The Mastery of Flight The Life of Birds

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Getting into the air...is not easy.

Indeed, for many birds it is the most exhausting part of flying.

But these shearwaters, here in Japan, have adopted a labour-saving way -

oops, there we go - a labour-saving way of doing so.

They've taken to climbing trees.

This particular tree is by far the most suitable for takeoff

and that bird may have come from as far as 30 or 40 yards away,

wandering across the forest floor, to climb it and reach the launch pad.

There's another.

A bend in the trunk is a perfect platform for a takeoff.

These shearwaters will spend most of their lives in the air.

They are true sea birds.

They only come to land to nest.

This species is exceptional in nesting in woodland.

Most of them nest on the edge of cliffs. All of them get into the air

by simply falling into space.

But the land birds, on the other hand, have much greater problems.

They get airborne with a standing start. That takes a lot more energy.

A pigeon begins by jumping vertically upwards.

As it leaves the ground, it opens its wings and sweeps them forward,

fanning the air downwards with maximum force.

The second stroke must be equally vigorous - pushing the bird upwards.

Now, it leans forward and starts to go ahead.

The effort involved has been huge.

Slightly bigger birds can't do this twice in quick succession.

And one as big as an albatross can't do it at all.

It has another way...

It taxies...along a runway.

It's a method we use, too, in our machines.

For the majority of birds, the most exhausting part of flight is over.

Those shearwaters climbed trees -

the pigeon jumped and flapped -

and aeroplanes and albatross ran and created a flow of air

over their wings.

When they do get into the air, a bird's flight seems effortless.

And superb flyers, like albatross, seem to defy the law of nature.

They are, after all, big heavy birds.

How can they withstand the gravity that keeps the rest of us firmly on the ground?

The secret is a wing with a thick, rounded front edge

that curves gently downwards towards the back edge, which is very thin -

as thin, in fact, as a feather.

As the bird glides forward,

the air flowing UNDER the wing is impeded by the wing's downward curve.

So, it becomes slightly compressed and that pushes the wing up.

And air flowing across the TOP of a wing is sent up by its front edge,

so reducing its pressure.

If the air is moving fast enough, the slight suction from above, combined with the push from beneath,

will lift the bird into the air - as it did during takeoff -

and keep it aloft - as it's doing now.

The trick is to ensure that air DOES flow over the wing quickly enough.

Upward air currents can also sustain a bird in flight

and that's what you get when breezes, blowing in from the sea, hit a cliff.

If they are really strong, such updraughts can be powerful enough to keep an albatross in the air.

Out at sea, the waves deflect the wind upwards

in somewhat smaller gusts.

The albatross is so skillful that it can sail on them for hours with scarcely a movement of its wings.

Most birds, however, once in flight, have to create that airflow across their wings by another method.

They drive themselves forward by flapping.

This knot is "rowing" through air -

stretching its wings forwards and beating them downwards.

It folds them to reduce surface area and air resistance on the upstroke.

The feathers on its wing slide smoothly over one another

so that as the wing changes shape, its surface remains perfectly smooth.

Its body is streamlined by its coat of feathers

and its feet are pressed against its tail to keep drag to a minimum.

This mallard is flying at nearly forty miles an hour

but its streamlining is so perfect that its feathers are hardly ruffled.

Only from behind, can you notice the flicks of its feathers over its tail and the back edge of its wings,

which show just how fast it is travelling.

To see how important streamlining is, and how much energy it can save,

watch this osprey as it goes fishing.

To take off again, with the fish in its talons, the bird must beat its wings with all its strength.

But even now it is in the air, the fish, hanging broadside,

creates so much drag that the osprey has difficulty in making any headway.

It knows how to solve the problem.

Gripping the fish with just one foot, it brings its other foot forward.

Now, using both feet,

the bird changes the position of the fish so that it faces ahead.

Its streamlined shape reduces its drag so much

that the osprey's wing beats verge on leisurely.

Flying in formation also saves energy.

A big bird, like a pelican,

creates a trail of turbulence in the air, giving a following bird lift.

The effect is at its greatest directly behind a bird's wing tip -

the best place for a following bird.

Pelicans save 20% of their energy by gliding as well as flapping.

Aerodynamically, it's better for a bird to time its flaps

with those of the bird ahead.

So it is, that pelicans give amazing displays of synchronised flying.

The most economical way of flying

is to draw almost all the energy you need directly from the sun.

As it warms the ground in the morning, the rocks reflect its heat

and shimmering columns of air - thermals - begin to rise.

Griffon vultures in Spain leave the ledges where they've spent the night

and launch themselves into the air.

As the thermals rise beneath their wings,

they sail effortlessly upwards.

All they have to do is ensure that they stay within the warm air column.

So, dozens of them spiral together in tight circles,

adjusting their flight with tiny movements of their wings and tail.

There can be no more economical flight than this.

The vultures' ability to read the air conditions above their landscape

and detect where the thermals are at their most powerful, seems uncanny.

But human beings have also mastered it.

When you hit a thermal in the glider, you really feel it.

Your stomach drops beneath your feet.

Oooh! Ride away! We're gonna roll into the thermal. Wow!

Glider pilots go round in circles a lot,

as birds of prey do, because a thermal's a rising column of air and to stay in it, you have to turn.

There's nothing to see, though, apart from what's on the ground.

There's nothing in the air to show a thermal. There's no cumulus cloud over this one, but often there is.

Under most of those cumulus clouds there, there would be a thermal.

That's one thing we look for and I'm sure birds look for it, too.

Look, feel that! There's a big, rocky outcropping

and there's our lift. Look at that.

The altimeter's winding up. Look.

How high could we go with this?

Probably to about 14-15,000 feet with no problem.

And do the birds go as high as that?

I've seen them up to 16-18,000 feet,

looking like they're flying for fun. How do you know?

Because how can you see a mouse from 18,000 feet?

And they do tricks and aerobatics. Look, we're really going up now.

The pressure on the wings is actually bending them, isn't it?

The spar is flexible, so as you develop lift, it bends upwards -

very similar to a bird.

Every flight has to end in a landing.

That requires less energy, but more skill if disaster is to be avoided -

particularly if you're a big bird like a pelican.

A swan, one of the heaviest of flying birds,

can only come down on water's smooth and forgiving surface.

There, you can use your feet as brakes.

Albatross are not so lucky. They have to alight on the ground.

Indeed, THEIR landings seem scarcely better than controlled accidents.

Most birds have to come down with much greater precision than that.

They may have to land, after all, on a narrow ledge or thin branch.

To do that, they have to put down the undercarriage and brake

so that they lose speed the moment they come alongside their perch.

That requires judgment and co-ordination.

A griffon vulture is able to exploit the position of its nest -

usually on the ledge of a cliff.

It descends towards it at speed.

It aims for a point BELOW its nest,

then brakes by swooping upwards

so that as it arrives at its ledge, its forward speed is zero.

Landing into the wind helps any bird. It keeps air flowing over the wings

and maintains lift until the last moment.

So, birds can complete an operation

fraught with danger, with virtually total success.

Anyone who has paid for excess baggage knows that if you fly, it's important to keep your weight down.

And this magnificent golden eagle manages to do that beautifully.

It's the same size as a bulldog, but I couldn't hold THAT on my wrist.

But this bird only weighs about a quarter as much.

How do birds manage to keep so light?

Well, the beak is not so heavy as the bony jaws and teeth of a mammal

or the birds' reptilian ancestors.

Its disadvantage is that a bird can't chew -

just pluck, crush or, like an eagle, tear and rip.

They also have weight-saving features INSIDE their bodies -

a skeleton with fewer bones than a mammal's, no tailbones, one wingbone instead of five fingers

and a pelvis fused to the backbone.

And the bones themselves are not solid like a mammal's, but hollow

with interior cross-struts strengthening them.

But the most remarkable weight-saving features

are the things only birds possess - feathers.

They look simple but they have a very complex structure.

The quills are hollow and light, yet resilient and strong.

The filaments attached to the quills are fringed with microscopic hooks.

They link, latching together to form a continuous surface.

That means that if a feather gets damaged or overstrained,

it can be repaired instantly - it can be zipped up.

Not surprisingly, all birds lavish a lot of attention on their feathers.

After all, their lives depend on them.

And since birds have no hands, they have little alternative

but to care for them with their beaks.

But one bird, uniquely, can't do that.

The swordbill hummingbird's beak is so long,

that its tip can't touch its feathers.

It has to comb its plumage with one foot, while balancing on the other.

Not easy!

A good bath is also important

to keep feathers clean and in first-class condition.

Most birds take one every day.

Watching them, it's difficult to avoid coming to the conclusion that they enjoy it just as much as we do.

Not all birds, of course, can get to water deep enough for bathing.

Then - like the quail

that lives on dry plains and in the summer seldom finds even a puddle -

they may have to use dust.

This may not exactly make them cleaner, but it dislodges parasites

such as lice that nibble their feathers, mites that scavenge dead skin and blood-sucking ticks.

Many parrots and cockatoos grow special feathers that fray at the end into a fine powder.

These are scattered throughout the plumage

and when a bird scratches after its toilet, the powder is dislodged

and caught in its ruffled feathers.

Exactly HOW this powder improves the feathers is not really certain -

it probably helps with waterproofing and discourages parasites.

Parasites are such a problem

that some birds may recruit assistants to help get rid of them...

..ants.

Crows and jays deliberately land on an ants' nest and stir up the colony

so that they swarm all over them.

In their irritation, the ants discharge formic acid -

a particularly powerful insecticide.

But we don't really understand this behaviour.

Maybe the birds are stimulating the ants to get rid of their formic acid

so that they are more digestible.

Regular, meticulous maintenance

is essential for safety in the air -

for everything that flies.

That was about 500 miles an hour

and birds can't equal that. But before planes were invented,

a bird was the fastest living thing in the air.

The peregrine holds the record.

Diving on its prey, it can exceed 200 miles an hour.

It achieves maximum aerodynamic efficiency by sweeping back its wings like the jet fighter.

Then, it accelerates by beating them.

The barn owl, on the other hand, owes its success as a hunter to its ability to fly extremely slowly.

It hunts voles and mice

and to find them in grass, it has to search intently - that takes time.

Its wings, therefore, are shaped very differently from a peregrine's.

They're rounded and much broader - giving maximum lift at slow speeds.

The barn owl also has a very special adaptation for this kind of hunting.

The rodents it seeks are often invisible from the air -

hidden beneath the matted grass.

The barn owl detects them by the rustling sounds they make.

It has acute hearing - sounds are focused by hair-like feathers

on discs on the sides of its head.

But if it is to hear them, it has to fly very quietly indeed

so its wings have silencers - fluffy margins to its wing feathers.

So, in the evenings, a barn owl can waft over the countryside as silent as a moth.

This little dot, suspended in the sky,

might seem to be the slowest flyer of all.

It's a kestrel. It's not, in fact, truly stationary.

It's facing a gentle wind, so an air current passes over its wings -

giving it all the lift it requires.

Silence is not as important for the kestrel as it is for the barn owl, for IT hunts by sight.

The wind has dropped a little.

Now, to keep its position relative to the ground

it must flap to keep air moving over its wings.

It has spotted something - a quick turn into the wind and a drop...

A turn back to face the wind for a stationary check...

Another quick look...

But whatever it was has gone.

Only one group of birds can manage to hover for any length of time

without the help of a headwind - the hummingbirds.

HUMMING

Their wings work in a way quite unlike that used by any other bird.

They beat routinely 25 times a second - so fast that they hum,

hence their name.

It's impossible to see how they operate without slowing the camera.

The wings have become twirling blades that create downdraughts -

rather like those that man produces with HIS hovering machines.

Helicopters, however, have a special device -

a wheel revolving continuously on an axle.

No bird or other animal has evolved a mechanism that can parallel this.

But hummingbirds have the next best thing -

wings which beat in a figure eight and flick over on the backstroke.

Uniquely, their wings have symmetry in cross section and work equally well with either surface uppermost.

By changing the angle of the beat,

the thrust can be directed forwards or backwards as well as downwards.

So, a hummingbird, steering with its tail, can move in any direction.

Beating wings at such speed, however, uses a lot of fuel.

Even at rest, hummers need lots just to keep their bodies ticking over.

They have to refuel very frequently.

But their fuel STATIONS - flowers - close at night. So, what do they do?

It's a problem in the Andes, where the nights can be very cold indeed.

As evening comes on, the hillstar hummingbird

makes its way to its regular roosting place in a cave.

After its regular toilet, it settles down for the night and, in effect, turns off all its motors.

Its heart, that in flight contracted 1,000 times a minute,

slows until its beat is virtually undetectable.

Its body temperature falls and its breathing seems to cease.

Like a hedgehog in winter,

it's hibernating.

But for a hummingbird, winter comes 365 times a year.

The sun returns and the temperature rises.

The hillstar starts up its motors.

Its heartbeat quickens, its muscles slowly warm to flying temperature.

A quick pre-flight check...

..and it's off again!

At higher altitudes, it seldom gets warm - even at mid-day.

This is the territory of the giant hummingbird.

It's as big as a thrush.

Its size helps retain body heat, but this is as big as a hummingbird gets.

Any larger and it couldn't beat its wings fast enough for flight.

And this is one of the smallest of all birds -

a purple-collared woodstar from Ecuador with a two-inch wingspan.

Small wings are easier to flap, but they must move faster

to produce sufficient downward thrust.

And this hummingbird beats its wings at an astonishing 75 times a second.

It's barely bigger than a moth.

This moth looks so like a tiny hummingbird

that people in the south of England, where it often appears, think that they have seen a real hummer.

GEESE HONK

The ability to fly gave birds the freedom of the planet.

Rivers, deserts, seas, even mountain ranges are no obstacle to them

as they are to land-bound creatures such as ourselves.

They can fly easily and quickly to collect a sudden glut of food.

And that's what has happened here.

I'm in northern Canada. It's June - the start of the short arctic summer.

Rising temperatures have caused the plants to put out leaves and roots,

and tens of thousands of snow geese have come here to graze.

They nested almost as soon as they arrived,

and many have already got families.

CHICKS CHIRRUP

Even hummingbirds have come to the far north to collect nectar

from the bushes that are now briefly blooming within sight of glaciers.

On the Arctic coast, little waders -

Western sandpipers - are collecting a rich harvest of small worms that are swarming in the mud.

In the middle of the continent, on the prairies,

grain crops are ripening in the summer sun.

Dickcissels, relatives of the common sparrow, are here for their share.

The warm weather has caused swarms of insects to hatch.

They provide young dickcissels with essential protein.

Hawks are also breeding here in the north -

attracted by the seasonal abundance of small mammals,

finches and songbirds that they need to feed their young.

But the superabundance of summer is brief.

By the end of July, days shorten.

Many trees are preparing to shed their leaves.

The birds that flew up for the summer banquet can no longer stay.

Across the northern hemisphere, the story is the same.

From Siberia, across Asia and Europe to the tundra of North America,

birds are starting to fly south.

The sandpipers are stocking up for the 6,000-mile journey ahead.

They eat so voraciously that they will nearly double their weight -

adding layers of fat to their flanks.

They even shrink their internal organs, partially absorbing them as though they were food reserves

and replacing them with fat.

They must wait for the right weather conditions.

Then, when the wind blows strongly from the north, they set off.

Hawks and vultures are now finding it harder to discover any food, too.

They, too, must prepare to leave.

But the weather THEY require is rather different.

They need a hot day, when thermals shimmer from the rocks that are still warming in the late summer sun.

As the last thermals of summer start to rise,

the birds circle up to great heights - 10,000 feet or more -

to get a good start for their long journey.

As they glide southwards, slowly losing height,

they will make for another thermal's base, so that again they will be lifted high enough to reach the next.

The snow geese are already on their way -

thanks to shortening days and dropping temperatures.

They will rely on straightforward muscle power.

They will travel continuously for great lengths of time,

both through the day and the night.

The raptors, however, have had to stop, to overnight in a roost.

Without thermals they can't go far.

But the snow geese fly on.

The exertion of continuously beating their wings creates body heat,

so travelling in the cool of the night does, in fact, suit them.

They navigate by the stars.

If the skies are overcast, they may get lost - but that is exceptional.

Day returns and the stars fade.

Now, they steer by the sun.

But the sun, of course, moves from east to west during the day,

so for navigation, they must have internal clocks and know fairly exactly what the time is.

Members of the same family travel together,

calling to one another as they go.

GEESE HONK

And the geese have made it.

One has been recorded as covering 1,700 miles in a mere 70 hours!

These Californian fields are now home

until they return north on their spring migration.

The sandpipers have gone even further - they have reached Mexico.

They will spend only a few days here,

for this is merely a refuelling stop.

They feed intensively, replacing fat reserves that they have lost.

The raptors, so conscious of the nature of the land beneath them that generates essential thermals,

also look to it for their signposts.

They are passing Mexico's highest mountain - the Pico de Orizaba.

There are no thermals over the sea, so they are tied to the land

and have to go right round the western side of the Gulf of Mexico.

There is, of course, a short cut - directly south, across the sea.

The ruby-throat hummingbird tackles that 500-mile journey.

It must be non-stop, for a hummingbird cannot land on water.

A feeder in Texas provides a final top-up of nectar for a ruby-throat.

Its cruising speed is about 27 miles an hour.

So, if conditions are good, it could make the crossing by flying for a little over 18 hours.

That's the limit of its endurance -

if there's even a light headwind, it will perish at sea.

Delphiniums blooming on the Mexican shore await with life-saving nectar.

A ruby-throat arrives after its epic journey

and feeds urgently, before it runs out of fuel and is fatally grounded.

But, even now, its journey is not finished.

It still has several hundred miles to go

and may go as far as the Panama border.

The hawks and vultures, travelling round the western side of the Gulf,

have now reached Panama City. They came from all over North America,

converged on the isthmus and flew together down that strip of land

so that now, for the only time each year, they form dense flocks.

Below, on the mud of Panama Bay, the sandpipers are feeding. This, at last, is the end of their journey.

The mud here never freezes, the sea enriches it

and each bird returns every year to exactly the same patch.

The raptors rise once more in an immense vortex.

They will take their separate ways all over South America -

some going as far south as Argentina.

Only by dispersing widely will each bird find enough prey to survive.

The dickcissels have also travelled down the isthmus of Panama.

They, too, have come from all over North America

and have now been funnelled together into gigantic, dense flocks.

This, surely, is the very acme of flying skill.

How they co-ordinate their flight in those extraordinary concentrations,

changing direction as if with one mind, is a mystery of ornithology.

Years ago, they, like the hawks and eagles, would have gone on south

and spread over the American plains to feed on the seeds of wild grasses.

But here in Venezuela, they find great fields of cultivated grain

exactly like they found in the north.

So, they have no need to disperse, but remain together and devastate the crops wherever they settle.

It seems they positively prefer one another's company -

flocks may be half a million strong.

And Man's practice of intensive cultivation

allows them to stay and feed together.

At night, they select a relatively small patch within a huge field,

where the whole half-million roost -

half a dozen birds to a single stem.

Flying, when all is said and done, takes a great deal of energy.

So, birds have huge appetites

and have to spend much of their lives in an unending search for food

to fuel their expensive lifestyle.

Just how they find it, we will be looking at in the next programme in The Life Of Birds.

Subtitles by Alison Haggart BBC Scotland - 1998