To Fly or Not to Fly? The Life of Birds

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Birds are the most accomplished aeronauts the world has ever seen.

They fly high...and low, at great speed...

and very slowly...

and always with extraordinary precision and control.

But birds are not the only creatures in the air.

There are also small furry mammals - bats, like these in Texas.

They are so competent in the air,

that they have just made a journey from Mexico, a thousand miles away,

to rear their young in this cave,

which is suitable for them as a nursery.

Now, they are out to catch their evening meal of insects.

But they had better be careful because above them, there lurks a creature that can outfly them.

It is, of course, a bird - a red-tailed hawk.

Bats, with their fluttering zig-zag flight, are not easy targets,

and a hawk needs all its aerobatic skills and powers of concentration

to snatch one out of the confusing multitude.

That is one bat that will not return to the roost tonight.

The red-tail lives beside the cave

and is well practised in bat-catching.

This prairie falcon, however, is a visitor, but it's learning fast.

Unlike the hawk, it chooses to eat its meals on the wing.

Bats are latecomers to the skies.

They've only been flying for a mere 60 million years.

The air was first colonised 200 million years earlier -

by the insects - but now they can't escape the birds either.

Some insects, of course, have powerful weapons

with which to defend themselves.

But a bee-eater certainly knows how to deal with a bee.

A rub against the perch usually discharges the sting.

And if that doesn't,

then a sharp nip will squirt the venom harmlessly into the air.

Dragonflies first flew around 350 million years ago;

and insects had the skies to themselves for 150 million years thereafter.

And then a different kind of animal joined them in the air.

As the dinosaurs dominated the land,

so the pterosaurs now ruled the skies.

Pterosaurs had wings of skin stretched between one enormously elongated finger and their flanks.

They flew over the sea as well as the land.

It seems likely that some roosted on cliffs

and launched themselves into the air as gannets do today.

They probably snatched fish from the surface of the sea,

and some certainly fell into it.

AARK!

Their bodies were buried by mud, the mud turned to limestone

and eventually became exposed in quarries like this one in Germany.

Today, separating the layers of sediment

is like searching through the pages of a visitors' book

that hasn't been opened for 150 million years.

Of course, nearly all the pages are absolutely blank.

Visitors, after all, were very few.

But now and again you come across a signature that is unmistakable.

A fish the size of a sardine.

A shrimp - even its antennae perfectly preserved.

One of the pioneering dragonflies - nearly six inches across.

And a pterosaur with skinny wings and teeth in its jaws.

With so many superb fossils, people thought that they had a complete list of the visitors to the lagoon.

And then, in the middle of the last century, a signature was discovered

that was wholly unexpected and totally amazing. This is it.

It's a feather.

Its barbs are narrower on one side of the quill than the other, as they are on a modern bird's wing.

This asymmetry is a sure sign that such feathers were used for flight.

But what animal at the time of the dinosaurs could have such a wing?

The answer was found the very next year in the same quarry.

A fossil with its feathers still attached to its body.

This is archaeopteryx.

It had three toes armed with claws and long, strong legs.

Clearly, it walked and perched like a bird.

But its head was very reptilian, with bony jaw bones.

And in those jaws - teeth.

Its spine was extended into a bony tail - again like that of a reptile.

But on either side of the tail bones,

it had those characteristic possessions of birds - feathers.

Feathers are made of keratin, as are the scales on the legs of many birds

and all over the bodies of reptiles. A scaly coat must be very hot,

so reptiles like this skink have to seek shade during the hottest part of the day.

But if the scales became fibrous, they could be fluffed up to let in cooling air during the day,

and closed down to trap insulating air for warmth at night.

So it is not hard to believe that scales eventually became feathers.

But why did they become so long that they enabled an animal to fly?

This Australian lizard has one answer.

When it is threatened by its enemies,

it responds by spreading the great frill it has around its neck.

But if that doesn't scare them off, it runs away...on its hind legs.

If such a reptile had developed feathery scales on its forelegs and then spread them out,

then it might easily lift into the air and escape a land-bound predator.

Here's another possibility.

Maybe that early reptile did not live on the ground, but in the trees,

as today, the little flying lizard of Borneo does.

It now glides from tree to tree.

It has developed wings

that are flaps of skin supported by elongated ribs.

If that early enterprising reptile with feathery scales

did have extra long ones on its arms,

then they too would have enabled it to glide from tree to tree.

Maybe its arm muscles were even strong enough to allow it to make a few flaps to help it on its way.

Archaeopteryx was certainly well-equipped for climbing,

for its wings still carried three fingers, each with a hooked claw.

And there are birds today with very similar ones that give a clear hint

as to how it might have used them.

These young hoatzin in South America are still guarded by their parents.

A hoatzin chick has an adventurous disposition

and starts clambering about when only a few days old.

The hooks on its front limbs are useful in keeping it secure

until they become feathered and reliable wings.

One can imagine that archaeopteryx used them in much the same way.

Sometimes, however, there's a disaster.

There are dangerous reptiles in the swamp - snakes and cayman.

But those claws on the wings are, once again, invaluable.

Mother returns. She has been feeding on leaves and will have a full crop.

Maybe there will be some for the chick.

The hoatzin, like all modern birds, doesn't have bony jaws with teeth like archaeopteryx,

but a lightweight beak. When did that important change take place?

This fossilised bird HAS a beak. It was found in China recently.

It's just younger than archaeopteryx, so the change took place quickly.

The beak prevented the bird from being nose-heavy

and significantly reduced its overall weight.

By 50 million years ago, the dynasty of birds was firmly established.

At that time, a great lake lay here in central Germany.

It dried out long ago and the layers of mud have turned into shales.

Excavations like the one that's going on here,

have revealed just how varied the birds had become.

This one's been set in yellow resin to make its details quite clear.

It had a horny beak, a fully-feathered wing,

a long feathered tail with no bony support,

and long legs. It probably looked like a...rail.

Other fossils from these shales show that several families of modern birds were already established.

This was a water bird - possibly an ancestor of today's jacana.

It would have found insects among the leaves on the lake.

There were birds with chisel-like bills. Perhaps they were woodpeckers

that even in this early period had started excavating insects from the trees in the surrounding forest.

Another resident of those woods had a stubbier, more all-purpose beak,

rather like finches do today.

There were tall birds with long, powerful legs

that probably hunted for small reptiles on the ground,

as the South American seriema does.

And there was a gigantic vulture with a wingspan of over 20 feet -

bigger than that of the Andean condor

and probably the biggest flying bird that has ever existed.

There were even birds which, judging from their skeletons,

were as agile as their probable descendants, the frigate birds.

So by 50 million years ago,

several families of modern birds were well established.

The rule of the reptiles was now over. Not only had pterosaurs disappeared from the skies,

dinosaurs had gone from the ground.

So the dominance of the land was up for grabs. There were two contenders: the mammals and the birds.

The biggest mammal found from this lake in Germany was a primitive horse, no bigger than a spaniel.

The biggest bird was very different.

That was the lower part of its beak,

and this was the upper.

If its skeleton still lay here, you would have to dig a huge pit to extract it.

This bird was immense.

It has been named - with good reason - the terror bird.

Its wings were tiny,

but it had long and powerful legs.

Flightless birds of a comparable size still exist

and can give us some idea of what it looked like.

This, the ostrich, is the biggest and heaviest bird alive today.

It is probably not closely related to those monstrous feathered hunters of prehistory,

but together with the Australian emu and the rhea of South America,

it belongs to a very ancient family of birds that abandoned flight a very long time ago.

It relies for its defence on speed,

and to increase its efficiency as a runner,

its toes have been reduced to two.

If pursued, it can sprint at over 40 miles an hour.

But although it is now flightless,

it still has many of the physical characters evolved by its ancestors

that enabled THEM to fly.

It still has feathers. They are still placed on its wings

in much the same position as those on the wings of a flying bird.

But they are now useless for flight.

Their filaments have no hooks, so cannot be zipped together into an unbroken blade.

Instead, they are loose and fluffy.

Their only function now is as insulation - to keep out the cold at night and the heat during the day.

Ostriches have become grazers -

the bird equivalent of antelope or horses.

But unlike them, they not only pick up leaves,

they swallow all kinds of other things too - and for a good reason.

Just as they inherited feathers from their flying ancestors, they also inherited a light, horny beak

instead of a heavy jaw laden with teeth.

And without teeth, they need another way to grind up their food.

A pebble can help them do just that.

It goes into a muscular compartment of the stomach - the gizzard,

a kind of mill where bits of vegetation are churned around and ground into a digestible pulp.

But while some birds were abandoning flight,

some mammals were becoming formidable hunters.

Ostriches, with their superb eyesight and tall necks,

can keep a sharp lookout for danger.

The cheetah has to calculate whether it's worthwhile chasing something...

and an ostrich, usually, is not. It is so fast,

and it has little meat on it compared with a similar sized mammal.

But birds that can't run are a tempting target for hunters.

If they couldn't fly, they wouldn't last long.

Flight has certainly enabled birds to colonise the entire globe.

But the compulsion that drove them into the air in the first place,

and has kept most of them there ever since, was probably safety.

Even so, the cost of flying is high.

Flapping wings takes a lot of effort and if there is no need to do so, birds save energy.

Those that live here on the isolated Galapagos Islands,

have no natural enemies from which to escape, so some birds don't bother to fly.

Have a look at these cormorants, for example.

At first sight, they look like any other cormorant to be found sitting on cliffs round the world.

But these wings are stunted and tattered.

CROAKS

This bird could never fly.

Its feathers serve only to keep it warm in the water.

It's for that reason alone that it keeps them well-oiled.

It is scarcely any better at walking than it is at flying.

Once in the water, however, it's a very efficient mover indeed.

The position of its legs right at the back of its body that made it so clumsy on land

is ideal for speeding through water

and helps it to catch all the fish it needs.

For the Galapagos cormorant, flight has become an irrelevance.

Other island birds have reacted in a similar way.

This is New Caledonia in the western Pacific - and this is its special bird, the kagu.

Its ancestors must have arrived here by air, but since New Caledonia had no ground predators until recently,

they gave up flying and today the kagu is virtually flightless.

It finds all its food on the ground in the leaf litter.

It's been here so long, it's not clear who its ancestors were, but they were probably herons.

Rails are a very widespread family of birds.

Wherever there is a big swamp, you are likely to find one.

On the continents, they tend to lurk shyly in the undergrowth,

but some have also managed to reach a great number of islands, and there they seem to have no fear at all.

This one - it's a weka - has also become flightless

because of its isolation on an island.

But its island...is immense.

It's a thousand miles long if you discount a narrow arm of sea crossing it in the middle,

and it contains mountains over 12,000 feet high. It's New Zealand.

The first land-living mammals to get here were human beings

and they didn't arrive until 1,500 years ago.

So here you can see what the world would have been like

if the birds had won that battle with the early mammals and now ruled the earth...for here they once did.

Many of New Zealand's birds flew here from Australia,

1,500 miles away across the sea.

They started to do so millions of years ago and they are still doing so today.

So if you know Australian birds,

you will recognise quite a lot, particularly those that are relatively recent arrivals.

The New Zealand pigeon is not very much different from Australian ones.

The saddleback, however, must have been living here for much longer,

for it's changed so much that no-one is sure what family it belongs to.

The tui has similarly mysterious origins.

No other bird has a costume like its lacy cape

and that little white throat bobble.

The kaka is clearly a parrot. There are lots of parrots in Australia

so it's not surprising that some have found their way here.

Most of these birds have still not learned that mammals are dangerous.

This saddleback is a fully wild bird

and certainly hasn't seen me before. But look how trusting it is.

This is a New Zealand robin. It's no relation to the European robin

and, if anything, it's even braver.

New Zealand is full of food, and as the birds once had it all to themselves,

some adopted diets and lifestyles that elsewhere were claimed by mammals.

The kokako eats much the same thing as squirrels -

fruit and leaves and insects.

European squirrels run along the branches;

Asian squirrels, using a skinny parachute,

are able to glide as well.

And that is very much how the kokako gets around in the trees.

Having glided down the branches,

it runs back up them

and jumps from one to the other.

So if the kokako, up in the trees,

feeds in the same way as a squirrel might do,

what lives and feeds like a mammal on the ground?

The leaf litter in these forests is full of food.

There are earthworms and insects and beetle larvae.

In any other land, there would be some small mammal burrowing around, seeking that food.

But not here in New Zealand. Here there's something different.

You will only see it after dark.

It is, of course, a bird,

but what an extraordinary one. The kiwi.

It's territorial and calls stridently to proclaim its ownership.

RATTLING CALL

It finds its prey by smell.

Uniquely, its nostrils are on the tip of its beak.

It's found a worm.

But once it drops it, its eyesight is so poor that it can't see it

and it has to smell for it - with its beak.

Its tiny vestigial wings are invisible, buried in its plumage,

and it has lost all sign of a tail.

If the kiwis live in forest close by the sea,

in the evening, they may come down onto the beach to look for these -

sandhoppers. They love 'em. And that will give us a chance,

a rare chance, to see them out in the open.

To do so properly, we have to use our special starlight camera.

The kiwi is hunting along the strandline

for hoppers feeding on the decaying seaweed.

Its sense of smell is so acute,

it can pick out the largest, juiciest hoppers deep in the sand

without even seeing them.

Our starlight camera can see much better than I can.

I need a torch to see this extraordinary creature properly...

but it doesn't seem to mind.

Its feathers are just filaments,

so that it almost looks as if it is covered with coarse fur.

Probing sand with your nostrils is all very well,

but it does clog them up, so you need to blow them clear now and then.

It's nocturnal and furry, it finds its way around by smell,

it lives in holes and digs for worms and grubs.

It's a bird equivalent of a badger.

But there's plenty of other food to be found in the New Zealand bush.

Here on the forest floor, there are lots of leaves.

They may be a bit twiggy and coarse but they are food. What could have browsed on these?

Well, not far from here,

bones like this have been dug up.

It's obviously a leg bone, and at first sight you might think it was the bone of a mammal, perhaps a cow.

But when you look closely,

you can see a honeycomb structure - a lightweight bone. It's a bird bone,

but the bone of a very big bird indeed,

as we know from the rest of its skeleton.

It had just three toes.

Its pelvis and its spine lead up to an extraordinarily long neck.

This bird stood over six feet - two metres tall.

The first human settlers here saw them alive and called them moas.

Among them were the tallest birds that ever existed,

that weighed over 200 kilos - 400lbs.

There were about a dozen different species of varying size and weight.

Up on the high moorlands,

there were smaller species with thicker feathers to keep them warm.

The absence of mammals didn't mean that the moas had no enemies.

They were hunted by, of course, another bird.

An immense eagle that could manoeuvre through the patchy forest.

The only prey abundant enough to feed such a giant were other birds,

and it could probably tackle even the biggest moa.

Its talons were long enough to stab through the flesh into its pelvis,

as some of the bones show.

Nevertheless, the moas survived for a million years or more

and spread all over New Zealand.

But eventually mammals DID reach these remote islands. Apart from bats which flew here,

the first to arrive were those most dangerous of all - human beings.

They hunted the moas for meat and soon hunted them to extinction.

But a different kind of flightless bird does still survive, up in these high mountains.

Like so many of New Zealand's native birds that had abandoned flight and nested on the ground,

it had no defence against the alien mammals that Europeans brought

and that soon escaped and ran wild.

Rats ate their eggs and killed the chicks,

cats and stoats massacred the adults.

There was a giant flightless coot that was originally very common.

But it got scarcer and scarcer and by the middle of the 19th century,

it was thought to be extinct.

And then, just 50 years ago, someone in these remote valleys

found something like this.

This is the dropping of a takahe,

and here...

the severed stems of tussock grass on which it's been feeding.

It's still here.

(And here is her nest.)

She's sitting tight,

hiding her brilliant red bill so she is not conspicuous.

Indeed, when only her lovely moss-green back is visible,

she is well camouflaged from her only native enemy - a bird of prey circling overhead.

Only about 40 pairs of takahe survive today in the wild,

so the eggs she is sitting on are very precious.

This high country was probably not the takahe's original home.

Most in the last century lived lower down where they fed on lush vegetation in the warm swamps.

But most of those were drained and turned into farmland.

So now these high empty valleys, scraped down to the rock by glaciers,

are the takahe's last refuge.

There is little to eat up here except tussock grass.

Extracting something nutritious from tussock is not easy.

The takahe's technique is to pull up a whole stem and then nibble the bottom inch.

That's where most of the minerals and sugars are and it's the only bit tender enough to be easily cut.

At first the chick doesn't know how to do this and has to be fed.

It will stay with its parent for a whole year.

Only then will it be skilled enough to feed entirely by itself.

Now it's summer, but when winter comes, life will be even tougher.

Then the pools freeze over and the tussock has no fresh shoots

even if they could be reached under the snow. Then the takahe has to dig for tubers in the freezing earth.

So the birds' continued survival up here in these barren moorlands is by no means assured.

One other flightless bird found refuge from mammals in these high mountains,

and in many ways, it was the most extraordinary of all. It was a giant parrot, the kakapo.

It lived in much the same way as rabbits do.

It made tracks through its territory, generation after generation,

trudging along here and feeding by plucking these grasses

and eating the succulent base.

This path runs under this bush

and continues upwards along the highest edge of this narrow ridge.

The track leads to this shallow bowl,

and there are others like it spaced out along the track.

They were excavated by the male kakapo whose territory this was.

In the night he would come here and crouching low,

would make a deep booming call which echoed out across these valleys,

summoning the females to come to him.

But there were no females seen after the 1970s up here.

One lone male continued trudging up here and calling...but in vain.

And in 1985, his call was heard no more.

When hope was almost gone, a new population of kakapos was discovered

on the southernmost island - Stewart Island.

They too were being harried by cats and stoats,

so the survivors were caught and taken to three small cat-free islets.

There were only 61 of them.

The kakapo's survival was on a knife edge.

A male, after slumbering all day in his burrow,

emerges for his evening meal.

His hearing is acute - he listens for danger.

He's following his regular track

that will lead him to the highest slopes of his mountain.

A female has clambered up into the top of the bushes,

looking for fresh shoots and fruit.

Her dappled plumage camouflages her against attacks from falcons,

but even so, she won't dare to venture into the topmost branches until it's dark.

Nightfall. Now we need our starlight camera to see what's happening.

By midnight,

the male has plodded his way right to the summit of his mountain.

He has reached one of his bowls

from which his call could echo out over the valley below.

He begins to tidy it up.

The female has found what she wants in the branches.

She'll need all the nourishment she can find if she is to produce an egg.

Even at the best of times, she will not accumulate enough bodily reserves to lay every year.

The male begins to inflate air sacs on his chest

that will act as resonators and so amplify his calls,

sending them booming out across the valley.

RHYTHMIC HISSING

DEEP HONKS

DISTANT HONKS

DEEP HONKING

There are probably only 12 fertile female kakapos left alive.

In the first ten years after they were moved to their new homes,

only three chicks were reared. But then in the last two seasons,

seven kakapo were successfully hatched.

Maybe they will come back from the brink of extinction after all.

Of course, only a minority of New Zealand's birds have become flightless.

Most, like these spotted shags, have retained that characteristic talent of birds -

the ability to travel by air.

Worldwide, birds have exploited that ability to an extraordinary degree.

Some can fly over 1,000 miles without landing, some can fly to altitudes of over 25,000 feet,

some can even fly backwards.

And how they manage to get into the air and sustain themselves there

is what we will look at in the next programme.

Subtitles by Gillian Frazer BBC Scotland 1998