Life on Earth

White storks. If you wanted to pick one bird

as a representative of all the birds in the world,

you could do worse than pick the white stork.

It's a marvellous flyer, an intrepid traveller.

This pair will have come from Africa to nest in this small town in Bavaria.

They have complicated courtship and greeting rituals

and they're devoted parents.

Just as they could stand for all the birds in the world, so this, a stork's feather,

could be seen as a key to everything that is most crucial about a bird.

The feather is a marvellous aerofoil.

Man has yet to invent anything as strong, weight for weight.

It's also an extremely efficient insulator, and that too is important to the bird.

And it's a very complicated structure.

This feather has several hundred separate filaments

on either side of the central quill.

They are held together to form one continuous surface

by several hundred thousand microscopic hooks.

The feather is the individual creation of the birds.

No other kind of animal possesses it.

And the oldest known feather in the world was discovered as a fossil

in the rocks only a few miles from here in Bavaria.

It's two and three-quarter inches long,

preserved in miraculous detail,

and to all intents and purposes it looks identical with a stork feather.

That feather had been found by men quarrying limestone near the little town of Solenhofen.

The stone had been laid down about 150 million years ago

at the bottom of a shallow lagoon,

and its fine, even texture made it ideal for use in lithographic printing.

That texture also makes it a superb preserver of fossils.

The feather tantalised the world of science.

What sort of creature could it have come from?

Well, immediately, there was a huge search mounted in these quarries.

Even now, it's almost impossible to resist the temptation

of pulling down almost every boulder you see

and then opening it like a book to look at each unopened page

to see whether maybe... it contains yet another fossil.

And after nearly a year, in 1861, in this very quarry,

they found what they'd been looking for.

A skeleton a foot or so long surrounded by feathers.

They called it archaeopteryx, "ancient bird".

Its skull was missing, but then another complete skeleton was found.

The creature had limbs of equal length,

so if it weren't for the feathers, it might have been taken for a four-legged runner

like a lizard.

Its head was reptilian, with tiny teeth along its bony jaws.

The front limbs had three toes apiece,

each toe ending with a claw.

The tail was supported by a bony rod, an extension of the backbone

running down the middle.

But all around, there are the miraculously detailed impressions of feathers.

In some places, it's possible to see the filaments on either side of a quill.

So those front limbs with claws are not the legs of a lizard,

but the wings of a bird.

So perfect are these fossils that we can make a pretty confident reconstruction.

But could this, the earliest of birds, flap its wings?

Its breastbone has no keel on it to carry large muscles,

so its wing-beats at best could only have been feeble.

It probably depended for the most part on gliding.

The long toes on its feet had a good grasp both forwards and backwards,

so it would have had no difficulty in perching on branches.

And those claws on its wing doubtless helped it to steady itself as it clambered around.

In all the world today, there is only one living creature with claws on its wings.

And this is it.

This is the hoatzin. Not an adult bird, but a chick.

It only possesses claws on its wings for a week or so.

While it does, it gives a vivid hint of how the first birds may have moved about

in their trees.

Maybe the ancestors of archaeopteryx took to the trees

because hungry reptiles roamed below looking for a meal.

The young hoatzin faces just such dangers today,

for its home in the swamps of Venezuela is haunted by crocodiles and cayman.

And reptiles very like them prowled swamps and forests 150 million years ago.

Although the adult hoatzins don't have claws on their wings,

they still have a very reptilian look to them,

with their glittering eyes surrounded with scaly skin

and an odd, bristly crest.

Nor do they seem to have mastered flight.

They can only cover short distances before they need a rest.

In the branches, they use their wings to help them keep their balance.

But they are true birds and much better adapted for flying

than archaeopteryx ever was.

Flyers must reduce their weight to a minimum,

and all birds today, including the hoatzin,

have lost the bony tail of the reptile and have instead a lightweight tail

constructed entirely of feathers.

Weight has been reduced at the front end too.

Bony jaws and teeth are particularly heavy, and no modern bird has them.

Instead, they have beaks of keratin,

the same horny substance as that from which feathers are made.

Keratin is not only light and strong, but it can easily be moulded into different shapes

and turned into the particular tool a bird needs to gather its food.

The hoatzin has an unusual diet.

It uses its beak to pluck leaves, which it then regurgitates for its chick.

Often the beak is elongated and used as a pair of forceps.

The white stork has a medium-sized model for picking up frogs and little fish.

Another stork, the shoebill, has a heavy-duty version for dragging lungfish from mud,

and the crane a short pair with which to pick up seeds and insects.

The butchers among birds have turned their beaks into hooks

with which to tear flesh: vultures.

The monkey-eating eagle from the Philippines.

Others, like the scarlet ibis, have long probes

that can pick out small invertebrates from their burrows.

The beak of a spoonbill is not in fact a spoon but a sieve,

with which its owner collects small creatures from the water.

The flamingo's bill is a filter pump.

The tongue works like a piston, drawing water in and squirting it out several times a minute.

Coarse hairs on the side of the bill prevent mud getting in,

and hairs inside trap microscopic plants and animals.

Pelicans.

Some dive on shoals of fish.

Others use their baggy bills as nets, and go fishing in teams.

Another fisherman.

Little fish, it seems, are eager to swim into the shade.

The black heron provides it with its wings

and then stabs with precision at any fish attracted to it.

The beak is not only a feeding tool.

It's also essential for keeping in trim those most valuable of a bird's possessions,

its feathers.

They need a lot of maintenance to be kept in good condition,

and the beak, even the biggest, is used with great delicacy

to preen the feathers one by one,

repairing breaks in their veins by zipping up the hooks on the filaments.

Storks, like most birds, have a preen gland on their rump.

They collect oil from it with their beaks and use it to waterproof feathers.

Waterproofing is particularly important for waterbirds,

and water only flows off a duck's back because its feathers are well oiled.

But being waterproof does cause problems.

Below water, unwetted feathers hold air trapped between them like a silver sheath,

and that's very buoyant, so ducks and ducklings have to paddle hard with their legs

to prevent themselves from bobbing back to the surface.

The business of getting the beak down into the mud

is made easier by having legs far back on the body, near the tail.

All the same, it's hard work.

As well as oiling their feathers, many birds, like the peacock,

ruffle them in dust.

The skin beneath the feathers makes an attractive home

for parasites like fleas and lice,

and dust bathing helps get rid of them.

Another way is to enlist the help of other insects: ants.

Many ants, when irritated, squirt acid,

and that will shift most insect parasites.

Jays and crows are particularly addicted to this habit.

By spreading its wings, the bird makes it easier for angry ants to swarm over its skin

between the feathers.

Often, the bird seems to enjoy the process so much that it goes into a kind of ecstasy.

So birds go to a lot of trouble to keep their feathers in good condition.

In truth, their lives depend on them, not only for support in the air.

Birds are warm-blooded.

They have to be to produce the energy they need for their active lives.

So insulation for their bodies is essential,

and nothing does it better than a coat of feathers.

Only a bird wrapped in a coat of fine feathers

can live in the coldest place on earth: the Antarctic ice cap in winter.

Some species of penguin survive temperatures of 40 degrees below freezing.

And for weeks on end.

The feathers of both adult and chick are specially adapted to provide warmth.

They're very fine and grow in a thick mat.

But the feathers of most birds have to serve another purpose: flight.

If beauty comes from perfection, if grace is a measure of skill,

then a bird in the air must be among the loveliest sights in nature.

Terns are among the most graceful of flyers,

responding to every tiny variation in the wind currents

with subtle adjustments to the contours of the tail and wings.

The tropic bird, by using updraughts and working hard,

can hang in the air to display to its mate.

Ocean-going birds like the frigate sail on long, thin wings,

the best shape for efficient gliding.

One of the most skilled gliders of all, the albatross,

which beats up and down the ocean supported by the lightest of breezes

with only the gentlest of flaps of its wings.

But long wings for gliding flight don't make for easy landings.

Having missed its touchdown, the booby has to labour to regain speed and avoid a stall.

The frigate bird has similar problems but solves them successfully.

The booby has another try.

There are helpful winds to be exploited over land as well as sea,

and above wild, mountainous country like the Grand Canyon

where the sun heats the rock,

there are hot thermal currents that sweep up the cliff faces.

So vultures can sail and soar with an economy that rivals that of the albatross.

Such birds as these, sliding effortlessly through the air, can reach great speeds.

But the airspeed record is held by a much smaller flyer.

The swift.

Here, it is in slow motion.

One species is said to reach speeds in level flight of 170km an hour.

The swift is the most aerial of birds, hardly alighting except to nest,

mating and even sleeping on the wing,

and every day flying up to 1,000km to gather insects from the air.

Other birds also feed on the wing,

and just as ground feeders have beaks modified for their diets,

so many of these flying hunters have beaks suited to their own techniques.

And what's more, ones that require great acrobatic skill to manage properly.

The skimmer has the oddest of beaks.

It's the only bird with the lower mandible longer than the upper.

To use it requires perfect flight control.

When the lower mandible strikes an object like a fish, the beak automatically snaps shut.

Some say there's more to it than that.

The furrow cut in the surface of the water sparkles and attracts surface-feeding fish.

Having made one run, the skimmer often turns and flies back along the same course,

perhaps to collect anything that has taken the lure.

In the Andes live other birds with superb aerial control