feast.
Then, every one of them will return south.
The opportunities brought by our tilted planet's journey
can be enormous yet fleeting.
After a month of long summer days,
temperatures in the North Pacific are reaching their peak.
An ocean predator detects the change
and sets a new course.
A tiger shark
on a thousand-kilometer odyssey across the Pacific.
Guided by the Earth's magnetic field,
she swims in a straight line towards her target...
Laysan Island.
And she's not the only one.
This species makes some of the longest journeys of all sharks.
And hunts a bigger range of animals than any other.
Including even
birds.
The Laysan albatross chick hasn't eaten
since he was abandoned by his parents two weeks ago.
To find food,
he must now leave the island.
But the world's largest seabirds need a lot of lift.
That could come at any moment.
In July, the high ocean temperatures generate tropical storms at sea...
bringing wind to Laysan
and the young albatross a ticket out of here.
The breeze continues to build
until there's enough to lift even an albatross.
And the black-footed chicks are always first to go.
A chick's first flight is never a long one.
And here, it's often their last.
The tiger sharks have arrived.
And they are very effective.
For the young albatross, it's not exactly encouraging.
- But it's now... -
...or never.
If he reaches the open ocean,
he may not touch land again for five years.
But there is no room for error.
Time to throw caution to the wind.
Time to fly.