Original subtitles

I'm several hundred feet up in the air.

Up here, I might encounter perhaps a flying insect,

though I haven't seen one yet, or maybe even a baby spider

clinging to a gossamer thread of silk,

which is their way of getting around.

But by and large, this is the kingdom of the birds.

The first birds flew about 150 million years ago.

They spread around the globe

and evolved into a multitude of different kinds.

Aerial acrobats.

Stealthy hunters.

And some of the fastest creatures on the planet.

Their extraordinary skill enabled them

to surpass Earth's original fliers - the insects.

But there is a vast kingdom that the birds do not control -

the night skies.

These are ruled by very different creatures -

flying mammals, bats.

And in one spectacular place

these two populations of the night and the day collide.

This is Segovia in central Spain.

Some of the inhabitants of this gorge allow us to see very clearly

how birds as a group have become so versatile in the air.

Through the ability to change the shape and the size

of their basic flying mechanism, the wing.

There's a wonderful example of that just over here.

You may think that birds are much the same when it comes to flight,

but in fact different species need to fly in their own particular way.

This vulture is an airborne scavenger.

It feeds on the bodies of dead animals,

so it needs to spot any fresh carcass very quickly

and get to it before others claim it.

Like most birds, it has superb eyesight.

So it climbs high in the sky,

constantly scanning the ground below,

for hours at a time if need be.

To fly in this highly specialised way,

it has evolved a very distinctive kind of wing.

To get up close to some of the many vultures that live in this area,

I'm visiting a place where they're regularly fed by conservationists.

These are griffon vultures,

one of the largest of all vulture species.

Each one can weigh up to 11 kilos.

Lifting an 11 kilo body high into the sky

takes a lot of energy,

but the vultures don't supply that energy directly themselves.

A clue of how they do so comes from observing their behaviour

at the start of the day.

Those vultures roost and nest on ledges up there.

They're not early risers.

That's because they rely on the sun to get airborne.

As the day warms up, patches of bare rock

reflect the heat of the sun

forming columns of rising hot air known as thermals.

The vultures know exactly how to exploit those thermals,

to be carried high in the sky with the minimum of effort.

They have wings that have been shaped over millions of years

to catch as much of that rising air as possible.

They're huge - very broad with a span of over two metres.

But riding thermals may not be as easy as it looks.

A thermal is quite a narrow column of rising air

and to stay within it, a vulture has to make quite sharp turns,

and that can lead to disaster.

In a tight spiral, a vulture's inside wing

travels a shorter distance than its outer wing.

If we were to measure the speed of this inner wing,

we would find that it moves much more slowly through the air.

This means it generates less lift.

So little, in fact, that the vulture could easily stall

and drop from the sky.

It avoids that by having special control

over the feathers at the ends of its wings.

They can be splayed so that they separate.

As a result, each feather acts as a small extra wing,

and together they increase overall lift.

This enables the vulture to turn in a tight circle

and so hold its place in the thermal and soar upwards.

Using this technique a vulture can climb to a height of a kilometre

above the ground with scarcely a flap of its wings.

Then, if it spots food down below,

it can switch its flying technique and descend at speed.

Once on the ground, it has to compete with other vultures

for a share of the feast.

And now, those broad wings are useful,

to help muscle out its rivals.

And that can put those all important wings at risk.

Bird bones, being hollow and lightweight,

are also usually very fragile.

And if a bird breaks its wings, that's usually a death sentence,

because most small birds have to feed every day or so.

But this is the wing bone of a vulture.

And vultures are so big and can fill their stomach with so much food,

that they can go without a meal for two or even three weeks.

And as a consequence, when these aggressive, quarrelsome vultures

have a row and perhaps injure one another,

a broken wing can heal itself.

And this is the wing bone of a vulture.

As you can see, it has been broken, and it has healed.

Its owner may well have lived to soar again.

This soaring technique can exploit not just the thermals,

but also winds deflected upwards by ridges and hills.

The same wing shape is used by other large birds to help them soar.

Eagles.

Pelicans.

And a bird that makes an immense journey here to Spain every summer.

Two young storks.

Their parents come here every year to this small town in northern Spain

in order to mate and nest and rear their young,

all the way from Africa.

Some from as far south as the Cape.

And they make that immense journey

by finding a thermal, a column of rising air,

circling in it, allowing it to carry them high into the sky,

and then gliding off on the next stage of the journey

to find another thermal to take them back up again.

It's an extraordinarily energy-efficient way of travelling.

So broad wings and splayed wing tips

enabled larger birds to stay airborne.

But other birds faced very different challenges,

and so evolved different specialities.

BELLS TOLL

To watch a bird that has evolved

into one of the world's most skilful hunters,

I've come to Italy and the city of Rome.

There's a bird that flies over these roofs

that finds its prey not on the ground but in the air.

It owes its success to its speed.

In fact, it's said to be the fastest moving animal on Earth -

the peregrine.

Peregrines hunt other birds.

Many different kinds of birds now live in cities,

attracted by the food and shelter that is so easily found here.

And a tall building like this is an ideal lookout for a hunter.

Flying prey can move in any direction it chooses,

so a hunter has to be both fast and agile

if its to get a meal.

A peregrine's wings have a very special shape.

They're pointed and swept back.

If wings have a blunt end,

air will swirl over that end, forming trails of turbulence.

These act like brakes, slowing a bird down.

But pointed wings shrink that edge and so reduce the turbulence.

Pulling the wings back towards the body

makes the bird even more streamlined.

And speed is crucial to a peregrine's success.

It also has acute vision

that enables it to spot prey over a mile away.

And for the peregrines that hunt in Rome...

...these birds are prime targets...

starlings.

They too are fast fliers.

And their smaller size makes them even more manoeuvrable.

So to catch a starling, a peregrine must be even faster.

In order to gain speed and surprise,

it attacks from above.

First, it climbs.

When it sees a group of its potential prey,

it turns...

dives...

and accelerates by beating its wings.

The starlings are still unaware of the danger hurtling towards them.

Finally, the peregrine draws its wings back.

This is called the stoop,

a superb streamlined shape

that slices through the air.

Now it can reach speeds of over 200mph.

As it nears its target,

it opens its wings to slow its descent

and makes its final lunge.

Starlings, in fact, are an abundant source of food

for the peregrines.

They come into the city in the winter,

attracted no doubt by the warmth in order to roost.

Every evening at dusk, the starlings start to arrive,

and they have a remarkable way of defending themselves

against peregrines.

One that relies on their ability to fly together

in tight formations as a flock.

Here they come - vast numbers of them.

Tens of thousands, hundreds of thousands.

It's like a great black hail storm, a blizzard of birds.

And now some start to fly closer together

and perform complex manoeuvres.

Look how these great flocks come together,

form a cloud, veer away, then split.

It's a quite extraordinary piece of aerial navigation.

We're still unsure exactly why they perform these elaborate dances,

but they're often triggered by the arrival of a predator.

And today is no exception.

Because over there on one of those buildings,

I've seen a peregrine.

Coming in in great numbers like this is in itself a defence,

because if you're surrounded by tens of thousands of others,

well, it's a good chance that the peregrine won't get you.

But the aerial ballet is part of a more complex defensive strategy.

When a peregrine does attack with its wings drawn back in its stoop,

the starlings, flying in their tight formation,

coordinate their escape.

Instead of scattering in different directions,

when a straggler might be picked off,

they stick together, even when they make the sharpest of turns.

Recent studies analysing the flight paths of these Roman flocks

have now revealed how they manage to do this.

Just how they achieve this was not understood until very recently.

But now, a team of physicists from Sapienza University in Rome

is beginning to find the answers.

We see these huge flocks of birds dancing in front of us.

And every time you look at them, you wonder how it is possible

that so many birds would be as only one entity.

They position a series of cameras along a rooftop

overlooking a favourite starling roost site,

just outside Rome's main railway station.

The cameras record the flock's complex manoeuvres

in three dimensions.

Bellissimo.

Advanced computer software

then locks on to the flight paths of thousands of individuals

with extraordinary precision.

Their painstaking work has produced a remarkable insight

into how the starlings coordinate their behaviour.

The main result that we found

is that each bird interacts with the seven birds around it,

regardless of the distance between these birds.

It is the key to have a stable flock.

An individual starling

is effectively linked to the seven around it

by an invisible web, even if they drift far apart.

This is the hidden glue binding the flock together.

But it may also act as a communication channel.

A bird that turns to evade a predator triggers a ripple effect,

that passes rapidly through the overlapping networks,

causing the whole flock to turn as one.

The special thing is that the information

can pass through the flock in milliseconds,

and this allows them to escape very quickly from predators.

Most of the time, the starlings' flock defence keeps them alive.

But now and again, the sheer speed and surprise

of the peregrines' diving attack proves too strong.

Out of the millions of starlings in the sky,

only a few will fall prey to the peregrines tonight.

As the light finally fades,

the flock suddenly descends into the trees

that will be their roost for the night.

The peregrines sharp eyesight doesn't operate

nearly so well in the dark.

So now the starlings are safe...

until tomorrow, that is.

6,000 miles away in South America,

there are other birds with a very different skill.

They also find their food on the wing.

In the cloud forests of Ecuador there is a plentiful supply

of a type of food produced by plants

to attract flying animals...

nectar.

Around 130 million years ago,

plants recruited insects to transport pollen,

from one flower to another,

by bribing them with a sugar-rich drink.

Birds, when they first evolved, were unable to collect it

because there was seldom something solid nearby

on which they could perch.

Then, around 30 million years ago,

a kind of bird appeared that had no need of such a perch.

Hummingbirds.

They could hover.

They do so by beating their wings extremely swiftly.

So fast, in fact, that they make a humming noise.

The largest hummingbird beats its wings around 14 times a second,

but some tiny species are able to do so 80 times a second.

To fly in this extraordinary way,

hummingbirds have changed the structure of their wings

and the way they beat them.

Here in Ecuador,

scientist Doug Altshuler is working to analyse

exactly how they do so.

Hummingbirds are remarkable animals.

They have extreme adaptations in physiology and anatomy

and they also have a very unique behaviour - they can hover.

The approach that we've taken is to study

how those physiological and anatomical adaptations

determine their hovering ability.

Using high-speed cameras,

he records the mechanics of their flight in minute detail.

He can slow down the action by around 40 times

and so observe exactly what's taking place.

Most birds flap their wings up and down.

But hummingbirds flap theirs more like insects.

They twist their wings around between strokes

and so can generate lift

when flapping both forwards and backwards.

Doing this at high speed puts a huge strain on their wings.

So to withstand it, the wings have a special structure.

The hummingbird wing is very stiff

and undergoes few changes in shape as it rapidly beats back and forth.

They owe this stiffness to a modification of the bones.

The arm bones have shrunk,

but the bones of the hand have elongated

and support most of the wing's surface.

Twisting this wing at the shoulder and at the wrist

produces the hummingbird's distinctive wingbeat.

Doug is also investigating

one of the great mysteries of hummingbird flight -

their ability to move sideways in mid-hover.

Hummingbirds are able to track flowers

that are moving back and forth in the wind

and this was something I always wanted to know more about.

To replicate the swaying motion of the flower,

Doug places a reservoir of nectar on a mechanical slider.

Before long, he has a volunteer.

Amazingly, it manages to track sideways

to keep up with the slider and still feed.

The bird is exploiting an unexpected feature of its wingbeat -

not the flapping itself,

but the twist at the end of each stroke.

During hovering flight, as the wings come forward,

they rotate symmetrically, so the forces remain in balance.

But if they instead rotate differently,

such that one wing rotates before the other,

then the forces are no longer in balance

and this asymmetry can be sufficient

to push them to one side or the other.

So a combination of modified wing bones

and precise control of wing motion

gives hummingbirds the aerial agility they need

to collect nectar.

And they need plenty of it -

hovering burns a huge amount of fuel.

All hummingbirds have to constantly top up their tanks

with high-energy nectar.

And when supplies are low, competition can be fierce.

Now, their flying skills are put to a very different use -

to fight off rivals.

So different birds adapted their wings

to fly in highly specialised ways.

Some began to hunt the earth's first fliers - the insects.

And in that battle, there is now no real contest.

But because most birds rely for so much of their success

on their exceptional eyesight,

there is one major habitat that is largely closed to them.

Not a place, but a time.

The night.

In the British countryside, however,

there is a bird that can fly in the dark.

A barn owl.

And one of its favourite meals is a fieldmouse.

But first, it has to find it in the dark.

A mouse is extremely alert to the approach of a predator.

But the barn owl has wings specially adapted for stealth...

...and senses that can penetrate darkness.

Its eyes are very sensitive in low light,

but even if the mouse is out of sight,

it's still not safe.

The owl's hearing is also very acute.

Those two discs on its face channel sound into its two ears,

which are on a slightly different level on the head.

And that difference enables the bird to pinpoint the source of the sound,

whether it's in the air, or down on the ground.

But in order to hear that sound,

its wingbeats have to be very, very quiet.

And the way it achieves that we can see when it goes hunting.

The key reason for its silent flight

lies in the nature of its wing feathers.

Along the back edge, their fringe is frayed and tatty.

Most birds' wings have a hard edge

and this can cause quite a loud noise.

The source is turbulence,

produced when air flowing over the wing rubs against its surface.

When this swirling air meets a hard back edge,

the sudden drop-off hugely amplifies the noise.

But the barn owl's tatty feathers avoid that -

by creating a softer edge, they cushion the turbulent air

and so reduce noise.

So silent flight allows the owl to hear its prey

and conceals its approach.

But to position itself for the kill, it needs to fly extremely slowly.

And to achieve that, it has particularly broad wings.

This slow, silent approach

leaves a fieldmouse little chance of escape.

On nights when there is thick cloud or no moon,

even an owl's sensitive eyes struggle.

But there are creatures that have such highly specialised senses

that they are able to navigate in total darkness.

Among insects, there are some moths

whose elaborate antennae are able to pick up

the sense of food, or a mate.

And there are those nocturnal animals,

the last group of flying creatures to appear on Earth - the bats.

To see how they battle with the insects

for dominance of the night skies,

we're heading into the rainforests of Borneo.

Many bats find their food not by sight or smell,

but by using a very different and highly advanced guidance system.

One way to find them is to search for their ideal home -

a place like that deep, black cave beneath me.

If you fly at night,

there's no better place to spend the day

than in a cave like that.

This is Gomantong.

The cave is a vast network of underground tunnels

and cathedral-sized caverns.

It was carved out by streams of water over millions of years.

And now, it's home to a remarkable community

of cave-dwelling specialists.

To find the creatures I'm looking for,

I'm being winched high up towards the ceiling,

where the towering walls make ideal roost sites for flying animals.

These little birds flying past me are swiftlets

that have made their nests on the walls of the cave.

They are active during the day

and they leave the cave to hunt insects.

The bats that I'm interested in are farther behind me,

in the semi-darkness,

and they're asleep now, during the day.

The bats are scarcely the size of mice.

Their wings are constructed with very long fingers

and they hang by their feet from the rock.

Although there are a few at the back there,

deeper in this cave, they exist in huge numbers.

To find their roosts, we're heading still deeper into Gomantong Cave.

High on the rocky cave ceiling above me, hidden in the darkness,

there are vast numbers of bats.

You can get some idea of how many there must be

because of this huge dune behind me.

That's formed of their droppings.

If you see a little moving glints on the surface,

that comes from an army of cockroaches

which are chewing their way through the bat droppings

to extract the last particles of nutriment.

Some people think there are a million bats up here in this cave.

It's impossible to see them in the gloom.

But special night vision cameras can reveal them,

densely packed, hanging from the ceiling.

Their tiny eyes are adapted to low light,

but they cannot penetrate the blackness.

Millions of years ago, however, these bats evolved

an extraordinary guidance system known as echo location, or sonar.

A bat produces extremely high-pitched sounds in its throat

and then projects them forward.

We have slowed the sounds down, but can still only hear them

by converting them to lower frequencies.

They bounce off the walls as echoes

and are detected by the bat's huge ears.

These are in constant movement and enable the bat

to map its surroundings with remarkable precision.

But these bats not only need to find their way in the dark,

they also need to find their food...

...night-flying insects.

And among them are moths.

Locking on to these moving targets

is a supreme test for the bat's echo location system.

As one homes in, its sonar beam switches into attack mode,

increasing the rate of its pulses.

This enables it to precisely pinpoint the location of its prey.

But in the battle for the night skies,

the bats don't have it all their own way.

A team of scientists in Borneo

is studying they way bats interact with their prey.

First, they capture bats in a trap

that uses thin wires to divert them into a pouch below.

There he is. Nice.

Beautiful. Yeah, he's gorgeous.

I'm shining through. Yeah. Ah, cool.

Bats and moths have co-evolved for almost 60 million years.

And so what we're doing here

with this giant tent and all these cameras

is to try and figure out what's happening in this ancient battle.

Trying to understand how moths survive a bat attack.

Set up here? Yeah, everything's ready to go.

Awesome.

The tent acts as a controlled flight arena

in which every movement and sound can be recorded in minute detail.

Filming these interactions with multiple cameras in 3D,

and ultrasonic microphones,

we can see how these interactions unfold

and to hear how they unfold.

These studies have revealed

that moths don't always fall prey to the bat attacks.

We know that mini moths have bat-detecting ears,

they can hear the bats coming.

They hear their echo-location cries,

and dive out of the sky, stop flying.

Got it? There you go.

But the team's work has identified a moth

with a very different defence strategy.

Playing recordings of bat sounds to this moth

reveals a remarkable ability.

Here in Borneo, we recently discovered that hawk moths

respond to these echo-location cries with their own sounds.

Oh, lots of sound production.

Hawk moths do it with the tip of their abdomen,

with modified genitals.

They rub the genitals against the inside of the abdomen

and reply to this bat attack.

The moth is tethered

to keep it in range of the cameras and microphones.

Then a bat is released.

As the bat approaches the moth,

its sonar pulses switches to attack mode.

But now the hawk moth responds,

sending its own rasping sound back with astonishing effect.

At the last moment, the bat appears to lose track of the moth

and fails to catch it.

We've shown that these moth sounds actually jam the bat's sonar.

They interfere with the returning echoes from the insect

and causes the bat to miss the moth.

The team has discovered that insects are fighting back

in the ongoing battle for the night skies.

But there are, of course,

plenty of other flying insects with no such defences.

And they live in vast numbers in the forests outside Gomantong cave.

So every evening, as dusk arrives,

the bats leave the safety of their secluded home to hunt.

And now the bats are beginning to use their echo location skill

to fly out from their roosts in the depths of the cave,

coming close to the ceiling

and then whizzing out through this little entrance here.

They don't collide with the roof,

they don't collide with one another, or even with me,

all through echo location. There they go.

But this is just a trickle.

The main exodus is taking place up a chimney that deeper in the cave.

To watch, close up,

the way the bats achieve their million strong mass departure,

I'm being hauled up 200 feet into the funnel

which serves as one of the cave's main exits.

At the top, there is a gaping hole.

And now the bats are preparing to leave.

They've assembled in a relatively small chamber close to the exit

and are flying round and round in a great swirling cloud,

waiting for daylight to fade.

And now off they go!

This brief hour of dusk is the moment when the two communities -

the day flyers and the night flyers -

may encounter one another in the air.

Outside, danger awaits.

Hunters belonging to that other great group of animals

with which they share the skies - birds.

Hawks, eagles and kites.

They are why the bats were reluctant to leave.

And why they now do so in one continuous torrent.

There is safety in numbers.

But some will pay the price.

The vast majority, of course, make it out over the forest canopy

and there they can use that skill of echo location to find food.

The way the different animals have colonised the skies is surely

one of the most remarkable stories in the natural world.

First to do so, over 320 million years ago, were the insects.

They had no competition for about 100 million years.

But then, much larger flying animals took to the air -

reptiles, the pterosaurs.

Around 70 million years later still,

one branch of the dinosaurs acquired feathers

and that enabled their owners to get airborne.

The birds had arrived.

And lastly, about 60 million years ago,

the night skies were invaded by mammals - the bats.

And here, in Gomantong cave, the three surviving groups of flyers -

insects, birds and bats -

are still locked together in an ongoing evolutionary struggle.

So the battle for the supremacy of the skies

that started over 300 million years ago

still continues every day around the world.

Subtitles by Sky Access Services www.skyaccessibility.sky