The Life of Birds - To Fly or Not to Fly
Transcript
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01:06The birds are the most accomplished aeronauts the world has ever seen.
01:11They fly high and low, at great speed and very slowly,
01:17and always with extraordinary precision and control.
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02:18But birds are not the only creatures in the air.
02:21There are also small furry mammals, bats like these in Texas.
02:27They are so competent in the air
02:29that they have just made a journey from Mexico, a thousand miles away,
02:34simply in order to rear their young in this cave,
02:37which is particularly suitable for them as a nursery.
02:41Just now, they're flying out to catch their evening meal of insects.
02:45But they had better be careful, because in the skies above them
02:49there lurks a creature that can outfly them.
02:52It is, of course, a bird, a red-tailed hawk.
02:58Bats, with their fluttering zigzag flight, are not easy targets,
03:02and a hawk needs all its aerobatic skills and powers of concentration
03:07if it is to snatch one out of the confusing multitude.
03:12That is one bat that will not return to the roost tonight.
03:20The red-tailed hawk lives beside the cave
03:23and is one of the few birds in the air.
03:26But it is not the only bird in the air.
03:29There is another bird in the air,
03:31but it is not the only bird in the air.
03:34It is a red-tailed hawk.
03:37The red-tailed hawk lives beside the cave
03:40and is well practised in bat catching.
03:46This prairie falcon, on the other hand, is a visitor,
03:50but it's learning fast.
03:53Unlike the hawk, it chooses to eat its meals on the wing.
04:03Bats are late comers to the skies.
04:06They've only been flying for a mere 60 million years.
04:10The air was full of them,
04:12but they've only been in the air for a short time.
04:15They've only been in the air for a short time.
04:18They've only been in the air for a short time.
04:21Yes, the air was first colonised 200 million years earlier still
04:26by the insects,
04:28but now they can't escape the birds either.
04:52Some insects, of course, have powerful weapons
04:55with which to defend themselves.
04:58But a bee-eater certainly knows how to deal with a bee.
05:03It's a bit of a pain in the arse.
05:06It's a bit of a pain in the arse.
05:09It's a bit of a pain in the arse.
05:12It's a bit of a pain in the arse.
05:15It's a bit of a pain in the arse.
05:18A bee-eater certainly knows how to deal with a bee.
05:32A rub against the perch usually discharges the sting.
05:37And if that doesn't,
05:39then a sharp nip will squirt the venom harmlessly into the air.
05:49BIRDS CHIRP
06:05Dragonflies first flew around 350 million years ago
06:10and insects had the skies to themselves for 150 million years thereafter.
06:17And then a different kind of animal joined them in the air.
06:23As the dinosaurs dominated the land,
06:26so the pterosaurs now ruled the skies.
06:34Pterosaurs had wings of skin
06:36stretched between one enormously elongated finger and their flanks.
06:42They flew over the sea as well as the land.
06:51It seems likely that some roosted on cliffs
06:54and launched themselves into the air as gannets do today.
07:06They probably snatched fish from the surface of the sea
07:09and some certainly fell into it.
07:21Their bodies were buried by mud.
07:23The mud turned to limestone
07:25and eventually became exposed in great quarries
07:28like this one in southern Germany.
07:32Today, separating the layers of sediment
07:35is just like searching through the pages of a visitor's book
07:39that hasn't been opened for 150 million years.
07:44Of course, nearly all the pages are absolutely blank.
07:48Visitors, after all, were very few.
07:51But every now and again, you come across a signature that is unmistakable.
07:56A fish the size of a sardine.
08:00A shrimp, even its antennae perfectly preserved.
08:05One of those pioneering dragonflies, nearly six inches across.
08:09And a pterosaur, with skinny wings and teeth in its jaws.
08:14With so many superb fossils,
08:16people thought that they had a complete list of the visitors.
08:20And then, in the middle of the last century,
08:22a signature was discovered that was wholly unexpected
08:26and totally amazing.
08:28This is it.
08:30It's a feather.
08:32Its barbs are narrower on one side of the quill than the other,
08:35just as they are on the feathers of a modern bird's wing.
08:39This asymmetry is a sure sign that such feathers exist.
08:44But what animal at the time of the dinosaurs could have such a wing?
08:48The answer was found the very next year in the same quarry.
08:52A fossil with its feathers still attached to its body.
08:59This is Archaeopteryx.
09:04It had three toes,
09:06and it was the largest bird of all time.
09:09Archaeopteryx.
09:14It had three toes, armed with claws and long, strong legs.
09:19Clearly, it walked and perched like a bird.
09:24But its head was very reptilian, with bony jaws.
09:28And in those jaws, teeth.
09:32Its spine was extended into a bony tail,
09:35again like that of a reptile.
09:38But on either side of the tailbones, clearly visible,
09:41it had those characteristic possessions of birds' feathers.
09:47Feathers are made of keratin,
09:49as are the scales that many birds still have on their legs
09:52and reptiles all over their body.
09:55A scaly coat must be very hot,
09:57so reptiles, like this skink,
09:59have to seek shade during the hottest part of the day.
10:04But if the scales became fibrous,
10:06they could be fluffed up to let in cooling air during the day
10:10and closed down to trap insulating air for warmth at night.
10:15So it's not difficult to believe
10:17that scales eventually became transformed into feathers.
10:20But why should they have become so long
10:23that they enabled an animal to take to the air?
10:27Well, this Australian lizard suggests an answer.
10:31When it is threatened by its enemies,
10:33it responds by spreading the great frill it has around its neck.
10:40But if that doesn't scare them off, it runs away on its hind legs.
10:46If such a reptile had developed feathery scales on its forelegs
10:50and then spread them out,
10:52then it might easily lift into the air
10:55and so escape a land-bound predator.
11:03There's another possibility.
11:05Maybe that early reptile did not live on the ground
11:08but climbed in the trees searching for food,
11:11as today the little flying lizard of Borneo does.
11:14It now glides from tree to tree.
11:17It has developed wings that are flaps of skin,
11:20supported by elongated ribs.
11:28If that early enterprising reptile with feathery scales
11:32did have specially long ones on its arms,
11:35then they too would have enabled it to glide from tree to tree.
11:39And maybe its arm muscles were even strong enough
11:42to allow it to make its way up the trees.
11:47It would take a few flaps to help it on its way.
11:52Archaeopteryx was certainly well-equipped for climbing,
11:55for its wings still carried three fingers,
11:58each ending with a hooked claw, ideal for clinging on to twigs.
12:02And there are birds today with very similar ones
12:05that give a clear hint as to how it might have used them.
12:09These are young hoatzin,
12:11sitting on their nest in the South American swamp,
12:14still guarded by their parents.
12:16A hoatzin chick has an adventurous disposition
12:19and starts clambering about when it's only a few days old.
12:30The hooks on its front limbs are obviously very useful
12:34in keeping it secure until such time
12:36as they become feathered and reliable wings.
12:39One can imagine that archaeopteryx used them in much the same way
12:43and for much the same reason.
12:47Sometimes, however, there's a disaster.
12:53There are dangerous reptiles in the swamp, snakes and caiman.
13:00But those claws on the wings are once again invaluable.
13:08Mother returns.
13:10She's been feeding on leaves and will have a full crop.
13:13Maybe there will be some for the chick.
13:30The hoatzin, of course, like all modern birds,
13:33doesn't have bony jaws with teeth like archaeopteryx,
13:36but a lightweight beak.
13:38When did that important change take place?
13:44Well, this fossilised bird has a beak
13:47and it was found in China recently.
13:49It's only a little younger than archaeopteryx,
13:52so it seems that the change took place very quickly
13:55and it must have made flight much more efficient,
13:58for it prevented the bird from being nose-heavy
14:01and significantly reduced its overall weight.
14:04By 50 million years ago,
14:06the dynasty of birds was firmly established.
14:09At that time, a great lake lay here in central Germany.
14:13It's long since dried out
14:15and the layers of mud from its floor have turned into shales.
14:22Excavations like the one that's going on here
14:25have revealed just how varied the birds have become.
14:30This one has been set in yellow resin to make its details quite clear.
14:35It had a horny beak, a fully feathered wing,
14:39a long feathered tail with no bony support,
14:43and long legs.
14:45It probably looked like a rail.
14:51Other fossils from these shales
14:53show that several families of modern birds were already established.
14:57This was a water bird, possibly an ancestor of today's jacana.
15:01It would certainly have found plenty of insects
15:04among the floating leaves on the lake.
15:07There were birds with powerful chisel-like bills.
15:10Perhaps they were woodpeckers that even in this early period
15:13had started excavating insects from the trees in the surrounding forest.
15:20Another inhabitant of those prehistoric woods
15:23had a stubbier, more all-purpose beak, rather like finches do today.
15:27There were tall birds with long, powerful legs
15:30that probably hunted for small reptiles on the ground,
15:34as the South American sariyama does.
15:45And there was a gigantic vulture with a wingspan of over 20 feet,
15:50bigger even than that of the Andean condor
15:53and probably the biggest flying bird that has ever existed.
16:05There were even birds which, judging from their skeletons,
16:08were as agile in the air as their probable descendants, the frigatebirds.
16:15So, by 50 million years ago,
16:19several families of modern birds were well established.
16:24The rule of the reptiles was now really over.
16:27Not only had pterosaurs disappeared from the skies,
16:30dinosaurs had gone from the ground.
16:33So the dominance of the land was up for grabs.
16:36There were two contenders, the mammals and the birds.
16:40The biggest mammal to be found from this lake in Germany
16:44was the primitive horse.
16:46It was no bigger than the spaniel.
16:48But the biggest bird was very different.
16:51That was the lower part of his beak,
16:54and this was the upper.
17:01If its skeleton still lay here,
17:03you would have to dig a huge pit to extract it.
17:07This bird was immense.
17:10It has been named, with good reason, the terror bird.
17:17Its wings were tiny, but it had long and powerful legs.
17:22Flightless birds of a comparable size still exist
17:25and could give us some idea of what it looked like.
17:30But what was it?
17:32What was it?
17:34What was it?
17:36What was it?
17:38What was it?
17:40What was it?
17:42What was it?
17:44What was it?
17:46What was it?
17:48What was it?
17:50We have a better idea of what it looked like.
17:53This, the ostrich,
17:56is the biggest and heaviest bird alive today.
17:59It's probably not closely related
18:02to those monstrous feathered hunters of prehistory,
18:05but, together with the emu of Australia
18:08and the rear of South America,
18:10it belongs to a very ancient family of birds
18:14that abandoned flight a very long time ago.
18:20It relies for its defence on speed.
18:23And in the interests of its efficiency as a runner,
18:26its toes have been reduced to two.
18:29If pursued, it can sprint at over 40 miles an hour.
18:42But although it is now flightless,
18:45it still has many of the physical characters
18:47evolved by its ancestors that enabled them to fly.
18:52It still has feathers,
18:54and they're still placed on its wings in much the same position
18:57as those on the wings of a flying bird.
19:01But they are now useless for flight.
19:04Their filaments have lost their hooks,
19:06so they can no longer be zipped together into an unbroken blade.
19:10Instead, they are loose and fluffy.
19:13Their only function now is as insulating blankets,
19:17to keep out the cold at night and the heat during the day.
19:24Ostriches have become grazers,
19:27the bird equivalent of antelope or horses.
19:39But unlike them, they not only pick up leaves,
19:43they swallow all kinds of other things as well.
19:46And for a very good reason.
19:49Just as they inherited feathers from their flying ancestors,
19:53so they also inherited a lightweight, horny beak,
19:57instead of a heavy jaw laden with teeth.
20:00And without teeth, they need another way to grind up their food.
20:07A pebble can help them do just that.
20:11Down it goes into a muscular compartment of the stomach, the gizzard,
20:16a kind of mill where bits of vegetation are churned around
20:20and ground into a digestible pulp.
20:26But while some birds were abandoning flight,
20:30some mammals were becoming formidable hunters.
20:34Ostriches, with their superb eyesight and tall necks,
20:37are able to keep a sharp lookout for approaching danger.
20:46The cheetah has to calculate very carefully
20:49whether it's worthwhile chasing something.
20:51And an ostrich usually is not.
20:54It is so careful that it can see the prey and not the prey.
20:58It's not a cheetah, it's a cheetah.
21:01An ostrich usually is not.
21:03It is so very fast, and even if it's caught,
21:05it has little meat on it compared with a similar-sized mammal.
21:11But birds that can't run are a tempting target for hunters everywhere.
21:25If they couldn't fly, they wouldn't last long.
21:31CHEETAH GROWLS
21:33CHEETAHS BARK
21:35CHEETAH GROWLS
21:37CHEETAHS BARK
21:39CHEETAHS BARK
21:41CHEETAHS BARK
21:43CHEETAHS BARK
21:45CHEETAHS BARK
21:47CHEETAHS BARK
21:49CHEETAHS BARK
21:51CHEETAHS BARK
21:53CHEETAHS BARK
21:56CHEETAHS BARK
21:58CHEETAHS BARK
22:00CHEETAHS BARK
22:02CHEETAHS BARK
22:04CHEETAHS BARK
22:07CHEETAHS BARK
22:09CHEETAHS BARK
22:11CHEETAHS BARK
22:13CHEETAHS BARK
22:15CHEETAHS BARK
22:17Flight has certainly enabled birds to colonise the entire globe.
22:22But the compulsion that drove them into the air in the first place,
22:25and has certainly kept most of them there ever since,
22:28was probably safety.
22:30Even so, the cost of flying is high.
22:36Flapping wings takes a lot of effort.
22:39And if there's no need to do so, birds save their energies.
22:43Those that live here on the Galapagos Islands,
22:46isolated in the Pacific,
22:47have no natural enemies from which to escape.
22:51So some birds don't bother to fly.
22:55Have a look at these cormorants, for example.
22:59At first sight, they look like many another cormorant
23:02to be found sitting on cliffs around the world.
23:06But these wings are stunted and tattered.
23:14This bird could never get into the air.
23:16Its feathers now serve only to keep it warm in the water.
23:20It's for that reason alone that it keeps them well-oiled.
23:31It's scarcely any better at walking than it is at flying.
23:51Once in the water, however, it's a very efficient mover indeed.
24:13The position of its legs right at the back of its body
24:16that allows it to fly is a bit of a challenge.
24:19The wing right at the back of its body, that made it so clumsy on land,
24:23is ideal for propelling it through the water at speed
24:26and helps it to catch all the fish it needs.
24:34For the Galapagos cormorant, flight has become an irrelevance.
24:41Other island birds have reacted in a similar way.
24:44This is New Caledonia in the western Pacific
24:47and this is its special bird, the cagou.
24:51Its ancestors must certainly have arrived here by air,
24:55but since New Caledonia had no ground predators until recently,
24:59they gave up flying and today the cagou is virtually flightless.
25:07It finds all its food on the ground in the leaf litter.
25:11It's been here so long that it's difficult to be sure
25:13exactly who its ancestors were,
25:16but they were probably herons.
25:37Rails are a very widespread family of birds.
25:40Wherever there's a big swamp, you're likely to find one.
25:44On the continents, they tend to lurk shyly in the undergrowth
25:47to keep out of trouble,
25:49but some, somehow, have also managed to reach a great number of islands
25:53and there they seem to have no fear at all.
25:57This one, it's a weka,
26:00has also become flightless because of its isolation on an island.
26:07But its island is immense.
26:11It's 1,000 miles long,
26:13if you discount a narrow arm of sea that crosses it in the middle,
26:17and it contains mountains over 12,000 feet high.
26:21It's New Zealand.
26:24The first land-living mammals to get here were human beings
26:28and they didn't arrive until a mere 1,500 years ago.
26:31So here you can still glimpse what the world would have been like
26:35if the birds had won that battle with the early mammals
26:38and now ruled the Earth, for here, they once did.
26:49Many of New Zealand's birds flew here from Australia,
26:521,500 miles away across the sea to the west.
26:56They started to do so millions of years ago
26:58and they're still doing so today.
27:08So, if you know Australian birds, you will recognise quite a lot,
27:13particularly those that are relatively recent arrivals.
27:20The New Zealand pigeon is not all that much different from Australian ones.
27:27The saddleback, on the other hand,
27:29must have been established here for much longer,
27:31for it has changed so much
27:33that no-one is quite sure what family it belongs to.
27:38The tui has similarly mysterious origins.
27:42No other bird has a costume to compare with its lacy cape
27:46and that little white throat bubble.
27:57The kakai is clearly a parrot.
27:59There are lots of parrots in Australia,
28:01so it's not surprising that some of them in the past
28:04should have found their way here.
28:09Most of these birds have still not learned that mammals are dangerous.
28:16This saddleback is a fully wild bird and certainly hasn't seen me before.
28:21But look how trusting it is.
28:33This is a New Zealand robin.
28:35It's no relation to the European robin
28:38and, if anything, it's even braver.
28:49The New Zealand bush is full of food of one kind or another.
28:53And as the birds once had it all to themselves,
28:56some were able to adopt diets and ways of life
28:59that elsewhere were claimed by mammals.
29:02The kokako eat much the same thing as squirrels.
29:06Fruit and leaves and insects.
29:13European squirrels run along the branches.
29:17Asian squirrels, using a skinny parachute stretched between their legs,
29:22are able to glide as well.
29:24And that is very much how the kokako gets around in the trees.
29:36KOKAKO
29:43Having glided down to the lower branches,
29:46it runs back up them and jumps from one to the other.
30:07KOKAKO
30:14So, if the kokako up in the trees
30:17feeds in the same way as a squirrel might do,
30:20what lives and feeds like a mammal on the ground?
30:25The leaf litter in these forests is full of food of one kind or another.
30:30There are earthworms and insects and beetle larvae.
30:35In any other land, there would be some small native mammal
30:39that was burrowing around, seeking that food.
30:42But not here in New Zealand.
30:44Here there's something quite different.
30:48You will only see it after dark.
31:00It is, of course, a bird.
31:02But what an extraordinary one.
31:05The kiwi.
31:10It's territorial and calls stridently to proclaim its ownership.
31:27It finds its prey by smell.
31:29Uniquely, its nostrils are on the tip of its beak.
31:36It's found a worm.
31:40But once it drops it, its eyesight is so poor that it can't see it
31:44and it has to smell for it with its beak.
31:53Its tiny vestigial wings are invisible, buried in its plumage,
31:58and it's lost all sign of a tail.
32:06If the kiwis live in a patch of forest close by the sea,
32:09then in the evening, they may come down onto the beach to look for these.
32:16Sandhoppers. They love them.
32:19And that will give us a chance, a rare chance, to see them out in the open.
32:28To do so properly, we have to use our special starlight camera.
32:36The kiwi is hunting along the strand line,
32:39where there are lots of hoppers feeding on the decaying seaweed.
32:46Its sense of smell is so acute,
32:48it can pick out the largest, juiciest hoppers
32:51deep in the sand without even seeing them.
32:54It's a very, very, very, very, very, very, very, very, very, very, very,
32:59very, very, very, very, very, very, very, very, very, very, very, very,
33:03without even seeing them.
33:08Our starlight camera can see much better than I can.
33:12I need a torch to see this extraordinary creature properly,
33:16but it doesn't seem to mind.
33:30Its feathers are just filaments,
33:32so that it almost looks as though it's covered with coarse fur.
33:44Probing sand with your nostrils is all very well,
33:47but it does clog them up,
33:49and so you need to blow them clear every now and then.
34:03It's nocturnal and furry.
34:05It finds its way around by smell.
34:08It lives in holes and digs for worms and grubs.
34:11It's a bird equivalent of a badger.
34:23But there's plenty of other food to be found in the New Zealand bush.
34:27Here on the forest floor, there are a lot of birds.
34:31On the forest floor, there are lots of leaves.
34:34They may be a bit twiggy and coarse, but they're food nonetheless.
34:38What could have browsed on these?
34:43Well, not far from here, bones like this have been dug up.
34:48It's obviously a leg bone,
34:50and at first sight, you might think it was the leg bone of a mammal,
34:53say, perhaps a cow.
34:55But when you look at it closely, you can see it's got a honeycomb structure.
34:59So, this is a very lightweight bone.
35:01It's a bird bone, but the bone of a very big bird indeed,
35:06as we know from the rest of its skeleton.
35:17It had just three toes.
35:20Its pelvis and its spine lead up to an extraordinarily long neck.
35:30This bird stood over six feet, two metres tall.
35:40The first human settlers on these islands saw these giants alive
35:44and called them mowers.
35:46Among them were the tallest birds that ever existed,
35:49that weighed over 200 kilos, 400 pounds.
35:53There were about a dozen different species of varying size and weight.
35:59MOWER
36:12Up on the high moorlands,
36:14there were smaller species with thicker feathers to keep them warm.
36:21The absence of mammals didn't mean that the mowers had no enemies.
36:25They were hunted by, of course, another bird.
36:37An immense eagle that could manoeuvre through the patchy forest.
36:42The only prey that was abundant enough to sustain such a giant
36:46were other birds,
36:48and it's probable that it was able to tackle even the biggest mower.
36:52Its talons were certainly long enough to stab right through a mower's flesh
36:56and into its pelvis, as some of the bones show.
37:02Nevertheless, the mowers survived for a million years or more
37:06and spread all over New Zealand.
37:08But eventually, mammals did reach these remote islands.
37:13Apart from bats, which flew here,
37:15the first to arrive were those most dangerous of all, human beings.
37:20They hunted the mowers for meat,
37:22and soon they had hunted them to extinction.
37:26But a different kind of flightless bird
37:28does still survive up in these high mountains.
37:35Like so many of New Zealand's native birds
37:37that had abandoned flight and nested on the ground,
37:40it had no defence against the alien mammals
37:43that Europeans brought with them,
37:45and that soon escaped and ran wild.
37:48Rats ate their eggs and killed the chicks,
37:51and cats and stoats massacred the adults.
37:58There was a giant flightless cook that was originally very common,
38:03but it got scarcer and scarcer,
38:05and by the middle of the 19th century, it was thought to be totally extinct.
38:10And then, just 50 years ago,
38:13someone in these remote valleys found something like this.
38:20This is the dropping of a takahē.
38:24And here, the severed stems of tussock grass on which it's been feeding.
38:30It's still here.
38:36And here is her nest.
38:43She's sitting tight,
38:46hiding her brilliant red bill so that she's not conspicuous.
38:59Indeed, when only her lovely moss-green back is visible,
39:04she is well camouflaged from her only native enemy,
39:07the giant fly.
39:13A bird of prey, circling overhead.
39:19Only about 40 pairs of takahē survive today in the wild,
39:23so the eggs she's sitting on are very precious.
39:27This high country was probably not the takahē's original home.
39:31Most of the population in the last century lived at lower altitudes,
39:35and there they had lush vegetation to feed on in the warm swamps.
39:40But most of those were drained and turned into farmland.
39:44So now these high, empty valleys,
39:46only recently scraped down to the rock by glaciers,
39:50are the takahē's last refuge.
39:52And there is little to eat up here except tussock grass.
40:11Extracting something nutritious from tussock is not easy.
40:15The takahē's technique is to pull up a whole stem
40:19and then nibble just the bottom inch or so.
40:22That's where most of the minerals and sugars are,
40:25and it's the only bit tender enough to be easily cut.
40:41At first, the chick doesn't know how to do this and has to be fed.
40:45It will stay with its parent for a whole year.
40:49Only then will it be strong enough and skilled enough
40:52to feed entirely by itself.
40:59Now it's summer, but when winter comes, life will be even tougher.
41:04Then the pools freeze over and the takahē's eggs hatch.
41:10The tussock has no fresh shoots,
41:12even if they could be reached beneath the snow.
41:15Then the takahē is reduced to digging for tubers in the freezing earth.
41:20So the bird's continued survival up here in these barren moorlands
41:24is by no means assured.
41:30One other sightless bird found refuge from mammals in these high mountains,
41:36and in many ways, it was the most extraordinary of all.
41:39It was a giant parrot, the kākāpō.
41:42It lived in very much the same way as rabbits do.
41:46It created tracks through its territory,
41:49generation after generation, trudging along here
41:53and feeding by plucking these grasses and eating the succulent base.
42:00This particular path runs under this bush
42:04and continues upwards along the highest edge of this narrow ridge.
42:10The track leads to this shallow bowl
42:15and there are others like it spaced out along the track.
42:19They were excavated by the male kākāpō, whose territory this was.
42:25He would dig them out and then in the night he would come here
42:29and, crouching low, make a deep, deep hole
42:33in the ground where he would feed the kākāpō.
42:36He would come here and, crouching low, make a deep, booming call
42:40which echoed out across these valleys,
42:43summoning the females to come to him.
42:49But there were no females seen after the 1970s up here.
42:53One lone male continued trudging up here and calling, but in vain.
43:00And in 1985, his call was heard no more.
43:07And then, when hope was almost gone,
43:10a new population of kākāpōs was discovered
43:13on the southernmost island of all, Stewart Island.
43:16They, too, were being harried by cats and stoats,
43:20so the survivors were caught and taken to three small, cat-free islets.
43:26There were only 61 of them.
43:28The kākāpō's survival was on a knife edge.
43:32A male, after slumbering all day in his burrow, emerges for his evening meal.
43:40His hearing is acute. He listens for danger.
43:48He has a keen sense of smell,
43:50and he can tell the difference between a cat and a kākāpō.
43:54His hearing is acute. He listens for danger.
43:58He listens for danger.
44:19He's following his regular track
44:21that will lead him to the highest slopes of his mountain.
44:29A female has clambered up into the top of the bushes,
44:33looking for fresh shoots and fruit.
44:39Her dappled green plumage camouflages her against attacks from falcons,
44:44but even so, she won't dare to venture into the topmost branches until it's dark.
44:51Nightfall, and now we need our starlight camera to see what's happening.
45:00By midnight, the male has plodded his way right to the summit of his mountain.
45:11He has reached the summit of the mountain,
45:14but he's not quite there yet.
45:20He has reached one of his bowls,
45:22from which his call could echo out over the valley below.
45:27He begins to tidy it up.
45:37The female has found what she wants up in the branches.
45:41She will need all the most nourishing food she can find if she is to produce an egg.
45:46For the rest of times, she will not be able to accumulate enough bodily reserves to lay every year.
45:59The male begins to inflate air sacs on his chest
46:03that will act as resonators and so amplify his calls,
46:07sending them booming out across the valley.
46:47There are probably only 12 fertile female kakapo's left alive.
46:53In the first ten years after they were moved to the safety of their new homes,
46:57only three chicks were reared.
46:59But then, in the last two seasons, seven young kakapo were successfully hatched.
47:05Maybe the species will come back,
47:08but we don't know for sure.
47:10Seven young kakapo were successfully hatched.
47:13Maybe the species will come back from the brink of extinction after all.
47:26Of course, only a minority of New Zealand's birds have become flightless.
47:31Most, like these handsome spotted shags,
47:34have retained that characteristic talent of birds, the ability to travel by air.
47:40And worldwide, birds have exploited that ability to an extraordinary degree.
47:45Some can make journeys of over 1,000 miles without coming down to earth.
47:49Some can fly to altitudes of over 25,000 feet.
47:53Some can even fly backwards.
47:56And how they manage to get into the air and sustain themselves there
48:00and do all those things is what we'll be looking at in the next programme.
49:00Subtitling by SUBS Hamburg