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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 skills enabled them to surpass Earth's original flyers, 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.

And 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 particularly 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 specialized 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 behavior 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.

And the vultures know exactly how to exploit those thermals

to be carried high in the sky with a 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 2 meters.

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

travel a shorter distance than its outer wing.

And 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 a thermal and soar upwards.

Using this technique, a vulture can climb to a height of a kilometer

above the ground, with scarcely of flap of its wings.

And 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 wingbone 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 wingbone of a vulture, and as you can see, it has been broken, and it has healed.

It's owner may well have lived to soar again.

This soaring tehnique 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 wingtips, enabled larger birds to stay airborne.

But other birds faced very different challenges and so evolved different specialities.

To watch a bird that has evolved into one of the worlds 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.

And 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 it's to get a meal.

A Peregrine 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 flyers,

and their smaller size makes them even more maneuverable.

So, to catch a starling, a Peregrine must be even faster,

and 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 toward 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 200 miles an hour.

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 the Peregrines.

One that relies on their ability to fly together in tight formations, as a flock.

And here they come, vast numbers of them, tens of thousands, hundreds of thousands.

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

And now, some start to fly closer together, and perform far more complex maneuvers.

Look how these great flocks, come together, form a cloud, veer away, and 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 defense,

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 analyzing the flight paths of these Roman flocks,

have now revealed how they manage to do this.

Just how they achieved 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 that you look at them you wonder,

how it is possible that so many birds could 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 flocks complex manouvers in three dimensions.

Advanced computer software then locks onto the flightpaths 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 allow 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 Peregrin's diving attack, proves too strong.

Out of the millions of Starlings in the skies,

only a few will fall prey to the Peregrins tonight.

As the light finally fades, the flock suddenly descends into the trees,

that will be their roost for the night.

The Peregrine's 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.

And 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 behavior, they can hover.

And 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 surface.

Twisting this wing at the shoulder and at the wrist,

produces the hummingbird's distinctive wing-beat.

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 a 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 wing-beat,

not the flapping itself, but the twists 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 flyers, 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:

The Barn Owl.

And one of its favorite meals, is a field mouse.

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 disks 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 wing beats 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 field mouse 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're able to navigate in total darkness.

Among insects there are some moths, whose elaborate antennae

are able to pick-up the scent 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 cabins.

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 are I'm interested in, are further behind me in the semi-darkness,

and they're sleep 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 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.

And if you see 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 crowds, hanging form 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 echolocation, 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 bats' echolocation 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 nightskies, the bats don't have it all their own way.

A team of scientists in Borneo is studying the way bats interact with their prey.

First, they catch a bat in a trap

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

There he is. - Nice.

Beautiful - Yeah he's gorgeous.

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

and so what we 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.

Everything set up here? - Yes, 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 many moths have bat detecting ears,

they can hear the bats coming, they hear their echolocation 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 echolocation cries with their own sounds.

All ... is now in 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 pulse 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 bats 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 defenses.

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 echolocation 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 that echolocation. There they go!

But this is just a trickle, the main exodus

is taking place up a chimney that's 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 crowd,

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'd shared 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 echolocation to find food.

The way that 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.

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