The Life of Birds - Mastery of Flight
Transcript
00:30Getting into the air is not easy. Indeed, for many birds, it's by far the most exhausting
00:58bit of the whole business of flight. But these shearwaters here in Japan have adopted a very
01:04labour-saving way... whoops, there we go... a very labour-saving way of doing so.
01:13They've taken to climbing trees.
01:22This particular tree is by far the most suitable for the take-off, and that bird may have come
01:30from as far as 30 or 40 yards away, wandering across the forest floor in order to climb
01:35it and get to the launch pad. There's another.
01:4240 feet up, a bend in the trunk makes a perfect platform for a take-off.
01:58These shearwaters will spend most of their lives in the air. They're true seabirds.
02:04They only come to land in order to nest. This particular species is exceptional in
02:10nesting in woodland. Most of them nest right on the edge of cliffs, and all of them get into the
02:16air by the simple method of simply falling into space. But land birds, on the other hand, have
02:23much greater problems. Most of them have to be able to get into the air from the ground with
02:27the standing start, and that takes a lot more energy. A pigeon begins by jumping vertically
02:36upwards. As it leaves the ground, it opens its wings and sweeps them forward, fanning the air
02:44downwards with maximum force. The second stroke must be equally vigorous, pushing the bird upwards.
02:53Now it leans forward and starts to go ahead, but the effort involved has been huge.
03:00Birds only a little bigger than a pigeon can't do this twice in quick succession.
03:07And a bird as big as an albatross can't do it at all. It has to use yet another way
03:13of getting into the air. It taxes along a runway.
03:21It's a method we use, too, in our machines.
03:37For the majority of birds, the most exhausting part of flight is now over.
03:42Those shearwaters achieved it by climbing trees, the pigeon jumped and flapped,
03:48and aeroplanes and albatross did it by running and creating a flow of air over their wings.
04:09When they do actually get into the air, a bird's flight seems almost effortless.
04:14And when you watch a really superb flyer like these albatross, then they seem almost to be
04:19defying the law of nature. They are, after all, big, heavy birds. How is it that they
04:25can withstand the pull of gravity that keeps the rest of us tied so firmly to the ground?
04:32The secret is a wing with a thick, rounded front edge that curves gently downwards towards the
04:38back edge, which is very thin. A thin wing is the most important part of a bird's flight.
04:44In fact, as a feather. As the bird glides forward, the air flowing under the wing is
04:51impeded by the wing's downward curve, so it becomes slightly compressed and that pushes the wing up.
05:00At the same time, the air flowing across the top of the wing is deflected upwards by the
05:05wing's front edge, so reducing its pressure. If the air is moving fast enough, then the
05:12slight suction from above, combined with the push from beneath, will be enough to lift the
05:17bird into the air, as it did during take-off, and ample to keep it aloft, as it's doing now.
05:23The trick is to make sure that the air does flow over the wing sufficiently quickly.
05:32Upward air currents can also sustain a bird in flight, and that's just what are created when
05:38breezes blowing in from the sea hit a cliff. If they're really strong,
05:43such updrafts can be so powerful that they alone will keep an albatross in the air.
05:52Out at sea, the waves deflect the wind upwards in somewhat smaller gusts, and the albatross
05:58is so skilful that it can sail on them for hours with scarce near movement of its wings.
06:04Most birds, however, once in flight, have to create that airflow across their wings
06:09by another method. They drive themselves forward by flapping. This knot is rowing itself through
06:17the air, stretching its wings forwards and beating them downwards. On the upstroke, it half-folds
06:23them to reduce their surface area, and therefore their resistance to the air. The feathers on its
06:29The feathers on its wing slide smoothly over one another, so that although the wing is
06:35continuously changing shape, its surface remains perfectly smooth and streamlined.
06:42Its body is also streamlined by its coat of feathers,
06:45and its feet are pressed against its tail so that drag is kept to a minimum.
06:51This mallard is flying at nearly 40 miles an hour,
06:55but its streamlining is so perfect that there is scarcely a ruffle to its feathers.
07:11It's only from behind that you notice the little flicks of the feathers over its tail
07:15To see how important streamlining is and how much energy it can save,
07:20just watch this osprey as it goes fishing.
07:33To take off, it has to take a long time.
07:37To take off again, with the fish in its tannins,
07:40the bird has to beat its wings with all the strength it can muster.
07:57But even now, the bird is still struggling.
08:06Even though it is in the air, the fish, hanging broadside, creates so much drag
08:11that the osprey has great difficulty in making any headway at all.
08:18But it knows how to solve the problem.
08:21Gripping the fish with just one foot, it manages to bring its other foot forward.
08:26Now, using both feet, the bird changes the position of the fish so that it faces ahead,
08:33and its streamlined shape reduces its drag so much
08:36that the osprey's wing beats become almost leisurely.
08:43Flying in formation also saves energy.
08:46A big bird like a pelican creates a trail of turbulence in the air,
08:51and this can give a following bird lift.
08:54The effect is at its greatest directly behind a bird's wingtip,
08:58so that is the best place for a following bird to fly.
09:02Pelicans also save some 20% of their energy by mixing flapping with gliding.
09:19Aerodynamically, it's more profitable for a bird to time its flaps
09:23with those of the bird ahead.
09:25So it is that pelicans give breathtaking displays of synchronised formation flying.
10:02The most economical way of flying, however,
10:06is to draw almost all the energy you need directly from the sun.
10:11As it warms the ground in the morning, the rocks reflect its heat,
10:15and shimmering columns of air, thermals, begin to rise.
10:23The sun is a powerful source of energy,
10:25but it is not the only source of energy.
10:27The thermals begin to rise.
10:32Gryphon vultures in Spain now leave the ledges on the cliffs
10:36where they spent the night, and launch themselves into the air.
10:42With the thermals rising powerfully beneath their outstretched wings,
10:47they sail effortlessly upwards.
10:57All they have to do is to make sure that they remain within the column of warm air.
11:08So dozens of them spiral together in tight circles, adjusting their flight with the tiniest
11:14movements of their wings and tail.
11:17There can be no more economical flight than this.
11:27The vultures' ability to read the air conditions above their landscape and detect exactly where
11:32the thermals are at their most powerful seems almost uncanny.
11:38But in recent years, human beings have also mastered it.
11:43When you hit a thermal in a glider, you really feel it.
11:47Your stomach drops beneath your feet.
11:49Right away.
11:50I'm going to roll into the thermal here, because there's one right there.
11:59Glider pilots spend a lot of time going around in circles, just like birds of prey do, because
12:04the thermal is a rising column of air in order to stay in if you have to turn.
12:12There's nothing to see there, is there, apart from what's on the ground.
12:15You can't see anything in the air to indicate this thermal.
12:18Right now, this thermal, there's no cumulus cloud over this thermal, but often there is.
12:22If you look over there, you can see all those cumulus clouds.
12:26Underneath most of those, there will be a thermal.
12:28And that's one of the indicators of lift.
12:30That's one thing we will look for.
12:32That's what birds look for also when they fly, I'm sure.
12:35Look, feel that.
12:36There's a big, rocky outcropping.
12:37And there's our lift.
12:38Look at that.
12:39Now, you can see that altimeter winding up.
12:40Look at that.
12:41How high could we go with this?
12:42We could probably go to about 14,000, 15,000 feet with no problem.
12:43And do the birds go as high as that?
12:44I've seen birds up at 16,000, 18,000 feet.
12:45They're just out there flying for fun.
12:46How do you know they're flying for fun?
12:47Because how can you see a mouse from 18,000 feet?
12:48Not hardly.
12:49I don't know.
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13:33Every flight has to end in a landing.
13:36That requires less energy, but perhaps more skill
13:39if disaster is to be avoided,
13:41particularly if you're a big bird like a pelican.
13:56A swan, one of the heaviest of flying birds,
13:59can only come down when cleared of algae...
14:02safely on the smoothest and most forgiving of natural surfaces, water.
14:08There, you can use your feet as brakes.
14:22Albatross are not so lucky.
14:24They have to alight on the ground.
14:27Indeed, their landings seem scarcely better than controlled accidents.
14:41Most birds, however, have to come down with much greater precision than that.
14:46They may have to land, after all, on a narrow ledge or a very thin branch.
14:51And to do that, they have to lower their flaps,
14:54they have to lower their flaps, put down the undercarriage
14:57and put on their brakes,
14:59so that they lose all airspeed at the precise moment
15:02that they come alongside their posts.
15:04And that requires great judgment and coordination.
15:11A griffon vulture is able to exploit the position of its nest,
15:15which is nearly always on the ledge of a cliff.
15:21It descends towards it at speed.
15:24It aims for a point below its nest
15:28and then brakes by swooping upwards,
15:31so that as it arrives at its ledge, its forward speed is zero.
15:38Landing into the wind helps any bird by keeping air flowing over the wings
15:42and maintaining lift until the last moment.
15:49So, one way or another,
15:51birds manage to complete an operation fraught with danger
15:55with virtually total success.
15:58As anyone who's had to pay for excess baggage at an airport knows,
16:02if you travel by air, it's important to keep your weight down.
16:06And this magnificent golden eagle
16:08manages to do that in the most thoroughgoing way.
16:12It's about the same size, I suppose, as a bulldog.
16:14Well, I couldn't possibly hold a bulldog on my wrist,
16:17but this bird only weighs about a quarter as much.
16:21So, how do birds manage to keep so light?
16:25Well, obviously, a beak is not so weighty
16:28as the bony jaws and teeth of a mammal like a bulldog
16:31or one of the bird's reptilian ancestors.
16:34Its disadvantage is that a bird can't chew.
16:38It can only pluck, crush or, like an eagle, tear and rip.
16:43They also have weight-saving features inside their bodies.
16:47A skeleton with fewer bones than a mammal's,
16:50no tail bones, one wing bone instead of five fingers,
16:53and a slim pelvis fused to the backbone.
16:58And the bones themselves are not solid like a mammal's, but hollow.
17:02Inside, they have a lattice of cross struts,
17:05so they are nonetheless very strong.
17:07But the most remarkable weight-saving features of all
17:10are those things that only birds possess, their feathers.
17:15They look simple, but, in fact, they have a very complex structure.
17:19The quills are hollow and very light, yet resilient and extremely strong.
17:25The filaments on either side of the quills
17:28are fringed with microscopic hooks that link them to their neighbours,
17:32so that they all latch together to form a continuous surface.
17:36And that means that if a feather gets damaged or overstrained,
17:40it can be repaired instantly.
17:43It can be zipped up.
17:48Not surprisingly, all birds lavish a great deal of attention on their feathers.
17:53After all, their lives depend on them.
17:56And since birds have no hands,
17:58they have little alternative but to care for them with their beaks.
18:14But one bird, uniquely, can't do that.
18:18The sword-billed hummingbird has a beak that is so long
18:22that there is no way that its tip can touch its feathers.
18:25It has a very long beak,
18:27and it has a very long tail,
18:29so it can't reach the tip of its beak.
18:32It has a very long beak,
18:34and it has a very long tail,
18:35so it can't reach the tip of its beak.
18:38It has a very long beak,
18:39that there is no way that its tip can touch its feathers.
18:44So it has to comb its plumage with one foot
18:48while balancing on its perch with the other.
18:50Not easy.
18:57A good bath is also important
19:00in keeping the feathers clean and in first-class condition.
19:04Most birds take one every day,
19:07and watching them doing so,
19:08it's difficult to avoid coming to the conclusion
19:11that they enjoy it just as much as we do.
20:07Not all birds, of course,
20:09can get to water that is deep enough for bathing.
20:13Then, like the quail that lives on dry plains
20:16and during the summer seldom finds so much as a puddle,
20:20they may have to use dust.
20:27This may not exactly make them cleaner,
20:30but it does apparently help in dislodging the dust
20:34from the surface of the water.
20:37It's also good for protecting parasites,
20:39such as lice that nibble their feathers,
20:42mites that scavenge bits of dead skin,
20:45and ticks that suck their blood.
20:57Many parrots and cockatoos grow special feathers
21:00that fray at the end into a fine powder.
21:04These feathers are scattered throughout the plumage,
21:07and if a cockatoo scratches itself at the end of its toilet,
21:10the powder is dislodged in clouds
21:13and is caught in its ruffled feathers.
21:26Exactly how this powder improves the feathers is not really certain,
21:30but it's probable that it helps with waterproofing,
21:33as well as by discouraging parasites.
21:38Parasites are such a problem
21:40that some birds are thought to recruit assistants
21:43to help in getting rid of them.
21:46Ants.
21:52Crows and jays deliberately land on an ant's nest
21:56and stir up the colony.
21:58This is done to ensure that the ants don't get in the way of the birds.
22:02Ants deliberately land on an ant's nest and stir up the colony
22:07so that the angry insects come rushing out and swarm all over them.
22:12In their irritation, the ants discharge formic acid,
22:15which is a particularly powerful insecticide.
22:22But we don't really understand this behaviour.
22:25It may be that the birds are stimulating the ants
22:28to get rid of their formic acid
22:30so that they are more digestible as meals.
22:40Regular, meticulous maintenance, of course,
22:42is essential for maximum efficiency and safety in the air,
22:46and that applies to everything that flies.
23:00WHIRRING
23:26Well, that was about 500 miles an hour,
23:29and, of course, birds can't equal that.
23:31But, nonetheless, before aeroplanes were invented,
23:35a bird was the fastest living thing in the air.
23:40The peregrine holds the record.
23:45Diving on its prey, it can exceed 200 miles an hour.
23:56It achieves maximum aerodynamic efficiency
23:59by sweeping back its wings like the jet fighter,
24:02and then it accelerates by beating them.
24:30The barn owl, on the other hand, owes its success as a hunter
24:34to its ability to fly extremely slowly.
24:38It hunts voles and mice, and to find them in the grass,
24:42it has to search very intently, and that takes time.
24:48Its wings, therefore, are shaped very differently from a peregrine's.
24:52They're rounded in outline and much broader,
24:54which gives the maximum lift possible.
24:59The peregrine is very swift at slow speeds,
25:02but the barn owl also has a very special adaptation
25:05for this kind of hunting.
25:10The rodents it seeks are often invisible from the air,
25:13hidden beneath the matted grass.
25:16The barn owl detects them by the rustling sounds they make.
25:20To do that, it has extremely acute hearing,
25:23the sounds being focused by the hair-like feathers of the discs
25:28But if it's to hear them, it has to fly very quietly indeed,
25:33and its wings are fitted with silencers,
25:36fluffy margins to its wing feathers.
25:39So, in the evenings, a barn owl can waft over the countryside
25:45as silent as a moth.
25:57BIRDS CHIRP
26:21This little dot, suspended in the sky,
26:25might seem to be the slowest flyer of all.
26:28It's a kestrel.
26:30It's not, in fact, truly stationary.
26:32It's facing into a gentle wind,
26:34so there's a current of air passing over its wings.
26:38And that gives it all the lift it requires.
26:42Silence is not as important for the kestrel as it is for the barn owl,
26:46for it hunts by sight.
26:55The wind has dropped a little.
26:57Now, to keep its position relative to the ground,
26:59it has to flap to keep the air moving over its wings.
27:18It's spotted something. A quick turn into the wind and a drop.
27:25A turn back to face the wind for a stationary check.
27:32Another quick look.
27:37But whatever it was has gone.
27:41Only one group of birds can manage to hover for any length of time
27:45without the help of a headwind.
27:47The hummingbirds.
27:55Their wings work in a way quite unlike that used by any other birds.
28:00They beat routinely, 25 times a second,
28:03so fast that they make the humming noise that gives them their name.
28:08It's impossible to see how they operate
28:10unless the camera slows them down.
28:13The wings have become, in effect, twirling blades.
28:17The birds can't see what they're doing.
28:21The wings have become, in effect, twirling blades
28:25that create downdrafts
28:27rather like those that man produces with his hovering machines.
28:37Helicopters, however, have a very special device.
28:41A wheel revolving continuously on an axle.
28:51No bird or any other animal has yet evolved a mechanism
28:55that can exactly parallel this.
28:59But hummingbirds have the next best thing.
29:02Wings which beat in a figure of eight and flick over on the backstroke.
29:07Unlike the wings of other birds, they're symmetrical in cross-section
29:11and work equally well with either surface uppermost.
29:21By changing the angle of the beat,
29:23the thrust can be directed not only downwards,
29:26but either forwards or backwards.
29:28So a hummingbird, steering with its tail,
29:31can move through the air in any direction.
29:38Beating wings at such speed, however, uses a lot of fuel.
29:42Even at rest, hummers need a great deal just to keep their bodies ticking over.
29:47So they have to refuel very frequently,
29:49but their fuel stations, flowers, close at night.
29:53What do they do then?
29:56This is a particular problem in the Andes
29:58where the nights can be very cold indeed.
30:02As evening comes on, the hill star hummingbird
30:05makes its way to its regular roosting place in a cave.
30:20After its regular toilet, it settles down for the night
30:24and, in effect, turns off all its motors.
30:31Its heart, that in flight contracted a thousand times a minute,
30:35slows until its beat is virtually undetectable.
30:41Its body temperature falls dramatically
30:43and its breathing seems to cease altogether.
30:46It's doing what a hedgehog does in winter,
30:49it's hibernating.
30:50But for a hummingbird, winter comes 365 times a year.
31:02The sun returns and the temperature begins to rise.
31:10The hill star starts up its motors.
31:12Its heartbeat accelerates, its muscles slowly warm to flying temperature.
31:19A quick pre-flight check.
31:26And it's off again.
31:29At higher altitudes, it seldom gets really warm, even at midday.
31:34This is the territory of the giant hummingbird.
31:39It's as big as a thrush.
31:41Great size helps in retaining body heat,
31:44but this is as big as a human body.
31:46Its size helps in retaining body heat,
31:48but this is as big as a hummingbird can get.
31:51Any larger and it couldn't beat its wings fast enough
31:54for this kind of flight.
32:00And this is one of the smallest of all birds.
32:03The purple-collared wood star from Ecuador,
32:06with a wingspan of scarcely more than two inches.
32:09Small wings are easier to flap,
32:12but the smaller they are, the faster they have to move
32:14to produce sufficient downward thrust.
32:17And this hummingbird can beat its wings at an astonishing 75 times a second.
32:22It's barely bigger than a moth.
32:29Indeed, this moth looks so like a tiny hummingbird,
32:33that some people in the south of England,
32:35where it appears regularly in the summer,
32:37think that they have been visited by a real hummer.
32:45The ability to fly gave birds the freedom of the planet.
32:50Rivers, deserts, seas, even mountain ranges
32:54are no obstacle to them,
32:56as they are to land-bound creatures such as ourselves.
32:59They can fly relatively fast,
33:02but they can't fly as fast as birds can fly.
33:05They can't fly as fast as birds can fly.
33:08They can't fly as fast as birds can fly.
33:11Birds, such as ourselves,
33:13they can fly relatively easily and quickly
33:16to collect a sudden glut of food.
33:20And that's exactly what has happened here.
33:22I'm in northern Canada.
33:24It's June, the beginning of the short Arctic summer.
33:28The rising temperatures have caused the plants
33:30to put out new leaves and roots.
33:32And tens of thousands of snow geese have come here to graze.
33:38They nested almost as soon as they arrived,
33:41and many have already got families.
34:07Even hummingbirds have come up to the far north
34:10to collect nectar from the bushes
34:12that are now briefly blooming within sight of glasses.
34:28On the Arctic coasts, little waders, western sandpipers,
34:33are collecting a rich harvest of small worms
34:36that are swarming in the mud.
34:46In the middle of the continent, on the prairies,
34:48sorghum and other grain crops are ripening in the summer sun.
34:52Dick's thistles, relatives of the common sparrow,
34:55have come up here to take their percentage.
35:06The warm weather has caused swarms of insects to hatch,
35:10and they provide the dick's thistles with the protein
35:13that's essential for the nourishment of their swiftly growing young.
35:37Hawks are also breeding here in the north.
35:40They were attracted by the seasonal abundance of voles
35:43and other small mammals, as well as the finches and songbirds
35:47that they need to feed their young.
35:50But the superabundance of summer is brief.
35:56By the end of July, the days are noticeably shortening.
36:00Many of the trees are preparing to shed their leaves.
36:05The winter is coming.
36:07It's time for the winter to come.
36:09It's time for the winter to come.
36:11It's time for the winter to come.
36:13It's time for the winter to come.
36:15It's time for the winter to come.
36:18The birds that flew up for the summer banquet can no longer stay.
36:26All across the northern hemisphere, the story is the same.
36:30From eastern Siberia, across Asia and Europe,
36:33to the woodlands and tundra of North America,
36:36birds are starting to fly south.
36:48The sandpipers are stocking up for the 6,000-mile journey
36:52that lies ahead of them.
36:54They eat so voraciously that they will nearly double their weight,
36:57putting on layers of fat on their upper thighs and their flanks.
37:01They even shrink their internal organs,
37:04partially absorbing them as though they were food reserves
37:07and replacing them with more readily available fat.
37:12They must wait for the right weather conditions,
37:15and then, when the wind blows strongly from the north, they set off.
37:46Hawks and vultures are also now finding it harder to discover any food.
37:50They, too, must prepare to leave.
37:53But the weather they require for their journeys is rather different.
37:56They need a good hot day
37:58when the thermals are shimmering upwards from the rocks
38:01that are still warming in the late summer sun.
38:08As the last thermals of summer start to rise,
38:11the birds circle up to great heights, 10,000 feet or more,
38:16to give themselves a good start for the long journey ahead.
38:20As they glide southwards, slowly losing height,
38:24they will look for another thermal and make for its base,
38:27so that, once again, they will be lifted high enough to reach the next.
38:35The snow geese are already on their way.
38:38Their departure triggered by the shortening days and the dropping temperatures.
38:42They will rely on straightforward muscle power.
38:53They will travel continuously for great lengths of time,
38:57both through the day and the night.
39:08The raptors, however, have had to stop to overnight in a roost.
39:12Without thermals, they can't travel far.
39:19The raptors are now on their way to their final destination,
39:23a place where they will have to spend the rest of their lives.
39:28The raptors, however, have had to stop to overnight in a roost.
39:33Without thermals, they can't travel far.
39:39But the snow geese fly on.
39:41The exertion of continuously beating their wings
39:44creates a lot of heat in their bodies,
39:46so travelling in the cool of the night does, in fact, suit them.
39:52They navigate by the stars.
39:54If the skies are heavily overcast for long periods,
39:57they may lose their way, but that is exceptional.
40:02Days are short.
40:05Day returns and the stars fade.
40:08Now they steer by the sun.
40:11But the sun, of course, moves from east to west during the day,
40:15so the fact that they're able to use it for navigation
40:18means that they must have internal clocks
40:21and know fairly exactly what the time is.
40:24Members of the same family travel together,
40:28calling to one another as they go.
40:31They are so close,
40:33I can barely think of what I'm going to do with my head.
40:36I can't see the stars clearly.
40:39I can't see them clearly.
40:41But no matter how close they are,
40:44I can feel that they are right there.
40:52The sun makes it clear,
40:55that if the stars are about to show up,
40:58it's only a matter of time before they do.
41:01And the geese have made it. One of them has been recorded as covering an astonishing 1700
41:28miles in a mere 70 hours. Now these fields in California will be their home until they
41:36return north on their spring migration.
41:45The sandpipers have gone even further. They have now reached Mexico.
42:02They will spend only a few days here, for this is merely a refuelling stop. They feed
42:07intensively, replacing the fat reserves that they've lost.
42:20The raptors, so conscious of the nature of the land beneath them that generates the thermals
42:24on which they depend, also look to it for their signposts. They're now passing Mexico's
42:31highest mountain, the Pico de Orizaba.
42:47There are no thermals to be found over the sea, so they are tied to the land. And that
42:52means that they have to go all the way round the western side of the Gulf of Mexico. There
42:58is, of course, a shortcut, directly south across the sea. Astonishingly, the little
43:05ruby-throat hummingbird tackles that 500-mile journey. It must necessarily be non-stop,
43:12for a hummingbird cannot land on the water.
43:16A feeder in Texas provides a ruby-throat with a final top-up of nectar. Its cruising speed
43:27is about 27 miles an hour, so if conditions are good, it could make the crossing by flying
43:34for a little over 18 hours. But that is right on the very limit of its endurance. If even
43:41a light headwind springs up, it will perish at sea.
43:52Flowers blooming on the Mexican shore await with life-saving nectar.
44:02A ruby-throat arrives after its epic journey and feeds urgently before it runs out of fuel
44:09and is fatally grounded.
44:15But even now, its journey is not finished. It still has several hundred miles to go and
44:21may travel as far as the Panama border.
44:34The hawks and vultures travelling round the western side of the Gulf have now reached
44:39Panama City. They came from all over North America, converged on the Isthmus and travelled
44:46together down that narrow corridor of land so that now, for the only time each year,
44:52they form dense flocks.
45:04Below on the mud of Panama Bay, the sandpipers are feeding. This, at last, is the end of
45:11their journey. The mud here will never freeze, the sea will enrich it daily, and each bird
45:16returns every year to exactly the same patch.
45:22The raptors rise once more in an immense vortex. From here, they will take their separate ways
45:29all over South America, some going as far south as Argentina. Only by dispersing widely
45:37will each bird find enough prey to sustain itself.
45:43The dickcissels have also travelled down the Isthmus of Panama. They too have come from
45:48all over North America and have now been funnelled together into flocks of gigantic size and
45:53density.
46:03This surely is the very acme of flying skill. How they coordinate their flight in these
46:09extraordinary concentrations, changing direction within it as if with one mind, is one of the
46:15unsolved mysteries of ornithology.
46:26Years ago, they, like the hawks and eagles, would have travelled on south from here and
46:30spread over the plains of North and South America to feed on the seeds of wild grasses.
46:39But here in Venezuela, they find great fields of cultivated grain, exactly like they found
46:45up in the north. So they have no need to disperse, but remain together and devastate the crops
46:52wherever they settle.
47:10It seems they positively prefer one another's company. Some of the flocks may be half a
47:16million strong, and man's practice of intensive cultivation allows them to stay and feed together.
47:33At night, they select a relatively small patch within a huge field of sugar cane, where the
47:39whole half million roost half a dozen birds to a single stem.
47:53Flying, when all is said and done, takes a great deal of energy. So birds have huge appetites
48:00and have to spend much of their lives in an unending search for food to fuel their expensive
48:07lifestyle. Just how they find it, we'll be looking at in the next programme, in The Life of Birds.