Birds of Prey The Wonder of Animals

Planet Earth.

Millions of species...

..but a few are special...

..thriving,

dominating.

The key to their success lies in their opportunism.

For others, it's down to their ability to collaborate.

And for some, it's all about surviving where others can't.

So, what is their secret?

In this series, we'll delve deep beneath the skin to reveal

the unique features that set some species apart.

New behaviour and the very latest scientific discoveries

will offer fresh insight into the wonder of animals.

One group of animals dominate the skies...

..winged predators.

They've conquered every environment on the planet...

..outdoing even their fellow birds...

..because they don't just fly.

They hunt in ways that

no other animal on Earth can.

What sets raptors apart from other birds is their strength...

..their manoeuvrability...

..and their speed.

In this episode, we're going to explore

the anatomical and physiological adaptations

that allow these birds to rule the aerial roost.

So, let's start with their strength.

Raptors are distinguished from other birds by their choice of prey...

..big, heavy vertebrates...

..that can weigh more than the bird itself.

They need strength both to catch their prey...

..and to carry it.

To capture it, raptors must channel their strength

to one part of their body...

..their talons.

Four claws,

three facing forward, and one back.

They're made of keratin,

similar to that found in human fingernails,

but with a structure that makes it considerably stronger.

BIRD CALL

The largest can be nearly seven centimetres long.

That's as long as a bear's claws.

The talons of each species are tailored for their prey.

The bald eagle's are perfect for catching fish.

Each year on North America's Pacific coast,

millions of salmon return to spawn.

The bald eagle is waiting.

It can catch fish weighing 6kg...

..but keeping hold of it requires gripping power.

Its curved talons function like fish hooks.

Pressure-sensitive pads on the feet tell them when to close.

Spicules, tiny spikes on the bottom of those feet,

help them grasp their slippery prey.

And once the grip is firm,

the talons actually lock shut.

To do THAT, they use tendons in their legs.

These are contained in sheaths.

Both tendons and sheaths have tiny ridges along their surface.

When the muscles contract,

two sets of teeth interlock, like a ratchet...

..clamping the talons shut.

The eagle can now maintain

huge pressure on its claws...

..even when its leg muscles are relaxed.

Ratchet talons are just as crucial

for birds of prey hunting on dry land.

Once this Galapagos hawk gets hold of the marine iguana,

it's unlikely to get away.

Researchers have found

that the talons are not always used as daggers.

Raptors sometimes use their feet to suffocate their prey.

Not all prey, though, can be eaten where it falls.

To carry their kill...

..and stay airborne,

raptors must summon...

a different form of strength.

This time, channelled through their wings.

The amount of power a bird needs to fly

is governed by its "wing loading".

That's the relationship between the area of the wings

and the weight of the raptor.

Big, powerful wings and a light body

mean that a bird can carry heavier loads.

The most powerful birds of prey are the eagles.

Anatomy and physiology work together to keep their weight down.

Some eagles have over 7,000 feathers

that together weigh less than a kilo

and their skeleton is even lighter

because it's full of air.

Their respiratory system

is directly connected to their skeleton.

When they breathe in,

air floods into hollow spaces in the bones,

keeping their weight down.

Their wings, too, are designed for maximum lift.

The golden eagle's are broad and long.

Its wingspan can reach over two metres...

..perfect for soaring and gliding on air currents...

..both of which use far less energy than powered flight.

Gaps between the feathers at their wingtips

allow air to rush through, increasing lift.

These long, broad wings, and lightweight structure

mean that eagles can support

a lot of extra weight when airborne...

..enabling them to carry the biggest prey of all the raptors.

In mountains across Europe,

golden eagles scour the cliffs for a meal...

..like young ibex and chamois.

BLEATING

A young chamois can weigh considerably more than

the eagle's own body weight.

But some birds of prey don't just rely on strength to get a meal.

They employ supreme wing control to catch their prey...

..especially when hunting in dense woodland.

Navigating through trees,

in pursuit of fast, evasive prey

takes manoeuvrability and agility.

The goshawk has both.

It uses the forest to conceal its approach.

Its ability to dodge obstacles at high speed

is down to its wing design.

Goshawks have a relatively short wingspan,

averaging just one metre...

..perfect for small spaces

and for fast responses...

..because short wings can be flapped quicker.

What the wings lose in length,

they make up for in breadth,

generating a lot of lift with each flap...

..and a long tail that acts like a rudder...

..enabling the goshawk to change direction fast.

But to cut through a space like THIS...

..the goshawk must actually fold away its wings...

..and stay airborne.

Viewed in a lab and shot in slow motion,

it becomes clear how it does it.

Its feet lead the way

and its wings fold neatly behind.

What keeps it flying is its tail.

As it clears the gap, the tail fans out,

like a third wing...

..giving it lift

in even the tightest of spaces.

Incredibly, as the goshawk makes these manoeuvres,

it manages to keep sight of its quarry.

No mean feat in dense woodland like this...

..following fast, evasive prey.

That's because raptors have the clearest vision of all birds.

It's difficult to imagine how a raptor sees the world,

but if our eyes were the same size as theirs,

relative to our skull, then we would have eyes

the size of oranges.

But it's not just size that's important, it's sharpness, too.

The resolution of raptor eyes

is higher than ours

because they have more receptors.

Many birds of prey have over half a million, per square millimetre,

in areas known as the foveae.

That's over twice the density found in the human eye.

And whereas we have one fovea per eye...

..birds of prey have two.

These are the areas of the eye where the image is most sharply focused.

In birds, the forward-facing foveae

are used for short-range vision.

These fields overlap to give birds

binocular vision, just like our own.

The other pair of foveae, set at 45 degrees from the front,

are used for long-range vision.

These three fields of vision

enable a raptor to focus on three things at once.

It's thought this adaptation

enables the goshawk to keep its eyes fixed,

both firmly on its prey...

..and firmly on the obstacles in its path...

..as it tears through the undergrowth.

But flying fast through sharp branches

leaves the goshawk's eyes vulnerable to injury.

So, to protect them, they are shielded by a third eyelid.

The nictitating membrane.

All birds have them.

They keep the eye moist and protect it from the elements...

..and from sharp branches.

But whilst most birds' membranes are opaque...

..the goshawk's are almost transparent.

So, even when its membrane is shut,

it may still be able to see the obstacles in its path.

But being able to keep their eyes locked onto prey

takes an extra adaptation.

Keeping the eyes steady when manoeuvring

is crucial to these high-speed hunters.

So much so, that raptors have an adaptation

that keeps their head motionless

whilst their body is moving.

In most vertebrates, when the body and head move,

the eyes roll in their sockets to stay level.

But raptors' eyes are relatively fixed in their sockets.

So, the only way to keep their eyes level

is to keep their head level, too.

If you look at this goshawk flying, its eyes stay horizontal,

no matter what the position of its wings.

They can do this

because birds' necks have more vertebrae than mammals

and are packed with over 200 muscles.

These enable fast reflexes

to isolate the head from the vibrations of the body,

keeping their eye-line completely horizontal.

Being able to maintain a steady pin-sharp focus

allows birds to function at speeds we can only dream of.

Imagine sticking your head out of the car window,

facing into the wind

and then trying to breathe normally whilst travelling

at speeds of over 200 miles an hour.

Let's face it, you simply couldn't do it

but birds of prey can.

For raptors, speed is crucial for catching prey unaware.

But flying rapidly requires huge amounts of energy

and, therefore, more oxygen.

What allows them to cope is their unusual respiratory system.

It takes up over one-fifth of a bird's body.

And is over seven times more efficient than ours

at getting oxygen into the bloodstream.

Because oxygen is extracted from air in their lungs

both when they breathe in and when they breathe out.

When a bird breathes in, air floods, not just into the lungs,

but into a chain of air sacs around its body.

These act as reservoirs, storing fresh, oxygenated air.

When the bird breathes out, this stored air is sent to the lungs,

supplying the blood with a second hit of oxygen.

These air sacs mean that air flows in one direction around the body...

..so inhaled air doesn't get polluted

by exhaled air, as it does in mammals.

This efficient system means that birds of prey are supplied

with a near constant flow of oxygen...

And more oxygen means more energy.

When it comes to moving fast, one bird of prey outdoes

every single animal on Earth.

The peregrine falcon.

Top speed - 220 miles an hour.

On the face of it, it looks like any other bird of prey.

Research by American ornithologists,

however, found that falcons come from an entirely different branch

of the family tree to their other raptor cousins.

Their closest relative is, in fact, the parrot.

This different evolutionary path has led to some novel adaptations,

which allow the falcon to fly faster than all other birds.

For a start, it has "baffles",

cone-shaped bones just inside the nostrils,

that moderate the air pressure as it enters the body,

helping it to breathe at high speed.

Its small intestine is, proportionally,

50% shorter than those of other birds of prey,

reducing its weight so it can accelerate faster.

But most important is how it uses its wings.

Unlike their eagle and hawk cousins,

falcons have long, thin, pointed wings.

This streamlined silhouette means less drag and faster flight.

But fast, horizontal flight is not enough to keep up

with some of its prey.

The humble pigeon is no easy meal.

It flies faster than the falcon on the flat

and has more stamina.

So, to catch it, the falcon has a trick up its sleeve...

Gravity.

In a dive or "stoop", it can reach its top speed.

To dive like this, falcons need to reduce drag.

They do this by adjusting their wing position...

..moving them closer and closer to their body.

For the final push, they fold their wings completely

against their body, like a vacuum pack,

preventing any air entering between the feathers.

But airflow is controlled by more than just wing shape.

The positioning of individual feathers is crucial.

When a solid object moves through air,

the flow of air around its surface forms regions of swirling eddies.

These create drag...

..but sometimes, drag can be decreased

by creating an uneven surface.

It's why golf balls are designed with dimples.

And aeroplane wings have small fins on their surface.

Falcons, however, have had their own

natural solution for millions of years.

New research from Germany has found that

at the point the peregrine's dive,

where drag should be a problem,

feathers change position

in the exact location of the drag.

The feathers react to the flow of air over the wing's surface.

If drag starts to develop, wing feathers will automatically pop up,

adjusting the flow of air on their wing

to enable the maximum speed needed...

..to capture its prey.

Giving the peregrine falcon the edge over its prey

and its raptor relatives.

From ratchet talons

to the highest resolution sight...

to drag-reducing feathers.

Birds of prey have a range

of anatomical and physiological adaptations

that give them strength, manoeuvrability and speed.

Essential attributes in their hunting armoury,

which make them the ultimate aerial assassins.

And THAT is the wonder of birds of prey.