The Life of Birds (1998) s01e06 Episode Script
Signals and Songs
It's spring in Sweden.
Fieldfares are industriously ferrying meals of worms to their ravenous chicks.
They nest in colonies, up to thirty or forty in a group, and that helps a lot with defence, which is important, for there are plenty of raiders around.
That was one.
It's a young raven.
He's after a nestling.
A fieldfare has spotted him and sounds the alarm.
0thers take up the call, and the defence force assembles.
The raven now knows that he's been spotted, but he's hungry.
The fieldfares, screaming with anger, converge on their enemy.
And now threat turns into direct action.
They mob him.
Thoroughly intimidated by the commotion, the raven retreats.
But the fighters press home their attack, and the raven is brought down.
And now they bomb him with their droppings.
Soiled feathers soon become waterlogged, and that could be crippling, even fatal Thoroughly cowed, the raven retreats.
The colony is saved, thanks to its members' highly effective system of communication, between themselves and with their enemy.
The messages proclaimed by those Scandinavian fieldfares could scarcely be misunderstood, even by us.
The first were calls to arms, the second were battle cries designed to intimidate the enemy.
But alarm calls aren't always so easily recognised by outsiders.
Sometimes it's better to sound the alarm more surreptitiously.
And that is something that birds in an English wood do very well.
(BIRDS CHIRP) That sound, for example, is a general alarm call.
It's short, very high-pitched, and that makes it very difficult to locate the bird that makes it.
It's a great tit.
Half hidden among the leaves, he continues sending furtive signals to his family, but all the birds around get the message.
An enemy would find it very hard to detect where that sound is coming from.
Another warning.
This time it's from a robin.
He's telling his mate to stay still until the danger has passed.
And that's the blackbird's version.
The begging cries of nestlings could put them in danger.
A male chaffinch tells them to keep quiet.
And they do as they're told.
This surreptitious call is like an international distress signal understood by everyone.
(BIRDCALL) But that's a different kind of message.
That's not one that's sent surreptitiously to others.
It's aimed directly at me.
It's a warning to tell me that I've been spotted.
I'm too near this blackbird's mate who's sitting on her nest.
His calls are almost continuous and much lower pitched, because he wants to be located, so that he can distract me away from her, and even unsettle me.
Sound is not the only way to spread the alarm or intimidate an intruder.
Some birds do the same thing visually.
Most of the time, this sunbittern is well-camouflaged and unobtrusive.
Even less conspicuous than the jacanas and the cayman that also haunt the river's edge here in Venezuela.
The river is continually bringing edible bits and pieces within range, and the sunbittern lives on them.
But he has competitors.
A hawk in the branches above has spotted something.
So has the sunbittern.
But the hawk gets there first .
.
and collects it.
A second hawk arrives.
If the sunbittern is to get anything at all, it will have to frighten the others off.
So it transforms itself.
A ferocious, hissing, two-eyed monster that doesn't exist is saving the day.
The hawk tries again.
But this startling display has convinced the hawks that the bird down there is dangerous, and they give up.
There is, of course, an alternative signal Instead of saying ''I am here and extremely formidable'', you could say ''I'm not here at all''.
That, of course, is a straightforward lie, but there's a bird in these Brazilian forests that tells the most convincing of lies.
Finding it is not easy.
Indeed, I'm quite sure I've walked past one many times without knowing it.
But this time, we're lucky.
It's sitting on the tree trunk.
It's a potoo, a kind of nightjar.
It hunts for insects at night, so, like any other nightworker, it needs to rest during the day, and it relies on the visual match between its feathers and the tree trunk to protect it.
The only thing that could give it away are its beak and its eyes.
Now I am getting quite close, so it decides to improve its disguise even further.
And it does that by changing its posture and closing those give-away eyes.
Now it's no more than the stump of a broken branch.
You might think it would be dangerous to shut your eyes just when danger approaches.
But, in fact, although its eyes are shut, it can still see me.
There are two little hitches in its upper eyelid, and its night-vision eyes are so sensitive that it can still see what's going on.
As it watches me going away, it relaxes and returns to its doze.
Most birds, of course, rely on their ability to fly to keep them out of trouble, and so, as you walk through an English wood, they too vanish.
But establish their confidence and they will soon come back.
And then you can see that they use their plumage to send very different messages.
0ne finch meeting another needs to know whether or not it's the same species.
If it is, it could be a rival, either for a mate or for territory.
If not, it can be largely ignored.
So finches with such similar body shapes wear uniforms that make plain who they are.
And what works for other birds will also work with us, provided we know the code.
Most bird-watchers do.
A grey-blue cap and reddish cheeks identify a chaffinch.
A brown head and grey colour, a hawfinch.
A completely green head, a greenfinch.
A black cap and red cheeks, a buLLFinch.
And a red face and forehead, a goldfinch.
So every finch knows whether another is a rival or not, and there are no pointless quarrels.
In the forests of Indonesia, hornbills also use colour codes.
Several species there have predominantly black and white plumage.
This one, however, the pied hornbill, has yellowish areas on its white wing patches.
These are not accidental smudges.
This bird uses make-up.
With its beak, it squeezes a yellow oil from a giant preen gland on its rump.
And it uses that oil to paint on those yellow blotches.
And it's not just on its wings.
It adds yellow patches to its neck as well, though they are more difficult to put on.
Even its huge bill owes its yellow colour to the preen oil Different kinds of hornbills paint themselves different colours.
Whether these cosmetics are used just for appearance's sake, or whether they have an additional purpose, we don't know.
But one thing is quite certain - birds take a lot of care over their appearance.
All birds have good eyesight, a necessity if they are to navigate at speed through the air.
In particular, they have excellent colour vision, and that enables some species to have the most gorgeous uniforms.
I can attract some of the most spectacular using this bottle of artificial nectar.
The particular glory of hummingbirds are their bibs and breast-shields.
Their colour is not pigment but an optical effect created by refraction, like the colours on a film of oil on water.
They are particularly attracted by red, which is why I have got red artificial flowers on this bottle.
They can also see in ultraviolet, a colour that lies beyond the range of the human eye.
It's recently been discovered that many of their feathers reflect ultraviolet, so the likelihood is that these brilliant costumes are even more vivid in their eyes than they are in ours.
Many birds that we might think are plain are almost gaudy in ultraviolet light.
If we look closely at starlings, we can se a sheen to their plumage, but that has an ultraviolet component that makes them appear much more vivid to one another.
Blue tits, in our eyes, are one of the more colourful of our garden birds.
But in ultraviolet they are much brighter still.
Their crests are particularly vivid and much brighter in males than females.
So to them, the sexes look very different, whereas to us blue tits all look the same.
We would all, I imagine, think that budgerigars are unusually colourful birds.
But ultraviolet radically changes the character of their costume.
Their feet glow.
And the spots on their cheeks, which are not very prominent to our eyes, positively blaze.
Indeed, a budgerigar's full-dress uniform is dramatic in the extreme.
Uniforms not only indicate an individual's regiment, but his rank within that regiment.
Male sparrows have black bibs, but the size varies.
The more vigorous birds have bigger bibs and therefore higher ranks.
Sparrows forage in flocks, and when there's lots of food in a small area, you might expect lots of quarrels, but there aren't.
This is a private, with no badges.
This one is somewhat senior, a sergeant, perhaps.
A captain.
And the colonel There could be disputes not only over food but over amenities like dust baths.
The privates are squabbling among themselves.
But watch what happens when a corporal steps in.
Junior ranks retreat.
0r when a corporal gets too close to a sergeant.
A sergeant, however, will give way to a captain.
And no one should think of parting a colonel from his lunch.
A quick flourish of his insignia is quite enough.
Among birds that don't live in flocks there's no need for the ranking system to be so multi-layered.
Moorhens may mingle, but each pair has its own territory.
Their badges are the red beak and head shield and the white patches on either side of the tail Rivals assess one another's strength by the size and brilliance of those head shields.
If they feel their ranks are equal, then they may not want to contest the position of the boundary, and they display the white tail patches to indicate that the confrontation is being broken off.
But here, the male on the right is standing boldly upright.
He reckons he is the senior, and he wants to enlarge his territory.
The time for sending messages is over.
This quarrel can only be settled by physical violence.
Birds can get badly injured in these battles, but they have to be fought if a senior bird is to establish and retain his rank.
Eventually the junior bird surrenders.
A new line has been drawn.
They won't need to fight again as long as it's not over-stepped.
Communication by visual signals, however, has one major limitation.
Except in completely open country, they only work at close range.
In forests, sound signals will travel much further.
So if a bird, in order to get enough food, needs a very large territory, it is likely to declare its territorial claims with sound.
There are, of course, many different ways of making a noise, and knocking on a resonant tree trunk is one of them.
In this part of the world, in Patagonia on the southernmost tip of South America, two knocks on a tree trunk has a very particular meaning, at least among birds.
If I do it, I might even get an answer.
(w00DPECKER KN0CKS 0N TREE) (w00DPECKER KN0CKS 0N TREE) (w00DPECKER KN0CKS 0N TREE) It's a Magellanic woodpecker, one of the largest of all woodpeckers, and he thinks he has heard a rival He comes in for a closer look.
(w00DPECKERS KN0CKING) And here's his mate, coming to support him.
Now she joins in the dispute.
He is now on my tree, and his mate is even closer.
I have stopped knocking, so it seems to them that their rival has disappeared.
So all is well.
There's another drummer in the bird world, Australia's palm cockatoo.
His beak is no good as a drum stick, so he uses a wooden one.
(vIBRATING BIRDCALL) And that noise too is made mechanically, by an African broadbill.
It makes its call in the same way as children do when they blow across a blade of grass.
But instead of grass, the broadbill has specially strengthened and shaped wing feathers.
But, of course, most of the sounds made by birds come from their throats.
The calls and songs that you hear in a tropical rainforest, however, are very different from those you might hear in a European or North American woodland, and there's a reason for that.
The leaves in a tropical rainforest have smooth and shiny surfaces that reflect sound.
So a complex call up here would have its notes slurred and confused.
As a result, birds that live up here tend to have calls that are simple, short, and often very, very loud.
(SHRILL BIRDCALL) That's the loudest of all .
.
from a bare-throated bell bird.
And this is a close rival, a screaming piha.
Toucans must also be close to the top ten.
All these birds call from high up in the canopy.
Lower down, where the foliage is less dense, the calls can be different.
For one thing, they can be longer.
(L0NG DRAWN-0UT CALL) That's a currasow.
And thisa wattled guan.
A longer call, of course, can contain more notes.
A kagu in New Caledonia.
This is a female.
Her mate, some distance away, is listening.
The family wandered apart as they foraged, and now they want to get together again.
Her son has also heard the message.
So the adult pair are reunited, and they greet one another as usual with a visual display.
But their son is still out there, somewhere.
And once again, the family group is complete.
The calls of the kagu can be heard half a mile away.
But some birds need to communicate over even greater distances, and the best way to do that is with very low-pitched notes.
An American bittern.
An air sac in his chest acts as a resonator, so he starts by gulping in air and pumping it up.
This call carries for over two miles, even through the thickets of reeds.
And when the performance is over, the air sac slowly deflates.
If you can get out of the reeds, then your calls are less impeded.
The Australian musk duck does just that in order to broadcast his messages.
And the smooth surface of the water also helps to reflect the sound far across the lake.
The flap on his chin is a visual signal for any birds that come over for a closer look.
However, if calls are directed to neighbours nearby, then they can become very elaborate.
This red bishop makes an almost constant stream of high-pitched notes as he hops around his territory.
And you can't get much more elaborate than this.
The extraordinary display of the 0ropendula includes one of the strangest songs of all.
So how do birds do it? How, for example, can canaries sing continuously for minutes on end? Slowing the singer down, which also lowers the pitch of the notes, allows us to understand what's happening.
Between the notes it takes mini-breaths to replenish its air supply, and in full song it may do so 30 times a second.
A bird's voice box can also produce two different notes simultaneously.
It's not high in the throat like ours, but deep in its chest.
Low notes come from one side.
High from the other.
By alternating between high and low notes, even short songs can carry very complex messages.
And this is the champion.
The cowbird uses over 40 different notes in his songs.
Some of them are so high that they are beyond the hearing of many of us.
0ne again, if we slow the action down, we can hear what's going on.
The left side is producing the low notes.
And the right, the high.
0thers are made by combining the sounds higher up in the throat.
It may take a cowbird two years to learn his song properly.
And it's important that birds should get their calls exactly right, for they can be just as significant in proclaiming identity as their uniforms.
Indeed, if a bird has a shy and retiring disposition, and lives in a secluded place like this English woodland, then its voice may be the only way that it can be recognised by another bird, or indeed by a bird-watcher.
There are two kinds of warblers here.
This is a chiff-chaff, fuelling up after its long flight from Africa.
And this is a willow-warbler.
To me it looks virtually identical But wait until they sing.
This is a chiff-chaff.
And this, a willow-warbler.
There's no mistaking who's who, as long as you can hear their calls.
But a bird's call can tell another bird more than just what kind of bird it is that's singing.
This patch of bush on a small New Zealand island belongs to a male saddleback.
He has held it throughout the year, and he knows who his neighbours are because their calls vary slightly.
And he can recognise each one individually.
And there he is.
Throughout the day he keeps in regular contact with his neighbours.
They each answer his call, and he can distinguish between them in the same way that we can distinguish between regional dialects.
A northerner.
A southerner.
And someone from the east coast.
If the right call comes from the right place, then he knows that his territory is safe, and he can happily go back to feeding.
But if the call is from a saddleback, one that he doesn't recognise, and if, as well as that, it comes from a completely new place, then he will react in a very different way.
And, of course, it's quite easy for me to make that happen.
I'm going to play him a recording of a saddleback from a different island.
His response is swift and very aggressive.
He comes down for a closer look.
He gives another warning.
This is a serious challenge to his territory.
It can't be tolerated.
Now, since his rival seems to be close by, he's making a visual threat, displaying the brown patch on his back.
He is ready to fight, if only he could find who it is who needs fighting.
Well, I guess that's enough of that.
We'll leave him in peace.
To may of us, however, this is the most delectable of natural sounds.
It's an hour before dawn, it's spring, this is an English woodland.
All around, the dawn chorus.
It's so familiar that perhaps we take it for granted.
But there's a lot we don't know about it still.
As first light brightens, different kinds of birds, one by one, join the choir.
(INCREASING DAWN CH0RUS) (M0RE AND M0RE BIRDS J0IN IN) Why should they all sing together at this time? Wouldn't it be better for some to sing later, by themselves? And that's not the only puzzle.
Why should it happen at this time of day? Well, at dawn it's still quite cold.
Insects are not yet up and about, so for many of the birds there's nothing to eat.
So they might as well sing.
There's another possible reason.
It's usually quite calm at dawn, and with no wind these messages will travel far and still be recognisable.
The chorus is the equivalent of our early morning news, except that it is broadcast in fifty languages.
By listening to it, this wren knows which of its neighbours is still alive.
He knows where they are.
And if there are any new males on the scene.
Each kind of bird listens to its own particular section of the sound spectrum.
The song thrush broadcasts in the mid-range.
The wood pigeon's calls are somewhat lower.
Smaller birds, like the firecrest, use the higher frequencies.
These spring-time messages from male birds not only say ''This is my patch'', they also say to passing females ''Why don't you come and join me?'' Robins have now extended their usual songs to carry this additional message.
The male chaffinch has done the same.
He may sing his song over 500,000 times in a season.
By late spring, migrants have arrived from southern Europe and Africa, and are adding their voices to the chorus.
A wood warbler.
A pied flycatcher.
And a redstart.
His mate, like many, will be impressed by the originality and complexity of his song.
The male sedge warbler can produce fifty different notes and never sings the same song twice.
He's like a jazz singer, continually improvising, and different males develop different singing styles.
And this is perhaps the most lyrical of all European songsters.
A nightingale.
He may have three hundred different love songs in his repertoire.
And he will sing for a mate all through the night.
What bird has the most elaborate, the most complex, the most beautiful song in the world? I guess there are lots of contenders, but this bird must be one of them, the superb lyrebird of southern Australia.
He clears a space in the forest to serve as his concert platform.
To persuade females to come close and admire his plumes, he sings the most complex song he can.
And he does that by copying the songs of all the other birds he hears around him, such as the kookaburra.
It's a very convincing impersonation.
Even the original is fooled.
He can imitate the calls of at least twenty different species.
He also, in his attempt to out-sing his rivals, incorporates others sounds in the forest.
That was a camera shutter.
And again.
And now a camera with a motor drive.
And that's a car alarm.
And now the sounds of foresters and their chain-saws working nearby.
That wonderful performance is only one example of the extent to which male birds will go in order to attract a female.
The range and sheer extravagance of their courtship displays can be quite astonishing.
The range of relationships between male and female that these displays lead to is also much more varied than you might suppose.
And it's that, the most crucial stage in the life of any bird, that we'll be looking at in the next programme.
Fieldfares are industriously ferrying meals of worms to their ravenous chicks.
They nest in colonies, up to thirty or forty in a group, and that helps a lot with defence, which is important, for there are plenty of raiders around.
That was one.
It's a young raven.
He's after a nestling.
A fieldfare has spotted him and sounds the alarm.
0thers take up the call, and the defence force assembles.
The raven now knows that he's been spotted, but he's hungry.
The fieldfares, screaming with anger, converge on their enemy.
And now threat turns into direct action.
They mob him.
Thoroughly intimidated by the commotion, the raven retreats.
But the fighters press home their attack, and the raven is brought down.
And now they bomb him with their droppings.
Soiled feathers soon become waterlogged, and that could be crippling, even fatal Thoroughly cowed, the raven retreats.
The colony is saved, thanks to its members' highly effective system of communication, between themselves and with their enemy.
The messages proclaimed by those Scandinavian fieldfares could scarcely be misunderstood, even by us.
The first were calls to arms, the second were battle cries designed to intimidate the enemy.
But alarm calls aren't always so easily recognised by outsiders.
Sometimes it's better to sound the alarm more surreptitiously.
And that is something that birds in an English wood do very well.
(BIRDS CHIRP) That sound, for example, is a general alarm call.
It's short, very high-pitched, and that makes it very difficult to locate the bird that makes it.
It's a great tit.
Half hidden among the leaves, he continues sending furtive signals to his family, but all the birds around get the message.
An enemy would find it very hard to detect where that sound is coming from.
Another warning.
This time it's from a robin.
He's telling his mate to stay still until the danger has passed.
And that's the blackbird's version.
The begging cries of nestlings could put them in danger.
A male chaffinch tells them to keep quiet.
And they do as they're told.
This surreptitious call is like an international distress signal understood by everyone.
(BIRDCALL) But that's a different kind of message.
That's not one that's sent surreptitiously to others.
It's aimed directly at me.
It's a warning to tell me that I've been spotted.
I'm too near this blackbird's mate who's sitting on her nest.
His calls are almost continuous and much lower pitched, because he wants to be located, so that he can distract me away from her, and even unsettle me.
Sound is not the only way to spread the alarm or intimidate an intruder.
Some birds do the same thing visually.
Most of the time, this sunbittern is well-camouflaged and unobtrusive.
Even less conspicuous than the jacanas and the cayman that also haunt the river's edge here in Venezuela.
The river is continually bringing edible bits and pieces within range, and the sunbittern lives on them.
But he has competitors.
A hawk in the branches above has spotted something.
So has the sunbittern.
But the hawk gets there first .
.
and collects it.
A second hawk arrives.
If the sunbittern is to get anything at all, it will have to frighten the others off.
So it transforms itself.
A ferocious, hissing, two-eyed monster that doesn't exist is saving the day.
The hawk tries again.
But this startling display has convinced the hawks that the bird down there is dangerous, and they give up.
There is, of course, an alternative signal Instead of saying ''I am here and extremely formidable'', you could say ''I'm not here at all''.
That, of course, is a straightforward lie, but there's a bird in these Brazilian forests that tells the most convincing of lies.
Finding it is not easy.
Indeed, I'm quite sure I've walked past one many times without knowing it.
But this time, we're lucky.
It's sitting on the tree trunk.
It's a potoo, a kind of nightjar.
It hunts for insects at night, so, like any other nightworker, it needs to rest during the day, and it relies on the visual match between its feathers and the tree trunk to protect it.
The only thing that could give it away are its beak and its eyes.
Now I am getting quite close, so it decides to improve its disguise even further.
And it does that by changing its posture and closing those give-away eyes.
Now it's no more than the stump of a broken branch.
You might think it would be dangerous to shut your eyes just when danger approaches.
But, in fact, although its eyes are shut, it can still see me.
There are two little hitches in its upper eyelid, and its night-vision eyes are so sensitive that it can still see what's going on.
As it watches me going away, it relaxes and returns to its doze.
Most birds, of course, rely on their ability to fly to keep them out of trouble, and so, as you walk through an English wood, they too vanish.
But establish their confidence and they will soon come back.
And then you can see that they use their plumage to send very different messages.
0ne finch meeting another needs to know whether or not it's the same species.
If it is, it could be a rival, either for a mate or for territory.
If not, it can be largely ignored.
So finches with such similar body shapes wear uniforms that make plain who they are.
And what works for other birds will also work with us, provided we know the code.
Most bird-watchers do.
A grey-blue cap and reddish cheeks identify a chaffinch.
A brown head and grey colour, a hawfinch.
A completely green head, a greenfinch.
A black cap and red cheeks, a buLLFinch.
And a red face and forehead, a goldfinch.
So every finch knows whether another is a rival or not, and there are no pointless quarrels.
In the forests of Indonesia, hornbills also use colour codes.
Several species there have predominantly black and white plumage.
This one, however, the pied hornbill, has yellowish areas on its white wing patches.
These are not accidental smudges.
This bird uses make-up.
With its beak, it squeezes a yellow oil from a giant preen gland on its rump.
And it uses that oil to paint on those yellow blotches.
And it's not just on its wings.
It adds yellow patches to its neck as well, though they are more difficult to put on.
Even its huge bill owes its yellow colour to the preen oil Different kinds of hornbills paint themselves different colours.
Whether these cosmetics are used just for appearance's sake, or whether they have an additional purpose, we don't know.
But one thing is quite certain - birds take a lot of care over their appearance.
All birds have good eyesight, a necessity if they are to navigate at speed through the air.
In particular, they have excellent colour vision, and that enables some species to have the most gorgeous uniforms.
I can attract some of the most spectacular using this bottle of artificial nectar.
The particular glory of hummingbirds are their bibs and breast-shields.
Their colour is not pigment but an optical effect created by refraction, like the colours on a film of oil on water.
They are particularly attracted by red, which is why I have got red artificial flowers on this bottle.
They can also see in ultraviolet, a colour that lies beyond the range of the human eye.
It's recently been discovered that many of their feathers reflect ultraviolet, so the likelihood is that these brilliant costumes are even more vivid in their eyes than they are in ours.
Many birds that we might think are plain are almost gaudy in ultraviolet light.
If we look closely at starlings, we can se a sheen to their plumage, but that has an ultraviolet component that makes them appear much more vivid to one another.
Blue tits, in our eyes, are one of the more colourful of our garden birds.
But in ultraviolet they are much brighter still.
Their crests are particularly vivid and much brighter in males than females.
So to them, the sexes look very different, whereas to us blue tits all look the same.
We would all, I imagine, think that budgerigars are unusually colourful birds.
But ultraviolet radically changes the character of their costume.
Their feet glow.
And the spots on their cheeks, which are not very prominent to our eyes, positively blaze.
Indeed, a budgerigar's full-dress uniform is dramatic in the extreme.
Uniforms not only indicate an individual's regiment, but his rank within that regiment.
Male sparrows have black bibs, but the size varies.
The more vigorous birds have bigger bibs and therefore higher ranks.
Sparrows forage in flocks, and when there's lots of food in a small area, you might expect lots of quarrels, but there aren't.
This is a private, with no badges.
This one is somewhat senior, a sergeant, perhaps.
A captain.
And the colonel There could be disputes not only over food but over amenities like dust baths.
The privates are squabbling among themselves.
But watch what happens when a corporal steps in.
Junior ranks retreat.
0r when a corporal gets too close to a sergeant.
A sergeant, however, will give way to a captain.
And no one should think of parting a colonel from his lunch.
A quick flourish of his insignia is quite enough.
Among birds that don't live in flocks there's no need for the ranking system to be so multi-layered.
Moorhens may mingle, but each pair has its own territory.
Their badges are the red beak and head shield and the white patches on either side of the tail Rivals assess one another's strength by the size and brilliance of those head shields.
If they feel their ranks are equal, then they may not want to contest the position of the boundary, and they display the white tail patches to indicate that the confrontation is being broken off.
But here, the male on the right is standing boldly upright.
He reckons he is the senior, and he wants to enlarge his territory.
The time for sending messages is over.
This quarrel can only be settled by physical violence.
Birds can get badly injured in these battles, but they have to be fought if a senior bird is to establish and retain his rank.
Eventually the junior bird surrenders.
A new line has been drawn.
They won't need to fight again as long as it's not over-stepped.
Communication by visual signals, however, has one major limitation.
Except in completely open country, they only work at close range.
In forests, sound signals will travel much further.
So if a bird, in order to get enough food, needs a very large territory, it is likely to declare its territorial claims with sound.
There are, of course, many different ways of making a noise, and knocking on a resonant tree trunk is one of them.
In this part of the world, in Patagonia on the southernmost tip of South America, two knocks on a tree trunk has a very particular meaning, at least among birds.
If I do it, I might even get an answer.
(w00DPECKER KN0CKS 0N TREE) (w00DPECKER KN0CKS 0N TREE) (w00DPECKER KN0CKS 0N TREE) It's a Magellanic woodpecker, one of the largest of all woodpeckers, and he thinks he has heard a rival He comes in for a closer look.
(w00DPECKERS KN0CKING) And here's his mate, coming to support him.
Now she joins in the dispute.
He is now on my tree, and his mate is even closer.
I have stopped knocking, so it seems to them that their rival has disappeared.
So all is well.
There's another drummer in the bird world, Australia's palm cockatoo.
His beak is no good as a drum stick, so he uses a wooden one.
(vIBRATING BIRDCALL) And that noise too is made mechanically, by an African broadbill.
It makes its call in the same way as children do when they blow across a blade of grass.
But instead of grass, the broadbill has specially strengthened and shaped wing feathers.
But, of course, most of the sounds made by birds come from their throats.
The calls and songs that you hear in a tropical rainforest, however, are very different from those you might hear in a European or North American woodland, and there's a reason for that.
The leaves in a tropical rainforest have smooth and shiny surfaces that reflect sound.
So a complex call up here would have its notes slurred and confused.
As a result, birds that live up here tend to have calls that are simple, short, and often very, very loud.
(SHRILL BIRDCALL) That's the loudest of all .
.
from a bare-throated bell bird.
And this is a close rival, a screaming piha.
Toucans must also be close to the top ten.
All these birds call from high up in the canopy.
Lower down, where the foliage is less dense, the calls can be different.
For one thing, they can be longer.
(L0NG DRAWN-0UT CALL) That's a currasow.
And thisa wattled guan.
A longer call, of course, can contain more notes.
A kagu in New Caledonia.
This is a female.
Her mate, some distance away, is listening.
The family wandered apart as they foraged, and now they want to get together again.
Her son has also heard the message.
So the adult pair are reunited, and they greet one another as usual with a visual display.
But their son is still out there, somewhere.
And once again, the family group is complete.
The calls of the kagu can be heard half a mile away.
But some birds need to communicate over even greater distances, and the best way to do that is with very low-pitched notes.
An American bittern.
An air sac in his chest acts as a resonator, so he starts by gulping in air and pumping it up.
This call carries for over two miles, even through the thickets of reeds.
And when the performance is over, the air sac slowly deflates.
If you can get out of the reeds, then your calls are less impeded.
The Australian musk duck does just that in order to broadcast his messages.
And the smooth surface of the water also helps to reflect the sound far across the lake.
The flap on his chin is a visual signal for any birds that come over for a closer look.
However, if calls are directed to neighbours nearby, then they can become very elaborate.
This red bishop makes an almost constant stream of high-pitched notes as he hops around his territory.
And you can't get much more elaborate than this.
The extraordinary display of the 0ropendula includes one of the strangest songs of all.
So how do birds do it? How, for example, can canaries sing continuously for minutes on end? Slowing the singer down, which also lowers the pitch of the notes, allows us to understand what's happening.
Between the notes it takes mini-breaths to replenish its air supply, and in full song it may do so 30 times a second.
A bird's voice box can also produce two different notes simultaneously.
It's not high in the throat like ours, but deep in its chest.
Low notes come from one side.
High from the other.
By alternating between high and low notes, even short songs can carry very complex messages.
And this is the champion.
The cowbird uses over 40 different notes in his songs.
Some of them are so high that they are beyond the hearing of many of us.
0ne again, if we slow the action down, we can hear what's going on.
The left side is producing the low notes.
And the right, the high.
0thers are made by combining the sounds higher up in the throat.
It may take a cowbird two years to learn his song properly.
And it's important that birds should get their calls exactly right, for they can be just as significant in proclaiming identity as their uniforms.
Indeed, if a bird has a shy and retiring disposition, and lives in a secluded place like this English woodland, then its voice may be the only way that it can be recognised by another bird, or indeed by a bird-watcher.
There are two kinds of warblers here.
This is a chiff-chaff, fuelling up after its long flight from Africa.
And this is a willow-warbler.
To me it looks virtually identical But wait until they sing.
This is a chiff-chaff.
And this, a willow-warbler.
There's no mistaking who's who, as long as you can hear their calls.
But a bird's call can tell another bird more than just what kind of bird it is that's singing.
This patch of bush on a small New Zealand island belongs to a male saddleback.
He has held it throughout the year, and he knows who his neighbours are because their calls vary slightly.
And he can recognise each one individually.
And there he is.
Throughout the day he keeps in regular contact with his neighbours.
They each answer his call, and he can distinguish between them in the same way that we can distinguish between regional dialects.
A northerner.
A southerner.
And someone from the east coast.
If the right call comes from the right place, then he knows that his territory is safe, and he can happily go back to feeding.
But if the call is from a saddleback, one that he doesn't recognise, and if, as well as that, it comes from a completely new place, then he will react in a very different way.
And, of course, it's quite easy for me to make that happen.
I'm going to play him a recording of a saddleback from a different island.
His response is swift and very aggressive.
He comes down for a closer look.
He gives another warning.
This is a serious challenge to his territory.
It can't be tolerated.
Now, since his rival seems to be close by, he's making a visual threat, displaying the brown patch on his back.
He is ready to fight, if only he could find who it is who needs fighting.
Well, I guess that's enough of that.
We'll leave him in peace.
To may of us, however, this is the most delectable of natural sounds.
It's an hour before dawn, it's spring, this is an English woodland.
All around, the dawn chorus.
It's so familiar that perhaps we take it for granted.
But there's a lot we don't know about it still.
As first light brightens, different kinds of birds, one by one, join the choir.
(INCREASING DAWN CH0RUS) (M0RE AND M0RE BIRDS J0IN IN) Why should they all sing together at this time? Wouldn't it be better for some to sing later, by themselves? And that's not the only puzzle.
Why should it happen at this time of day? Well, at dawn it's still quite cold.
Insects are not yet up and about, so for many of the birds there's nothing to eat.
So they might as well sing.
There's another possible reason.
It's usually quite calm at dawn, and with no wind these messages will travel far and still be recognisable.
The chorus is the equivalent of our early morning news, except that it is broadcast in fifty languages.
By listening to it, this wren knows which of its neighbours is still alive.
He knows where they are.
And if there are any new males on the scene.
Each kind of bird listens to its own particular section of the sound spectrum.
The song thrush broadcasts in the mid-range.
The wood pigeon's calls are somewhat lower.
Smaller birds, like the firecrest, use the higher frequencies.
These spring-time messages from male birds not only say ''This is my patch'', they also say to passing females ''Why don't you come and join me?'' Robins have now extended their usual songs to carry this additional message.
The male chaffinch has done the same.
He may sing his song over 500,000 times in a season.
By late spring, migrants have arrived from southern Europe and Africa, and are adding their voices to the chorus.
A wood warbler.
A pied flycatcher.
And a redstart.
His mate, like many, will be impressed by the originality and complexity of his song.
The male sedge warbler can produce fifty different notes and never sings the same song twice.
He's like a jazz singer, continually improvising, and different males develop different singing styles.
And this is perhaps the most lyrical of all European songsters.
A nightingale.
He may have three hundred different love songs in his repertoire.
And he will sing for a mate all through the night.
What bird has the most elaborate, the most complex, the most beautiful song in the world? I guess there are lots of contenders, but this bird must be one of them, the superb lyrebird of southern Australia.
He clears a space in the forest to serve as his concert platform.
To persuade females to come close and admire his plumes, he sings the most complex song he can.
And he does that by copying the songs of all the other birds he hears around him, such as the kookaburra.
It's a very convincing impersonation.
Even the original is fooled.
He can imitate the calls of at least twenty different species.
He also, in his attempt to out-sing his rivals, incorporates others sounds in the forest.
That was a camera shutter.
And again.
And now a camera with a motor drive.
And that's a car alarm.
And now the sounds of foresters and their chain-saws working nearby.
That wonderful performance is only one example of the extent to which male birds will go in order to attract a female.
The range and sheer extravagance of their courtship displays can be quite astonishing.
The range of relationships between male and female that these displays lead to is also much more varied than you might suppose.
And it's that, the most crucial stage in the life of any bird, that we'll be looking at in the next programme.