♪♪ ATTENBOROUGH: I've recorded many songs of the natural world over the years.

Just listen to this.

And I've learned that there is, perhaps surprisingly, no scientific definition of song.

[ Bird singing ] And there are some songs that it's impossible for me to imagine life without.

But this avian choir does not sing for us.

What we hear is not what the bird hears.

Birds live on a different timescale, and they can hear details in their song that are impossible for us to hear.

It was the ability to take recorded sound and then turn it into a visual picture that enabled us to analyze that sound and reveal its full complexity.

DALZIELL: It's extremely exciting.

And it really forces us to reconsider what we think of as song.

KREBS: Our understanding of song is continually developing.

ATTENBOROUGH: How exciting it is to think of the discoveries that are about to be made.

[ Bird singing ] ♪♪ ♪♪ [ Birds singing ] [ Singing continues ] ATTENBOROUGH: On a clear spring morning like this, the dawn chorus is at its peak.

[ Singing continues ] There are surely few more enchanting natural soundscapes than this.

[ Singing continues ] But this avian choir does not sing for us.

[ Singing continues ] These are songs of seduction... and weapons of war.

Males are defending territories and attracting mates.

[ Birds singing ] Singing is dangerous.

[ Singing continues ] It reveals the bird's location to predators.

But it also offers a huge reward -- the chance to attract a female and pass on genes to the next generation.

That, Charles Darwin said, is why song evolved.

[ Singing continues ] It was an example of what he called sexual selection.

[ Singing continues ] But today, new discoveries are transforming those long-held ideas.

For this program, I have chosen some of my favorite recordings from the natural world that have revolutionized our understanding of song.

♪♪ There are seven recordings of it that have particular interest for me.

Some are lovely, some are surprising, and one almost broke my heart.

But all of them broke new ground.

[ Birds singing ] I've recorded many sounds of the natural world over the years, and I've learnt that there is, perhaps surprisingly, no scientific definition of song.

♪♪ We tend to use the word to describe sounds that seem, to our ears, beautiful.

[ Birds singing ] It is, in truth, a somewhat whimsical label.

But it has been attached to some of nature's greatest marvels.

Just listen to this.

[ Whales singing ] ♪♪ [ Singing continues ] ♪♪ Each of my seven chosen songs was recorded in my lifetime.

[ Bird singing ] The oldest was made when I was 5, the most recent, just a few years ago.

[ Birds singing ] None is closer to my heart than the first one, which I made back in 1960.

[ Laughs ] [ Indris singing ] ♪♪ Recording audio in the field in remote parts of the world was almost unheard of.

There was certainly no money in the BBC budget for a sound recordist.

But I did manage to get hold of a very rare thing indeed, a battery-driven portable tape recorder, so that I could record sounds myself.

♪♪ It was a cumbersome piece of kit, but cutting-edge at the time, and I was determined to use it to record a singer that no one had ever recorded before -- Madagascar's largest lemur, the indri.

[ Birds chirping ] [ Indris singing ] This noise was no bird call.

I had never heard anything like it before.

It must be the voice of the indris.

[ Singing continues ] Using my new equipment, I made the first-ever audio recording of the indri.

[ Singing continues ] But could we also capture them on camera, as well?

The song was so loud that it seemed impossible that the animals could be more than 20 or 30 yards away.

But where were they?

[ Singing continues ] Until now, no one had even managed to photograph a living one.

let alone film it.

Infuriatingly, the bush was so thick that I could see no sign of them whatever.

[ Singing continues ] So, the question was, how could we get close enough to get a clear view of them without frightening them?

Well, I thought, "What about doing it the other way around and trying to persuade them to get closer to us by playing their calls?"

[ Birds chirping ] [ Indris singing ] And they did exactly what I hoped they would do.

[ Indris singing ] They called in return, came down close to us, stared at us, still calling.

[ Singing continues ] I was thrilled.

We had recorded their song and filmed them singing.

[ Singing continues ] But why had this trick worked?

[ Singing continues ] Well, because they thought that the song I was playing meant a competitor was close by.

And their response was to sing.

[ Indris singing ] And this suggested one thing -- there are such things as battle songs... [ Singing continues ] ...songs that say, "Get out, this is my territory."

[ Singing continues ] ♪♪ It seemed to be a clear example of Darwin's theory of sexual selection -- a male singing to defend his breeding patch.

But this was still just guesswork.

We suspected that songs could be weapons of war.

But it was the next recording that proved it.

♪♪ [ Great tit singing ] [ Great tit singing ] This is the song of a male great tit, a call, a very simple song.

[ Singing continues ] It was recorded by a scientist who's been a pioneer in understanding animal song -- Professor John Krebs.

He was the one who proved for the first time exactly why my playback trick in Madagascar had worked.

♪♪ [ Birds chirping ] Spring 1975.

[ Chirping continues ] Back then, John was a young lecturer at the University of Oxford, researching the birds in woodland just outside the city.

First he plotted the territories of all the pairs of great tits.

Next, he recorded the songs of the males.

[ Great tit singing ] Female great tits do not sing.

KREBS: Then, on a particular morning in February, I came out with mist nets to catch the birds, and I caught all the pairs of birds that were in the wood.

ATTENBOROUGH: Now there were no great tits, just empty great tit territories.

In some, John replaced the great tits with loud speakers that played the sounds of the birds that he'd temporarily removed.

[ Great tit singing ] If the song was a "keep out" signal, then new great tits would avoid these patches.

[ Singing continues ] Other territories were left silent.

No speakers, no great tit song.

New great tits should realize these spaces were empty and available and settle there quickly.

[ Great tit singing ] But how would John know that it was due to a great tit song that newcomers had been deterred?

Perhaps any sound would have had the same effect.

So in some territories he played a recording of a tin whistle.

[ Tin whistle playing ] He could have chosen anything, as long as it sounded nothing like a great tit.

KREBS: And then I watched to see what happened.

And new birds came into the wood because this is a prime breeding area, so birds love to come here to breed.

♪♪ ATTENBOROUGH: John discovered that the first territories that were taken were the silent territories and the ones with the tin whistle.

Crucially, the territories in which the great tit song was played remained empty of new great tits.

KREBS: By looking at the detail of that pattern of settling, I was able to show, experimentally, that song is an effective "keep out" signal.

[ Great tit singing ] ATTENBOROUGH: Professor Krebs' predictions were exactly right.

The song was a "keep out" signal to other great tits.

[ Great tit singing ] For the very first time, there was scientific evidence that song was being used to intimidate rivals, just as I had seen with the indris in Madagascar in the '60s.

How astonishing that it was only in 1975 that this was proved for the first time.

The song of the great tit had made history.

♪♪ [ Great tit singing ] [ Birds singing ] Open a window in spring, and these are the singers that serenade us... [ Singing continues ] ...songbirds.

They make up about half of the 10,000 species of birds in the world.

Five of my seven songs are sung by them.

[ Singing continues ] Birds have the most advanced vocal organs in the entire natural world.

We have our voice box at the top of our windpipe.

But their equivalent is at the base of theirs.

It's an organ called a syrinx.

They're the only creatures on Earth to have one of these.

And the syrinx of the songbird is the most complex of them all.

As breath passes through it, muscles contract and vibrate, creating the sounds we call song.

It can produce different notes from the left and the right sides simultaneously.

[ Bird singing ] It's like having two voice boxes that can operate at the same time.

[ Birds singing ] So this is how songbirds can perform such unparalleled feats of vocal gymnastics.

But working out why they do, is far more complex.

[ Singing continues ] Each note lasts just a tiny moment and then disappears.

[ Singing continues ] That presents anyone who wants to study it with a problem.

[ Birds singing ] But there are two relatively recent inventions that have helped us to capture songs.

It was with a gun mic like this -- inside its windshield -- that John Krebs caught the fleeting sound of the great tit.

But it was the ability to take that recorded sound and then turn it into a visual picture that enabled us to analyze that sound and reveal its full complexity.

[ Great tit singing ] This is an animation of the male great tit's song, generated by a machine called a spectrograph.

[ Great tit singing ] It changes the song into an image that's possible to read.

Slowed down, we can see that the great tit's song is relatively simple.

[ Great tit singing ] It's composed of two notes, one high and one low, repeated.

[ Great tit singing ] What we hear is very similar to what we can see on the spectrograph.

But compare that to the song of the male wren.

[ Wren singing ] What we hear is not what the bird hears.

Birds live on a different timescale, at a different pace from us.

And they can hear details in their song that are impossible for us to hear.

It's only when we use a spectrogram that these details are revealed.

A trill... [ Trill plays ] ...a connecting note... [ Note plays ] ...another trill... [ Trill plays ] ...and then a rapid burst of trills at the end.

[ Trills playing ] Now we can see that there are, perhaps, a hundred notes or more in a song that may last only a few seconds.

[ Wren singing ] [ Bird singing ] [ Singing continues ] What the spectrograms show are these songs are far more complicated and complex than we could possibly have imagined.

But why complicate things so?

The calls of the indri and the great tit work perfectly well.

The answer, of course, is sex.

♪♪ [ Birds singing ] ♪♪ That is the reason why so many birds have such complex mating songs.

[ Singing continues ] Females seem to prefer them.

The more intricate and detailed the song is, the better the male's chances.

[ Singing continues ] And my third song helps to explain why.

♪♪ [ Nightingale singing ] It's a recording of a nightingale that is my next chosen song.

[ Nightingale singing ] This is a male singing for a mate.

[ Singing continues ] Just as with the great tit, the females do not sing.

But they are the ones the males are singing for.

So, why do they prefer more complex songs?

[ Nightingale singing ] ♪♪ It's a question that has puzzled scientists for centuries.

♪♪ Charles Darwin's answer was that the females have an aesthetic sense.

After all, human beings appreciate beauty.

Why shouldn't other animals do so, too?

♪♪ But the question Darwin didn't answer was, why should the females have an aesthetic sense?

Here on the streets of Berlin, we might just find the answer.

[ Birds singing ] Spring attracts over 2,000 nightingales to this city each year.

They flock to Berlin because it's filled with green space, much of which has been left untamed.

And it's these wild places that they love.

[ Nightingales singing ] In one of its most popular parks -- a male nightingale.

[ Nightingale singing ] He, like the other males, arrived from Africa a few weeks ago and has spent his time singing to defend a breeding patch.

[ Nightingale singing ] But tonight, when the lights go out, he will change his tune... because the females have arrived.

[ Nightingale singing ] Nightingales migrate at night.

And when the females fly in under cover of dark, they are met with songs of seduction.

[ Nightingales singing ] Dr. Conny Landgraf made our recording of the nightingale's song.

[ Nightingale singing ] LANDGRAF: So, right now there's one male.

No.

There is a second male So, it's already two males in a vocal contest.

[ Singing continues ] What the females do is that they do not take the first male that comes their way, but they prospect and inspect different male territories.

And this is like a speed-dating that is actually going on in the middle of the night.

[ Singing continues ] ATTENBOROUGH: Each male sings his heart out.

And the females listen.

[ Ding! ]

Time's up.

What does the next potential mate sound like?

[ Ding! ]

Time's up again.

Does the next one have a better song?

[ Nightingale singing ] [ Ding! ]

[ Nightingale singing ] [ Ding! ]

Nightingales have extremely large song repertoires, up to 250 songs.

LANDGRAF: And also they are able to produce really challenging syllables and phrases, thinking of trill or buzz elements.

So, these are very fast, repeated single notes.

So, this is remarkable.

ATTENBOROUGH: The more complex his song, the more beautiful it seems to the female.

But why?

Does the female have an aesthetic sense, as Darwin suggested?

Conny and her team set up an experiment to find out.

First, they recorded males singing at the beginning of spring.

Then they set up cameras to record nightingale nests later in the season.

♪♪ Male nightingales play a crucial role in feeding chicks.

♪♪ [ Nightingale singing ] LANDGRAF: What we found in the end was that there was indeed a very strong correlation between the song and feeding rates at the nest.

So, it's like a promise that the males give to the females to be good fathers.

So, for example, a male could be saying, "Hey, there.

I'm a healthy, strong man.

"I know this area and all the best feeding places very well, so come settle down and mate with me."

[ Nightingale singing ] ATTENBOROUGH: Females, Darwin said, chose the males with the most beautiful songs.

And my third recording has demonstrated why -- better singers are better fathers.

[ Nightingale singing ] Song can be a promise of devoted parenthood.

[ Nightingales singing ] ♪♪ There are some songs that it's impossible for me to imagine life without... [ Bird singing ] ...songs that accompany our daily lives.

It's one of the most characteristic sounds and, to my ears, one of the most delightful of the English winter -- the song of the robin.

[ Robins singing ] These are the songs that Charles Darwin would have listened to as a boy.

And much of our research into song has centered on British species.

Indeed, for much of the last century, we thought birdsong originated here, in northern latitudes.

[ Birds singing ] But what if Darwin had been raised on the other side of the world?

Would his theories about song have been different?

[ Birds singing ] The forests of Australia.

[ Singing continues ] ♪♪ Here, scientists are in the process of changing many of our old ideas about song.

[ Singing continues ] They've found fossil and DNA evidence of early songbirds which show that this, in fact, is where song began.

[ Birds singing ] It was here in Australia that the ancestors of all songbirds first evolved.

[ Birds singing ] ♪♪ Song number four is from one of the original songbird's most remarkable descendants.

And it's one that amazes me every time I hear it.

[ Superb lyrebird singing ] It was recorded in the forests just outside Melbourne.

And was the first time that wild birdsong had ever been broadcast in Australia.

[ Superb lyrebirds singing ] The song of the lyrebird, in my view, is one of the most complex and beautiful in the whole of the natural world.

And what gives it its complexity, is the talent that the lyrebird has for mimicry.

Crimson parrot.

[ Superb lyrebird singing ] The kookaburra.

[ Superb lyrebird singing ] The descendants of that lyrebird, I'm happy to say, are still singing in those forests.

And I have been lucky enough to listen to one of them myself.

It was the 1990s when the BBC series "Life of Birds" took me to Australia.

♪♪ I wanted to film a lyrebird who was the most astounding mimic.

[ Superb lyrebird singing ] To persuade females to come close and admire his plumes, he sings the most complex song he can manage, and he does that by copying the songs of all the other birds he hears around him, such as the kookaburra.

[ Superb lyrebird singing ] He can imitate the calls of at least 20 different species.

[ Singing continues ] He also, in his attempt to out-sing his rivals, incorporates other sounds that he hears in the forest.

[ Superb lyrebird mimicking clicking ] That was a camera shutter.

[ Superb lyrebird singing ] [ Superb lyrebird mimicking clicking ] And again.

[ Superb lyrebird singing ] [ Superb lyrebird mimicking clicking ] And now a camera with a motor drive.

[ Superb lyrebird mimicking clicking, whirring ] [ Superb lyrebird singing rhythmically ] And that's a car alarm.

[ Singing continues ] And now the sounds of foresters and their chainsaws working nearby.

[ Superb lyrebird mimicking buzzing, clacking ] Since this remarkable clip was broadcast, millions of people have watched it online.

But scientists have discovered a lot more about lyrebird song since that recording was made.

That particular bird was accustomed to the presence of human beings nearby.

But much wilder birds produce something even more fascinating.

♪♪ Sunrise, Sherbrooke Forest today.

[ Birds singing ] One voice in the dawn chorus is louder than any other -- the male lyrebird.

[ Superb lyrebird singing ] ♪♪ It's the breeding season, and they're busy trying to attract mates.

♪♪ [ Superb lyrebird singing ] Clearing undergrowth from the forest floor creates an arena for his mating display.

[ Singing continues ] Once the stage is set, the show can begin.

[ Singing continues ] It's unlike any other in the natural world.

[ Singing continues ] Since I heard this song, scientists have analyzed it and broken it down into parts.

[ Superb lyrebird singing ] Song A comes first.

[ Singing continues ] Now, Song B, a loud, low shriek which alternates with Song C, a quiet, very soft clicking sound.

[ Singing continues ] A female arrives.

♪♪ But, no, she loses interest.

♪♪ What he does next will need to be even more impressive.

[ Birds singing ] ♪♪ Few have analyzed the grand finale of the male lyrebird song more intensively than Dr. Anastasia Dalziell.

She and her team were the first ones to film it properly.

DALZIELL: We thought, "No one's gonna believe us until we've actually filmed it."

It's really not like anything else that has been described previously in birds or any other animals.

♪♪ ATTENBOROUGH: It was by studying the recordings from their remote camera that the team were able to understand the real secret of the male lyrebird song.

And it is this footage, together with our own, that enables us to show this remarkable behavior on television for the first time.

Having sung Song A, B and C -- and been rejected -- the male now begins Song D. DALZIELL: So, this final song -- it's a sound of a flock of small birds... [ Superb lyrebird singing ] ...mobbing reacting to a stationary or a hidden predator like a snake.

And this was totally bizarre.

ATTENBOROUGH: So, why is he imitating a flock of birds?

[ Singing continues ] Well, a predator means danger, so at the sound of an alarm call, the female freezes.

[ Singing continues ] The forest is no longer a safe place.

[ Singing continues ] Now the male takes advantage of her panic.

He jumps on top of her to mate.

♪♪ It's hard for us to see her under his feathers.

And it's hard for her to see out from under them.

He is doing everything he can to disorientate and confuse her.

♪♪ DALZIELL: The male is actually telling her a big fib -- "Don't leave me because out there there's a hidden predator that you haven't seen."

To put it another way, it's like saying, "Baby, it's dangerous outside.

Come back here with me, where it's nice and safe."

♪♪ ATTENBOROUGH: The lyrebird is, in fact, a bird that tells lies.

His song is an acoustic illusion.

Up to now, scientists had thought that song was an honest signal from the male.

But it seems song can also be manipulative and false.

♪♪ [ Birds singing ] Not far away, in the Blue Mountains near Sydney, scientist Victoria Austin is studying the female lyrebird.

♪♪ What she's listening for is something very few people have heard.

AUSTIN: So, what I'm doing now is just preparing my recorder because we're about to head to a nest and we're hoping to be able to record the female lyrebird that resides in that area.

ATTENBOROUGH: The female lyrebird is the opposite of her flashy mate.

She's secretive.

When she is seen, she's often mistaken for a juvenile male.

To most people, she's invisible.

But Victoria knows how to find her.

This old nest suggests that she may be nearby.

And there she is.

[ Superb lyrebird singing ] This rarely heard sound is her whistle song.

[ Singing continues ] In Darwin's theory of sexual selection, males sing to attract females and females drive the evolution of song by preferring ever more complex songs.

The females themselves do not sing.

Yet here we are listening to a female lyrebird singing.

[ Singing continues ] So, what is the function of her song?

[ Singing continues ] Well, the female raises her chicks alone.

The male plays no part.

So it's extremely important for her to maintain her territory and the food within it.

She uses song to let everyone know that it's her patch.

[ Singing continues ] And Victoria is also researching whether song helps her in other ways.

In the last few years, she's been recording their songs across the Blue Mountains.

♪♪ This is one of them -- a female mimicking a goshawk.

[ Superb lyrebird mimicking goshawk ] AUSTIN: It's very accurate.

If you were play this call alongside an actual goshawk, it would be very difficult to tell the difference, if you were able to tell the difference at all.

ATTENBOROUGH: So, what is the purpose of this perfect impersonation?

She is using it to deceive predators into thinking she's more dangerous than she actually is.

♪♪ She is just as talented as the more famous male.

[ Superb lyrebird singing ] AUSTIN: So, the biggest purpose of what we're doing working with these female lyrebirds is to dispel this long-held myth that only recently was shown not to be true, and that is the idea that female birds don't sing or that it's very rare.

And that's just simply not the case.

[ Birds singing ] ATTENBOROUGH: This surely challenges Darwin's theory... as does my next, revolutionary song.

♪♪ [ Superb fairy-wren singing ] My fifth song was also recorded in Australia.

It, too, like the song of the female lyrebird, has rarely been heard.

[ Superb fairy-wren singing ] It's important because its existence changes our idea of what song actually is.

[ Superb fairy-wren singing ] This is a fairly common Australian bird called a fairy-wren.

[ Singing continues ] We filmed these birds for a BBC series back in the 1990s.

[ Superb fairy-wren singing ] ♪♪ We went there to film it because it's extremely promiscuous.

Both the male and female will mate with many different partners.

But what few people realized at the time -- and I certainly didn't -- was that it's not just the male that sings.

♪♪ The female does, too.

[ Superb fairy-wren singing ] ♪♪ This is Canberra Botanical Gardens.

♪♪ Professor Naomi Langmore was the scientist who made our fairy-wren recording.

She was one of the first to realize the significance of female song.

[ Birds singing ] A male fairy-wren with glorious iridescent blue and striking black plumage.

Rather difficult to miss.

[ Superb fairy-wren singing ] So, where is the female?

Well, not at the top of a perch like the male, but instead here, hiding in the bushes.

♪♪ She is, comparatively, rather dull -- a drab brown.

LANGMORE: Because females are often less flashy and eye-catching than males, it's very easy to miss female song.

[ Superb fairy-wren singing ] ATTENBOROUGH: But sing she does.

[ Singing continues ] Just as male song is used in competition with other males, female song seems to be in competition with other females.

But why didn't we hear her before now?

Is it really just because she is less noticeable than the male?

LANGMORE: In the history of the study of birdsong, most research was done in the northern hemisphere, in Europe and North America, and in those regions, female song is comparatively rare.

And so researchers working in those regions generalized from what they were observing in their local birds and assumed that male song was the norm throughout the world.

ATTENBOROUGH: A male-biased view of birdsong had, to an extent, deafened us to female song.

KREBS: So, when I was doing my research, it was basically assumed that it's the male that sings and the female that doesn't.

Maybe that's because most of the scientists were males who were studying birdsong.

LANGMORE: But now there's a new generation of female scientists coming through, studying birdsong all around the world, and discovering that actually female song is very common and occurs in more species than not.

ATTENBOROUGH: And it's only now that they're properly being heard.

Naomi and her colleagues have discovered that in 64% of all songbird species, females sing.

and that, in the distant past, the ancestors of all songbirds would have had both male and female singers.

♪♪ When birdsong began, perhaps all female birds sang.

[ Birds singing ] So, why don't all female birds sing today?

The answer is migration.

Male migrants, like the nightingale, arrive in their breeding grounds before the female.

They set up and defend the breeding territories, so the females don't need to sing.

♪♪ In species that don't migrate, like the lyrebird and the fairy-wren, then often the females will have their own territory [ Bird singing ] And they will sing to defend it.

[ Birds singing ] This female song challenges the way scientists have thought about sexual selection for the last hundred years.

DALZIELL: This recent discovery is a paradigm shift.

It's extremely exciting.

And it really forces us again to reconsider what we think of as birdsong.

[ Birds singing ] LANGMORE: Birdsong was fundamental to the formulation of Darwin's theory of sexual selection.

And the discovery that females sing as well makes us challenge that theory.

ATTENBOROUGH: Because our perception of birdsong has been biased towards the northern hemisphere, we have been unaware of some of the most thrilling birdsongs on the planet.

But that view is changing.

♪♪ KREBS: Our understanding of song is continually developing, and all the time we're learning new things.

It's a mystery that needs real research to unravel it.

And we're still learning.

♪♪ ATTENBOROUGH: How exciting it is to think of the discoveries that are about to be made about birdsong.

♪♪ [ Birds singing ] ♪♪ This next groundbreaking recording reminds us just how thrilling those new discoveries could be... ♪♪ ...because it revealed to us an entirely different world of song.

♪♪ Few animal songs are more beautiful than the ones that are recorded on this disc.

And yet, had it not been for recent technological advances, we would never have known that such songs existed.

♪♪ The waters off the coast of Bermuda.

U.S. Navy engineers are using underwater microphones, called hydrophones, to listen for enemy submarines.

This is what they pick up.

[ Low humming ] It's not a submarine or, indeed, any kind of man-made noise.

[ Humming continues ] I'll always remember the first time I heard those songs.

It brought back to my mind the stories of sailors in the old sailing ship days, out at sea in their bunks, hearing these wonderful, eerie sounds resonating through the ship.

It didn't come, of course, from a mermaid, but something perhaps even more extraordinary -- a gigantic creature, weighing many tons -- a whale.

[ Humpback whale singing ] Almost unbelievably, this was the first time that anyone had ever identified the sound of the whale.

When biologist Dr. Roger Payne heard it, he was thrilled.

PAYNE: It was back in 1967, about, that I met a fellow named Frank Watlington, who became a great friend.

And he played a sound to me of humpback whales.

[ Humpback whale singing ] It was the most beautiful thing I had ever heard from nature.

[ Singing continues ] The first time I ever went swimming with a whale that was singing, it was an incredible experience.

It's completely shattering.

[ Humpback whale singing ] It feels like, when you get close to one, that something has put its hands on your chest and is shaking you until your teeth rattle.

My first thought was, "I wonder if I can stand this.

I wonder if this is actually gonna kill me somehow."

[ Humpback whales singing ] ATTENBOROUGH: The question was, would we call these sounds "songs"?

Some were short, like bird calls, but others were longer, some up to half an hour.

[ Humpback whale singing ] Speeded up, this is what they sounded like.

[ Rapid, high-pitched singing ] They sounded like birdsong.

[ Singing continues ] Roger called them songs.

In the late 1970s, I, too, went swimming with humpback whales.

[ Humpback whale singing ] I remember seeing this creature below me and then hearing its song.

[ Singing continues ] It was thought they sing for the same reason as birds -- males singing to rivals and potential mates.

But no one has ever seen a female listening.

[ Singing continues ] In truth, no one really knows why whales sing.

[ Singing continues ] But one thing is certain -- the sound of their song saved them from us.

[ Singing continues ] In 1970, Roger released an album called "Songs of the Humpback Whale."

At the time, we had been killing whales, mainly for their oil, for centuries.

[ Harpoon gun fires ] We were very close to exterminating them out of sheer greed.

Then we heard their song.

[ Humpback whale singing ] PAYNE: Whole bunches of people in several countries began making organizations to save the whales, and the Save the Whales movement was born.

And in many ways, that was sort of the beginning of the conservation movement.

[ Singing continues ] ATTENBOROUGH: The conscience of the world was awoken by this song of the whale.

We heard it just in time to save them.

[ Singing continues ] But for my next and final singer, there was no such reprieve.

[ Kaua'i 'o'o singing ] There are few songs more haunting than this.

[ Kaua'i 'o'o singing ] It's a male Hawaiian 'o'o bird calling for a mate.

[ Singing continues ] But is he singing into silence?

[ Singing continues ] Habitat destruction and the introduction of invasive species have decimated many Hawaiian songbirds, including the 'o'o.

It may well be that by the time this recording was made there were no females left alive.

It's the sound of a male singing for a mate who no longer exists.

The 'o'o has since been declared extinct.

[ Kaua'i 'o'o singing ] He was the last of an entire bird family found nowhere else on Earth, now gone.

[ Singing continues ] There is no more dramatic reminder of this loss than this sound.

[ Singing continues ] ♪♪ And how many more songs have we lost in other parts of the planet?

Here in Britain, it's estimated that 38 million birds have disappeared from our skies in the last 60 years, one in five, gone.

♪♪ Climate change, habitat deterioration, and the resulting decrease in food and other resources are thought to be the main factors behind this catastrophic decline.

It's now up to us to decide how many more songs we will allow to fade into silence.

[ Birds singing ] ♪♪ These songs enrich our lives, too.

They are surely amongst the loveliest in the universe, and without them our lives would truly be impoverished.

[ Singing continues ] And what is lost when the songs fall silent is more than just an enchanting operatic backdrop to our own lives -- because, for the creatures that sing them, songs are far more than that.

They are a weapon of war, a serenade, a promise of parenthood, a daring deceit, or perhaps something even more astonishing that we are yet to discover... [ Animals singing ] ...each one a marvelous example of the spectacular survival strategies that animals have developed in order to stay alive.

[ Humpback whale singing ] That is why I'll never cease to wonder at the beautiful sounds we call song.

♪♪ [ Bird singing ] ♪♪ ♪♪ ♪♪ ♪♪ To learn more about what you've seen on this "Nature" program, visit PBS.org.

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