Original subtitles

Over 1,000 kilometres northwest of Australia's mainland,

a speck of an island rises out of the Indian Ocean.

A tiny refuge to plants and creatures

found nowhere else in the world.

Birds rule its skies,

magnificent, bullying, and strange,

while hungry giants patrol its seas.

And its land is the stage for a miracle migration of millions.

A spectacular red army marches to the sea...

under attack by monsters both big and small...

bound for amorous assignations

and a midnight moon dance to ensure their future.

This is the fantastic story of Christmas Island.

[♪]

Deep in the island's central rainforest

a season of waiting has begun for its most famous species.

[♪]

The flashy red crab.

Its ancestors were sea crabs.

As they slowly moved inland over 10 million years ago,

taking advantage of new habitats with few predators,

their gills gradually shrank and they evolved modified lungs.

Though now a dedicated land creature,

the plate-sized crab must still keep its lungs moist

in order to breathe.

So it's trapped here, in the deep humidity of the forest,

by its ocean heritage.

Dry days and sunshine are its great enemies.

It sticks to streams and its moist burrows in the rainforest.

In this muggy haven, the red crab has thrived.

There are tens of millions of them.

The rainforest floor is pockmarked with their burrows.

They are like the forest's gardeners,

constantly excavating,

tilling and aerating the soil.

A keystone species,

they eat the damp litter from the forest floor,

excreting it as nutrient-rich fertilizer,

enriching and propagating their comfy home.

But the red crab has a dilemma, and it's a big one.

They want to reproduce,

but their young can only develop in the ocean

five kilometres away.

An immense journey for a critter used to moving

only a few metres a day.

And the rainforest isn't uniformly wet.

The crabs must run a deadly gauntlet

of dry, rocky stretches.

Their only chance: the monsoon.

The season of drenching rains

will provide a temporary wet highway to the ocean,

and a future generation.

But the monsoon is weeks away.

For now, they wait.

Christmas Island itself,

all 135 square kilometres of it,

is just the very tip of an enormous volcano

that erupted out of the sea floor some 60 million years ago,

with a limestone cap formed by aeons of coral growth.

The island juts dramatically out of the sea,

steep cliffs rising to a plateau of rich rainforest.

Because it formed in the middle of an ocean,

every living thing here

is an immigrant that flew, blew, floated, or rafted in

over the millennia.

Isolated hundreds of kilometres from the nearest land mass...

[Birds chirping]

...it's become a vital breeding site for seabirds.

Some 60,000 of them come to nest here each year.

[Birds chirping]

Boobies, frigate birds, and magnificent tropic birdscan all be seen feeding together just offshore.

Diving, wheeling, and plunging into the rich currents.

Three different species of booby nest on Christmas Island,

but only the brown boobies nest here all year-round.

Graceful gliders and powerful fliers,

their webbed feet are good for paddling

and even incubating their eggs,

but not ideal for running take-offs or landings.

That's why they prefer to nest on cliffs,

where they can launch directly into strong updrafts.

Both parents are dedicated to taking care of their young.

While most couples lay two eggs,

this firstborn chick

will have pushed its younger, weaker sibling out of the nest

to its death long ago.

Now it has all the food to itself.

Its mother has just returned

with a cropful of fish and squid,

which she regurgitates directly into the greedy beak

of her youngster.

At almost three months old,

he's nearly the same size as his mother.

In another month, he'll be ready to fly on his own.

But it will be a further five months

before the chick stops pestering its parents for food

and leaves the nest for good.

Such devoted parenting is made easier

thanks to the bountiful waters of Christmas Island.

Since their parents don't have to travel far to feed,

the hungry chicks still scan the skies

for the arrival of their next meal.

[Thunder rumbling]

With the first rains,

the restlessness in the interior rainforest intensifies.

At first, only a few crabs stir.

But soon, there are millions,

making a determined march toward the edge of the forest.

The males leave first, creating great traffic jams

with their synchronized departure.

Red crabs join the march to the sea

when they reach four or five years of age.

They will continue marching annually

during their 20- to 30-year lifespan.

But the journey is long and full of enemies.

An armoured giant lurks along their path.

The largest land crab species on the planet: the robber crab.

Like something straight out of a sci-fi movie

or your worst nightmare.

Just its midsection can be over one metre wide.

But they may take an amazing 50 or 60 years

to reach their largest.

They're called robber crabs here

for their penchant for stealing shiny objects

and taking them to their lairs.

That pair of constantly moving antennae

are some of the most sensitive sniffers in the world,

able to smell hundreds of times more keenly than a dog.

The smell of carrion or rotting coconuts

especially grabs their attention.

And with those massive claws,

they can shred the tough coconuts with ease.

They'll also drag the tough husk fibres home

to use as bedding in their burrows.

In other places, they're known as coconut crabs.

Christmas Island has the largest and densest population

of these monsters in the world.

There are around 10,000,

though their numbers can't touch those of the red crab.

And while coconuts may be on the menu, given the chance,

these omnivores won't turn down a meal of red.

Usually, the smaller reds

are too agile for the robber crabs to catch,

but in the crowded melee of the migration,

it's much easier picking them off.

Even if the little reds can evade the colossal robber crabs,

they're just the first of many nightmares they'll encounter

on their long march to the sea.

Christmas Island is truly ruled by crabs.

There are 14 different species of land crab here,

more than anywhere else in the world.

They arrived here as sea crabs, like the scaly rock crab...

but evolved over the millennia

to take advantage of just about every nook and cranny

on the island.

The blue crab moved up the cliffs

and in toward the interior

by becoming a freshwater specialist.

It haunts streams and seepages

like a beautiful blue-white ghost.

Both sexes have hairy legs and moustaches,

dense mats of hair on either side of their mouth parts

that help them filter their food.

Once thought to be a subspecies of a crab

found throughout the Indo-Pacific,

it has now been discovered to be its own species,

found only on Christmas Island.

Once nearly wiped out by islanders

who harvested it for its delicate meat,

the blue crab is now protected.

Not everything the red crabs pass on their migration

is a threat.

In the treetops above

is one of the island's other permanent residents:

the endemic Christmas Island goshawk.

This clever predator has an extensive menu,

feeding on other birds,

small mammals, reptiles, and insects,

and is a specialist forest hunter.

There may be only 50 breeding pairs on the island,

and this is the only place they're found.

Its smaller, lighter cousin, the nankeen kestrel,

also hunts here.

While the goshawk is agile enough to hunt in the forest,

the kestrel prefers to use its extraordinary hovering ability

in the more open areas.

It can remain almost motionless in the sky,

looking for the slightest signs of movement

of its favourite prey:

giant grasshoppers.

Once the insect is in sight,

the kestrel makes a silent but pinpoint-accurate descent.

Another great master of the skies

is the Christmas Island flying fox.

Named for its fox-like face, it's actually a bat,

one of the few species that is active during the day.

This 350-gram mammal feeds on native fruit and flowers,

and plays an important role in pollination on the island.

Papaya, recently introduced here,

is also a particular favourite.

Unlike insectivorous bats,

these fruit-eaters have no sonar.

Feeding during the day,

they rely on sharp eyesight and a keen sense of smell instead.

Their roosts are close to the coast,

for easy take-off and access to updrafts

that carry them across the island

in search of fruits and flowers.

While Christmas Island itself

supports many species found nowhere else on the planet,

its strategic position between great ocean currents

means it's also a Grand Central Station for ocean travellers.

The south-flowing powerful South Equatorial Current

acts like a conveyor belt for plankton and fish eggs.

One of the world's most endangered fish,

the southern bluefin tuna, spawns here,

at the western edge of its breeding ground

in the path of the current,

which then carries their tiny young south

into temperate waters.

But it's the east-flowing South Java Current

that has the greatest impact on the island.

It brings the rains that trigger the crab migration.

It may also play a role in another great migration

that is under way,

one that could not be more different than that of the crab,

but their destinies are on a collision course.

The whale shark,

the largest fish in the world reaching 20 metres in length

and weighing 20 tonnes,

makes its way from the reefs off western Australia

to the waters of the Indian Ocean

and Christmas Island,

a 1,600-kilometre journey.

They arrive at Christmas Island at a very specific time,

one that experts think

is directly tied to the annual ritual

taking place right now on the tiny island.

The monsoon season has triggered

one of the greatest migrations on earth.

The red crabs of Christmas Island

are on their perilous march to the sea.

If there is a break in the rain, they risk dehydration.

But there's no turning back once committed to the march.

They keep to the shade,

moving in the morning and evening if they can.

But now they're approaching the most treacherous part

of their journey.

A killing field unlike anything their kind has faced

over the aeons their ancestors have made this arduous journey.

An invader against which they are nearly defenseless.

The most insidious threat on the island:

yellow crazy ants,

named for their frenetic movements.

Having hitched a ride to the island

on cargo ships decades ago,

these aliens have proceeded to wreak ecological havoc.

They've created super-colonies of billions,

and they show no mercy to the unfortunate creatures

they encounter.

When a red crab enters their territory,

they swarm it,

spraying it with corrosive formic acid from their abdomens.

Any single ant can spray only a tiny amount,

but they work in concert,

finding the chinks in the crab's armour,

eyes, mouth, joints,

blinding it and rendering it helpless.

The crab responds by foaming moisture

out of its mouth parts,

which does little

but waste their precious internal supply of water.

It's a slow, torturous death.

The merciless ants won't even wait for them to die

to begin eating them.

The crazy ants' toll on the red crabs

is almost unimaginable.

It's estimated that they've cut the population

from 50 million to half that in just a few decades.

Ant control programs

have only managed to reduce the ant numbers and slow the decline of the red crabs.

The yellow crazy ant killing fields

don't stop them all.

Millions still make it through,

driven on by instinct to an appointment with destiny.

Not all creatures here have such trouble

with the yellow crazy ants.

The golden orb-weaver weaves an impressive web

that glistens like gold in the sunlight.

The orb-weaver's silk is strong, flexible, and sticky.

As the spider produces its silk,

it covers it in a layer of spider glue,

ensuring its prey sticks fast.

But it's what's actually within the silk itself

that protects the spider from the angry ants.

It comes complete

with a compound called pyrrolidine alkaloid,

a built-in ant repellent.

While she is a formidable defender of her web

and will attack any invaders,

the ant deterrent is enough to keep the crazy ants

at a safe distance.

It turns out that the big spider has tiny groupies,

literal hangers-on who hover at the edges of her web,

alert for any insect

that's too tiny for the queen to be bothered with.

These little thieves, called dewdrop spiders,

are commonly found on the outskirts

of most golden orb-weavers' webs.

Fortunately for them, she has a larger meal in mind.

And quickly sets upon a grasshopper

ensnared in the filaments of the web.

She injects it with venom

then carefully wraps her dinner for safekeeping

to eat when she's in the mood.

[♪]

Within a week, the surviving front lines of the red army

have nearly reached their destination:

the beaches where they were born.

But this final leg of the journey won't be an easy one.

Craggy, treacherous cliffs ring the island.

The only way to the water is down.

They cling for dear life, sometimes upside-down.

A fall could crack a shell, a death sentence.

Finally, they reach the waters of their birth.

Here, they can take a refreshing dip,

cleaning up, hydrating, and filling up on salts and minerals

they've used up on the long journey.

They need it after their marathon,

and to face another odyssey ahead.

The females are already arriving

for one of the most remarkable

breeding spectacles in the world,

and they're expecting the males to be ready for them.

The red crab's distant cousins, hermit crabs,

are year-round beach dwellers,

and pay little attention to the march unfolding around them.

Unlike most crabs,

hermits have soft abdomens instead of hard shells.

To protect their vulnerable underparts from predators,

they take up residence in abandoned seashells,

usually snail shells.

As they grow, they need to find larger and larger shells,

and spend endless hours on the beach

inspecting potential new homes.

Sometimes trying them on for size.

The best shells are at a premium,

and it's common to see hermit crabs

tussling over a particularly appealing shell.

Almost unbelievably,

these little hermits are closely related

to the giant robber crabs,

which also begin their lives in hermit fashion,

carrying their shell homes around with them.

It's only when the robber crab is about a year old

that it sheds its hermit ways,

abandons life in its borrowed shell,

and hardens its body with layers of calcium.

Only then to they begin to grow to their enormous size.

Returning to their cliffside homes with full bellies

to feed their chick,

the brown boobies seem completely oblivious

to the massive scarlet migration going on around them

as crabs shuttle up and down the beach.

While crabs and boobies

don't always make getting around look easy,

one bird, unique to Christmas Island,

is known for its elegance:

the golden bosun.

The island's beautiful, yellow-tinged variation

on the more widely distributed white-tailed tropic bird.

With its long yellow tail streamers,

it is the angel of the island's skies.

They prefer to nest inland in the forest trees,

away from the hustle and bustle of the crowded cliffs.

After their reviving dip in the sea,

the males have clambered back up the cliffs of Christmas Island

to the terraces above

to prepare their welcome for the ladies.

Using their claws as earth-movers,

they methodically dig burrows.

They will serve as enticement to the females.

They're shallow and only about 30 centimetres long,

but a lot depends on them.

With limited real estate to work with,

staking out claims can become quite competitive.

Some crabs would rather steal than build.

There's a lot more bluster than violence involved,

with smaller males usually backing down

before any damage can be done.

Still, this is serious business.

Without a suitably impressive burrow,

a male is unlikely to seduce a female,

and his long trek

from forest to sea

will have been in vain.

In the end, the terraces become crowded beach condo complexes,

with a burrow or two in each square metre.

With the real estate tussles settled,

some of the males retreat to the moisture of their burrows.

They're only a cliff away from the refreshing ocean,

but hormones bid them to stay,

guarding their hard-won homes

and waiting to welcome the females.

[♪]

Up above the fray,

another booby species makes its home in the treetops.

Clinging with their big pink webbed feet to the branches,

the red-footed boobies have just returned

from a long journey out to sea to feed.

Preening is the order of the day.

These seabirds travel great distances

and dive hard into the ocean for food,

both of which require their feathers

to be kept in perfect shape.

An unwelcome visitor sits too close for comfort.

A Christmas Island frigatebird,

the bane of the boobies' existence.

As the boobies head out to feed,

the frigatebirds often provide an unwanted escort.

Described by some as muggers and thieves,

they're sometimes called pirate birds.

The boobies are plunge-divers,

folding back their wings and daggering into the water

at speeds of up to 50 kilometres per hour

to catch fish up to 50 metres deep in the water.

All that plunging is hard work.

Gullets full, they head home for the terraces,

all too often with a frigatebird on their tails.

It's now that these pirates live up to their name.

Why do all that work of diving and fishing

when the booby is doing it for you?

It harasses the booby relentlessly,

hoping to force it to regurgitate what it's caught.

But this booby is not so easily intimidated,

and returns to the terraces with its crop still full.

It's been 10 days

since the monsoon rains

touched off the red crab migration,

and the rear-guard of the red army is picking its way

over the steep cliffs to the beach.

Now it's the females' turn to dip in the ocean

to refresh and replenish.

[♪]

The sea is not always gentle with them.

Because millennia of evolution in the forested interior

have turned them into land animals

with lungs and only remnants of gills,

these crabs can drown if dragged under.

They can also fall prey to large fish,

like this giant trevally patrolling the shallows.

[♪]

The survivors turn their backs on the sea.

They have an appointment up in the terraces,

a strenuous climb away.

[♪]

In the village of burrows above the sea,

it's a buyer's market in the real estate game.

The females busily inspect the males' architectural offerings.

There's urgency now.

They're responding to the cycles of the moon,

urging them to mate and brood their eggs now

while there's still time.

A female who finds a male

with a well-appointed burrow engages with him.

They mate face-to-face in clenches

that can last 25 minutes or more.

[♪]

Then she moves into her new home.

She'll produce up to 100,000 eggs within the next three days,

brooding them in a pouch slung under her abdomen.

She'll incubate them for 12 or 13 days.

Having finished his annual mating duties,

the male immediately turns for home,

starting on the long and dangerous trek

back into the interior.

Once again, the red army will face the implacable crazy ants,

and the hungry robber crabs,

as they run the gauntlet in reverse.

If they don't make it back,

at least they'll have made a heroic effort

to pass their genes onto the army's next generation.

Many of Christmas Island's creatures take advantage

of opportunities at sea and on land.

[♪]

A female great frigate bird flies inland.

Unlike most seabirds, which drink salt water,

frigate birds will take advantage of freshwater

when it's available.

And when they do,

they use their graceful diving skills to drink on the wing.

[♪]

Back at the coast, the waning moon

announces that the time has come.

The female red crabs,

having brooded their eggs for about two weeks,

begin to emerge from their burrows.

Each carries in her brood pouch some 100,000 eggs.

Slowly, carefully, they make their way

from the forest terraces above the beach to the cliffs,

carrying their precious and cumbersome burdens.

They're not so agile now, and some will pay the price.

Robber crabs can smell them coming.

Their caviar is an added bonus,

and the robbers prey on them mercilessly.

[♪]

The lucky survivors pick their way back down the cliffs.

When they reach the shore,

they dip again to rehydrate and refresh.

And then they wait.

On beaches all around the island,

they redden the coast

with their urgent need to reproduce,

and timing is everything.

Tonight will be the neap tide,

when the waning moon's tug is the gentlest

and the difference between high and low tide is smallest.

[♪]

As each female enters the water,

she raises her claws and does a big of a jig,

releasing her eggs.

A moon dance on which the future of her kind depends.

Soon the shallows are darkened, saturated with billions of eggs.

[♪]

The eggs hatch immediately into larvae

on contact with the seawater,

before being swept out to sea.

In just one month, they'll morph through several life stages

until they're ready to return to land as baby crabs.

That is, if they survive.

Dawn reveals a beach full of females, their pouches empty,

setting off on the long march home.

There's no way to predict

how many of their offspring will return.

At the mercy of the weather, currents, and predators,

their fate is yet to be determined.

Whale sharks arrive just in time for the spawning,

when billions of tiny crab larvae fill the waters.

Other ocean giants also seem in on the secret.

Manta rays fly through the plankton, filling up as they go.

With each mammoth mouthful,

it seems more likely the red crabs' incredible migration

will have been in vain.

This is why it's a numbers game.

It has been a month

since the dancing red crab mothers

released billions of eggs into the ocean

and returned to the rainforest at the centre of the island.

By the time they return here to the shores of Christmas Island,

they have morphed into another life stage known as megalops.

In some years, just a handful of them return,

and other years it's a bonanza.

This year is one of the miracle years

for this most amazing of migrations.

Millions of tiny crablings scrabble up the beach,

turning the shoreline pink with the promise of life.

After three to five days at the water's edge,

they have molted and transformed

into their air-breathing crab stage.

Instinct drives them to follow their journey up the cliffs,

through the terraces, the gauntlet of predators,

and into the interior rainforest.

There's no way to predict

how many will fall victim along the way,

but survivors will spend four or five years

in their forest home

before they'll hear the call of their ocean origins.

Then they too

will join the yearly march to the sea

to burrow

and fight

and mate

and dance,

carrying on the ancient tradition etched in their genes.

A spectacle of reproduction unlike anything else on earth.

Here, on the remarkably restless sanctuary

of Christmas Island.