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00:30It's spring in Sweden.
00:45Field fairs are industriously ferrying meals of worms to their ravenous chicks.
00:56They nest in colonies up to 30 or 40 in a group, and that helps a lot with defence,
01:02which is important for there are plenty of raiders around.
01:10That was one.
01:18It's a young raven.
01:20He's after a nestling.
01:28A field fair has spotted him and sounds the alarm.
01:39Others take up the call and the defence force assembles.
01:45The raven now knows that he's been spotted, but he's hungry.
01:53The field fairs, screaming with anger, converge on their enemy.
02:11And now threat turns into direct action.
02:14They mob him.
02:21Thoroughly intimidated by the commotion, the raven retreats.
02:26But the fighters press home their attack.
02:29The raven is brought down.
02:31And now they bomb him with their droppings.
02:46Soiled feathers soon become waterlogged, and that could be crippling, even fatal.
02:54Thoroughly cowed, the raven retreats.
02:56The colony is saved thanks to its members' highly effective system of communication between
03:01themselves and with their enemy.
03:09The messages proclaimed by those Scandinavian field fairs could scarcely be misunderstood
03:15even by us.
03:17The first were calls to arms.
03:19The second were battle cries designed to intimidate the enemy.
03:25But alarm calls aren't always so easily recognised by outsiders.
03:30Sometimes it's better to sound the alarm more surreptitiously.
03:38And that is something that birds in an English wood do very well.
03:44That sound, for example, is a general alarm call.
03:53It's short, very high-pitched, and that makes it very difficult to locate the bird that
03:58makes it.
04:05It's a great tit.
04:07Half hidden among the leaves, he continues sending furtive signals to his family.
04:12But all the birds around get the message.
04:21An enemy would find it very hard to detect where that sound is coming from.
04:30Another warning.
04:33This time it's from a robin.
04:41He's telling his mate to stay still until the danger has passed.
04:52And that's the blackbird's version.
05:00The begging cries of nestlings could put them in danger.
05:10A male chaffinch tells them to keep quiet.
05:16And they do as they're told.
05:20So this surreptitious call is like an international distress signal understood by everyone.
05:34But that's a different kind of message.
05:37It's not one that is sent surreptitiously to others.
05:40It's aimed directly at me.
05:42It's a warning to tell me that I've been spotted.
05:49I'm too near this blackbird's mate who's sitting on her nest.
05:53His calls are almost continuous and much lower-pitched because he wants to be located so that he can
05:59distract me away from her and even unsettle me.
06:11Sound is not the only way to spread the alarm or to intimidate an intruder.
06:16Some birds do the same thing visually.
06:34Most of the time, this sun-bitten is well camouflaged and unobtrusive, even less conspicuous
06:40than the jacanas and the caiman that also haunt the river's edge here in Venezuela.
06:49The river is continually bringing edible bits and pieces within range, and the sun-bitten
06:54lives on them.
06:55But it has competitors.
06:56A hawk in the branches above has spotted something.
07:02So has the sun-bitten.
07:03But the hawk gets there first and collects it.
07:19A second hawk arrives.
07:22If the sun-bitten is to get anything at all, it will have to frighten the others off.
07:28So it transforms itself.
07:32A ferocious, hissing, two-eyed monster that doesn't exist is saving the day.
07:55The hawk tries again.
08:00But this startling display has convinced the hawks that the bird down there is dangerous,
08:05and they give up.
08:11There is, of course, an alternative signal.
08:15Instead of saying, I am here and extremely formidable, you could say, I'm not here at
08:21all.
08:22That, of course, is a straightforward lie.
08:25But there's a bird in these Brazilian forests that tells the most convincing of lies.
08:32Finding it is not easy.
08:33Indeed, I'm quite sure I've walked past one many times without knowing it.
08:38But this time, we're lucky.
08:41It's sitting on the tree trunk.
08:45It's a potu, a kind of nightjar.
08:48It hunts for insects at night, so, like any other night worker, it needs to rest during
08:52the day.
08:53And it relies on the visual match between its feathers and the tree trunk to protect
08:58it from disturbance.
09:02The only thing that could give it away are its beak and its eyes.
09:14Now I'm getting quite close.
09:16So it decides to improve its disguise even further, and it does that by changing its
09:21posture and closing those give-away eyes.
09:25Now it's no more than the stump of a broken branch.
09:38You might think it would be a dangerous thing to do, to shut your eyes just when danger
09:42approaches.
09:43But in fact, although its eyes are shut, it can still see me.
09:47There are two little hitches in its upper eyelid, and its night-vision eyes are so sensitive
09:52that it can still see what's going on.
09:57As it watches me going away, it relaxes and returns to its does.
10:06Most birds, of course, rely on their ability to fly to keep them out of trouble.
10:10And so, as you walk through an English wood, they too vanish.
10:16But establish their confidence, and they will soon come back.
10:22And then you can see that they use their plumage to send very different messages.
10:31One finch meeting another needs to know whether or not it's the same species.
10:36If it is, it could be a rival, either for a mate or for territory.
10:40If it isn't, it can be largely ignored.
10:45So finches with such similar body shapes wear uniforms that make plain who they are.
10:52And what works for other birds will also work with us, provided we know the code.
11:00Most bird watchers do.
11:06A grey-blue cap and reddish cheeks identify a chaffinch.
11:11A brown head and grey collar, a hawfinch.
11:17A completely green head, a greenfinch.
11:22A black cap and red cheeks, a bullfinch.
11:28And a red face and forehead, a goldfinch.
11:35So every finch knows whether another is a rival or not, and there are no pointless quarrels.
11:48In the forests of Indonesia, hornbills also use colour codes.
11:53Several species there have predominantly black and white plumage.
12:00This one, however, the pied hornbill, has yellowish areas on its white wing patches.
12:10These are not accidental smudges.
12:12This bird uses make-up.
12:20With its beak, it squeezes a yellow oil from a giant preen gland on its rump.
12:33And it uses that oil to paint on those yellow blotches.
12:39And it's not just on its wings.
12:42It adds yellow patches to its neck as well.
12:44Now, they are more difficult to put on.
12:48Even its huge bill owes its yellow colour to the preen oil.
13:00Different kinds of hornbills paint themselves different colours.
13:04Whether these cosmetics are used just for appearances' sake or whether they have an
13:08additional purpose, we don't know.
13:13But one thing is quite certain.
13:16Hornbills take a lot of care over their appearance.
13:22All birds have good eyesight, as they have to have if they are to navigate at speed through
13:26the air.
13:27But in particular, they have excellent colour vision, and that enables some species to have
13:32the most gorgeous uniforms.
13:34And I can attract some of the most spectacular using this bottle of artificial nectar.
13:43The particular glory of hummingbirds are their bibs and breast shields.
13:54Their colour is not pigment, but an optical effect created by refraction, like the colours
13:59you see on a film of oil on water.
14:20They're particularly attracted by red, which is why I've got red artificial flowers on
14:25this bottle.
14:27We can also see an ultraviolet, and that's a colour that lies beyond the range of the
14:32human eye.
14:33It's recently been discovered that many of their feathers reflect ultraviolet, so the
14:38likelihood is that these brilliant costumes are even more vivid in their eyes than they
14:44are in ours.
14:47Many birds that we might think are plain are almost gaudy in ultraviolet light.
14:52If we look closely at starlings, for example, we can see a sheen to their plumage, but that
14:57has an ultraviolet component that makes them appear much more vivid to one another.
15:06Blue tits in our eyes are one of the more colourful of our garden birds, but in ultraviolet
15:14they're much brighter still.
15:16Their crests are particularly vivid and much brighter in males than females, so to them
15:22these sexes look very different, whereas to us blue tits all look the same.
15:31We would all, I imagine, think that budgerigars are unusually colourful birds, but ultraviolet
15:37radically changes the character of their costume.
15:41Their feet glow, and the spots on their cheeks, which are not particularly prominent to our
15:48eyes, positively blaze.
15:53Indeed, a budgerigar's full-dress uniform is dramatic in the extreme.
16:05Uniforms not only indicate an individual's regiment, but his rank within that regiment.
16:15Female sparrows have black bibs, but the size varies.
16:19The more vigorous birds have bigger bibs, and therefore higher ranks.
16:25Sparrows forage in flocks, and when there's lots of food in a small area, you might expect
16:29lots of quarrels, but there aren't.
16:34This is a private with no badges.
16:38This one is somewhat senior, a sergeant perhaps, a captain, and the colonel.
16:53There could be disputes not only over food, but over amenities, like dust baths.
17:10The privates are squabbling among themselves, but watch what happens when a corporal steps
17:16in, junior ranks retreat, or when a corporal gets too close to a sergeant.
17:31A sergeant, however, will give way to a captain, and no one should think of parting a colonel
17:37from his lunch.
17:41A quick flourish of his insignia is quite enough.
17:49Among birds that don't live in flocks, there is no need for the ranking system to be so
17:54multilayered.
17:58More hens may mingle, but each pair has its own territory.
18:02Their badges are the red beak and head shield, and the white patches on either side of the
18:06tail.
18:09Rivals assess one another's strength by the size and brilliance of those head shields.
18:26If they feel their ranks are equal, then they may not want to contest the position of the
18:30boundary, and they display the white tail patches to indicate that the confrontation
18:35is being broken off.
18:41But here, the male on the right is standing boldly upright.
18:44He reckons he's the senior, and he wants to enlarge his territory.
18:50The time for sending messages is over.
18:53This quarrel can only be settled by physical violence.
19:04Birds can get badly injured in these battles, but they have to be fought if a senior bird
19:09is to establish and retain his rank.
19:34Eventually, the junior bird surrenders.
19:55A new line has been drawn.
19:57They won't need to fight again as long as it's not overstepped.
20:04Communication by visual signals, however, has one major limitation.
20:09Except in completely open country, they only work at close range.
20:13In forests, sound signals will travel much further.
20:17So if a bird, in order to get enough food, needs a very large territory, it's likely
20:21to declare its territorial claims with sound.
20:31There are, of course, many different ways of making a noise, and knocking on a resonant
20:36tree trunk is one of them.
20:39In this part of the world, in Patagonia, on the southernmost tip of South America, two
20:43knots on a tree trunk has a very particular meaning, at least among birds.
20:48If I do it, I might even get an answer.
21:18It's a Magellanic woodpecker, one of the largest of all woodpeckers, and he thinks
21:33he's heard a rival.
21:44He comes in for a closer look.
22:08And here's his mate coming to support him.
22:16Now she joins in the dispute.
22:40He's now on my tree, and his mate is even closer.
22:57I've stopped knocking, so it seems to them that their rival has disappeared.
23:02So all is well.
23:08There's another drummer in the bird world, Australia's palm cockatoo.
23:12His beak is no good as a drumstick, so he uses a wooden one.
23:35And that noise too is made mechanically, by an African broadbill.
23:47It makes its call in the same sort of way as children do when they blow across a blade
23:51of grass.
23:52But instead of grass, the broadbill has specially strengthened and shaped wing feathers.
24:05But of course, most of the sounds made by birds come from their throats.
24:10The calls and songs that you hear in a tropical rainforest, however, are very different from
24:14those you might hear in a European or North American woodland.
24:18And there's a reason for that.
24:21The leaves in a tropical rainforest have smooth and shiny surfaces that reflect sound.
24:27So a complex call up here would have its notes slurred and confused.
24:33As a result, birds that live up here tend to have calls that are simple, short, and
24:39often very, very loud.
24:47That's the loudest of all, from a bare-throated bellbird.
25:03And this is a close rival, a screaming pihar.
25:13The chookwings must also be close to the top ten.
25:22All these birds call from high up in the canopy.
25:25Lower down, where the foliage is less dense, the calls can be different.
25:30For one thing, they can be longer.
25:46That's a curaçao.
25:54And this, a wattled guan.
26:03A longer call, of course, can contain more notes.
26:09A cagou in New Caledonia.
26:11This is a female.
26:14Her mate, some distance away, is listening.
26:24The family wandered apart as they foraged, and now they want to get together again.
26:52Their son has also heard the message.
27:09So the adult pair are reunited and they greet one another, as usual, with a visual display.
27:26But their son is still out there, somewhere.
27:56And once again, the family group is complete.
28:10The calls of the cagou can be heard half a mile away.
28:13But some birds need to communicate over even greater distances, and the best way to do
28:18that is with very low-pitched notes.
28:30An American bittern.
28:47An air sac in his chest acts as a resonator, so he starts by gulping in air and pumping
28:54it up.
29:04This call carries for over two miles, even through the thickets of reeds.
29:27And when the performance is over, the air sac slowly deflates.
29:44If you can get out of the reeds, then your calls are less impeded.
29:48The Australian musk duck does just that in order to broadcast his messages.
29:56And the smooth surface of the water also helps to reflect the sound far across the lake.
30:16The flap on his chin is a visual signal for any birds that come over for a closer look.
30:29However, if calls are directed to neighbours nearby, then they can become very elaborate.
30:38This red bishop makes an almost constant stream of high-pitched notes as he hops around his
30:43territory.
30:51And you can't get much more elaborate than this.
30:58The extraordinary display of the arapendula includes one of the strangest songs of all.
31:04So how do birds do it?
31:13How, for example, can canaries sing continuously for minutes on end?
31:21Slowing the singer down, which also lowers the pitch of the notes, allows us to understand
31:26what's happening.
31:29Between the notes, it takes mini-breaths to replenish its air supply, and in full song,
31:36it may do so 30 times a second.
31:47A bird's voice box can also produce two different notes simultaneously.
31:52It's not high in the throat like ours, but deep in its chest.
31:59Low notes come from one side, high from the other.
32:20By alternating between high and low notes, even short songs can carry very complex messages.
32:30And this is the champion.
32:35The cowbird uses over 40 different notes in his songs.
32:39Some of them are so high that they are beyond the hearing of many of us.
32:45Once again, if we slow the action down, we can hear what's going on.
32:55The left side is producing the low notes, and the right, the high.
33:05Others are made by combining the sounds higher up in the throat.
33:11It may take a cowbird two years to learn his song properly.
33:22And it's important that birds should get their calls exactly right, for they can be just
33:27as significant in proclaiming identity as their uniforms.
33:32Indeed, if a bird has a shy and retiring disposition, and lives in a secluded place like this English
33:40woodland, then its voice may be the only way that it can be recognized by another bird,
33:46or indeed by a birdwatcher.
33:51There are two kinds of warblers here.
33:53This is a chiffchaff, fueling up after its long flight from Africa.
34:05And this is a willow warbler.
34:06To me, it looks virtually identical, but wait until they sing.
34:15This is a chiffchaff.
34:25And this, a willow warbler.
34:44There's no mistaking who's who, as long as you can hear their calls.
34:55But a bird's call can tell another bird more than just what kind of bird it is that's singing.
35:03This patch of bush on a small offshore New Zealand island belongs to a male saddleback.
35:10He's held it throughout the year, and he knows who his neighbors are, because their
35:15calls vary slightly, and he can recognize each one individually.
35:20And there he is.
35:23Throughout the day, he keeps in regular contact with his neighbors.
35:36They each answer his call, and he can distinguish between them in the same way that we can distinguish
35:42between regional dialects.
35:47A northerner, a southerner, and someone from the east coast.
36:02If the right call comes from the right place, then he knows that his territory is safe from
36:06intruders, and he can happily go back to feeding.
36:15But if the call is from a saddleback, and it's one that he doesn't recognize, and if,
36:20as well as that, it comes from a completely new place, then he will react in a very different
36:26way.
36:27And, of course, it's quite easy for me to make that happen.
36:37I'm going to play him a recording of a saddleback from a different island.
36:45His response is swift and very aggressive.
36:54He comes down for a closer look.
37:00He gives another warning.
37:10This is a serious challenge to his territory.
37:12It can't be tolerated.
37:26Now, since his rival seems to be somewhere close by, he's making a visual threat, displaying
37:31the brown pat on his back.
37:33He's ready to fight, if only he could find who it is who needs fighting.
37:39Well, I guess that's enough of that.
37:44We'll leave them in peace.
38:00To many of us, however, this is the most delectable of natural sounds.
38:08It's an hour before dawn.
38:10It's spring.
38:11This is an English woodland, and all around, the dawn chorus.
38:16It's so familiar that perhaps we take it for granted, but there's a lot we don't know about
38:28it still.
38:34As first light brightens, different kinds of birds, one by one, join the choir.
39:04Why should they all sing together at this time?
39:07Wouldn't it be better for some to sing later, by themselves?
39:12And that's not the only puzzle.
39:16Why should it happen at this time of day?
39:19Well, at dawn, it's still quite cold.
39:22Insects are not yet up and about, so for many of the birds, there's nothing to eat, so they
39:29might as well sing.
40:00There's another possible reason.
40:05It's usually quite calm at dawn, and with no wind, these messages will travel far and
40:10still be recognisable.
40:15The chorus is the equivalent of our early morning news, except that it's broadcast in
40:2050 different languages simultaneously.
40:36By listening to it, this wren knows which of his neighbours is still alive.
40:44He knows where they are.
40:51And if there are any new males on the scene.
40:59Each kind of bird listens to its own particular section of the sound spectrum.
41:07The song thrush broadcasts in the mid-range.
41:28The wood pigeon's calls are somewhat lower.
41:34Smaller birds, like the firecrest, use the higher frequencies.
41:41These springtime messages from male birds not only say, this is my patch, they also
41:47say to passing females, why don't you come and join me?
41:54Robins have now extended their usual songs to carry this additional message.
42:18The male chaffinch has done the same.
42:26He may sing this song over 500,000 times in a season.
42:41By late spring, migrants have arrived from southern Europe and Africa and are adding
42:46their voices to the chorus.
43:02A wood warbler.
43:08A pied flycatcher.
43:11And a redstart.
43:24His mate, like many, will be impressed by the originality and complexity of his song.
43:34The male sedge warbler can produce 50 different notes and never sings the same song twice.
43:56He's like a jazz singer, continually improvising, and different males develop different singing
44:02styles.
44:19And this is perhaps the most lyrical of all European songsters.
44:26A nightingale.
44:33He may have 300 different love songs in his repertoire.
44:47And he will sing for a mate all through the night.
45:12What bird has the most elaborate, the most complex and the most beautiful song in the
45:19world?
45:20Well, I guess there are lots of contenders, but this bird must be one of them.
45:25The superb lyrebird of southern Australia.
45:43He clears a space in the forest to serve as his concert platform.
45:59To persuade females to come close and admire his plumes, he sings the most complex song
46:04he can manage, and he does that by copying the songs of all the other birds he hears
46:09around him, such as the kookaburra.
46:16It's a very convincing impersonation.
46:26Even the original is fooled.
46:36He can imitate the calls of at least 20 different species.
46:44He also, in his attempt to out-sing his rivals, incorporates other sounds that he hears in
46:49the forest.
46:52That was a camera shutter.
47:00And again.
47:05And now a camera with a motor drive.
47:15And that's a car alarm.
47:26And now the sounds of foresters and their chainsaws working nearby.
47:51That wonderful performance is only one example of the extent to which male birds will go
47:57in order to attract a female.
47:59The range and sheer extravagance of their courtship displays can be quite astonishing.
48:09And the range of relationships between male and female that these displays lead to is
48:14also much more varied than you might suppose.
48:20And it's that, the most crucial stage in the life of any bird, that we'll be looking at
48:26in the next program.
48:50Thank you.
49:20.