0:00:02 > 0:00:07*
0:00:52 > 0:00:55Getting into the air...is not easy.
0:00:55 > 0:01:01Indeed, for many birds it is the most exhausting part of flying.
0:01:01 > 0:01:06But these shearwaters, here in Japan, have adopted a labour-saving way -
0:01:06 > 0:01:11oops, there we go - a labour-saving way of doing so.
0:01:14 > 0:01:17They've taken to climbing trees.
0:01:21 > 0:01:26This particular tree is by far the most suitable for takeoff
0:01:26 > 0:01:32and that bird may have come from as far as 30 or 40 yards away,
0:01:32 > 0:01:38wandering across the forest floor, to climb it and reach the launch pad.
0:01:40 > 0:01:43There's another.
0:01:43 > 0:01:47A bend in the trunk is a perfect platform for a takeoff.
0:01:57 > 0:02:03These shearwaters will spend most of their lives in the air.
0:02:03 > 0:02:06They are true sea birds.
0:02:06 > 0:02:08They only come to land to nest.
0:02:08 > 0:02:13This species is exceptional in nesting in woodland.
0:02:13 > 0:02:18Most of them nest on the edge of cliffs. All of them get into the air
0:02:18 > 0:02:21by simply falling into space.
0:02:21 > 0:02:26But the land birds, on the other hand, have much greater problems.
0:02:26 > 0:02:32They get airborne with a standing start. That takes a lot more energy.
0:02:34 > 0:02:39A pigeon begins by jumping vertically upwards.
0:02:39 > 0:02:44As it leaves the ground, it opens its wings and sweeps them forward,
0:02:44 > 0:02:48fanning the air downwards with maximum force.
0:02:48 > 0:02:54The second stroke must be equally vigorous - pushing the bird upwards.
0:02:54 > 0:02:58Now, it leans forward and starts to go ahead.
0:02:58 > 0:03:01The effort involved has been huge.
0:03:01 > 0:03:06Slightly bigger birds can't do this twice in quick succession.
0:03:07 > 0:03:12And one as big as an albatross can't do it at all.
0:03:12 > 0:03:14It has another way...
0:03:19 > 0:03:22It taxies...along a runway.
0:03:26 > 0:03:30It's a method we use, too, in our machines.
0:03:41 > 0:03:46For the majority of birds, the most exhausting part of flight is over.
0:03:46 > 0:03:49Those shearwaters climbed trees -
0:03:49 > 0:03:52the pigeon jumped and flapped -
0:03:52 > 0:03:57and aeroplanes and albatross ran and created a flow of air
0:03:57 > 0:04:00over their wings.
0:04:13 > 0:04:19When they do get into the air, a bird's flight seems effortless.
0:04:19 > 0:04:25And superb flyers, like albatross, seem to defy the law of nature.
0:04:25 > 0:04:28They are, after all, big heavy birds.
0:04:28 > 0:04:35How can they withstand the gravity that keeps the rest of us firmly on the ground?
0:04:35 > 0:04:40The secret is a wing with a thick, rounded front edge
0:04:40 > 0:04:45that curves gently downwards towards the back edge, which is very thin -
0:04:45 > 0:04:48as thin, in fact, as a feather.
0:04:48 > 0:04:51As the bird glides forward,
0:04:51 > 0:04:56the air flowing UNDER the wing is impeded by the wing's downward curve.
0:04:56 > 0:05:01So, it becomes slightly compressed and that pushes the wing up.
0:05:01 > 0:05:06And air flowing across the TOP of a wing is sent up by its front edge,
0:05:06 > 0:05:09so reducing its pressure.
0:05:09 > 0:05:17If the air is moving fast enough, the slight suction from above, combined with the push from beneath,
0:05:17 > 0:05:21will lift the bird into the air - as it did during takeoff -
0:05:21 > 0:05:25and keep it aloft - as it's doing now.
0:05:25 > 0:05:30The trick is to ensure that air DOES flow over the wing quickly enough.
0:05:32 > 0:05:37Upward air currents can also sustain a bird in flight
0:05:37 > 0:05:42and that's what you get when breezes, blowing in from the sea, hit a cliff.
0:05:42 > 0:05:49If they are really strong, such updraughts can be powerful enough to keep an albatross in the air.
0:05:52 > 0:05:56Out at sea, the waves deflect the wind upwards
0:05:56 > 0:05:59in somewhat smaller gusts.
0:05:59 > 0:06:06The albatross is so skillful that it can sail on them for hours with scarcely a movement of its wings.
0:06:06 > 0:06:13Most birds, however, once in flight, have to create that airflow across their wings by another method.
0:06:13 > 0:06:17They drive themselves forward by flapping.
0:06:17 > 0:06:20This knot is "rowing" through air -
0:06:20 > 0:06:24stretching its wings forwards and beating them downwards.
0:06:24 > 0:06:30It folds them to reduce surface area and air resistance on the upstroke.
0:06:30 > 0:06:35The feathers on its wing slide smoothly over one another
0:06:35 > 0:06:41so that as the wing changes shape, its surface remains perfectly smooth.
0:06:42 > 0:06:46Its body is streamlined by its coat of feathers
0:06:46 > 0:06:52and its feet are pressed against its tail to keep drag to a minimum.
0:06:52 > 0:06:57This mallard is flying at nearly forty miles an hour
0:06:57 > 0:07:03but its streamlining is so perfect that its feathers are hardly ruffled.
0:07:13 > 0:07:20Only from behind, can you notice the flicks of its feathers over its tail and the back edge of its wings,
0:07:20 > 0:07:23which show just how fast it is travelling.
0:07:25 > 0:07:30To see how important streamlining is, and how much energy it can save,
0:07:30 > 0:07:33watch this osprey as it goes fishing.
0:07:45 > 0:07:52To take off again, with the fish in its talons, the bird must beat its wings with all its strength.
0:08:05 > 0:08:10But even now it is in the air, the fish, hanging broadside,
0:08:10 > 0:08:16creates so much drag that the osprey has difficulty in making any headway.
0:08:18 > 0:08:21It knows how to solve the problem.
0:08:21 > 0:08:26Gripping the fish with just one foot, it brings its other foot forward.
0:08:26 > 0:08:29Now, using both feet,
0:08:29 > 0:08:34the bird changes the position of the fish so that it faces ahead.
0:08:34 > 0:08:38Its streamlined shape reduces its drag so much
0:08:38 > 0:08:43that the osprey's wing beats verge on leisurely.
0:08:43 > 0:08:47Flying in formation also saves energy.
0:08:47 > 0:08:50A big bird, like a pelican,
0:08:50 > 0:08:55creates a trail of turbulence in the air, giving a following bird lift.
0:08:55 > 0:09:00The effect is at its greatest directly behind a bird's wing tip -
0:09:00 > 0:09:03the best place for a following bird.
0:09:03 > 0:09:08Pelicans save 20% of their energy by gliding as well as flapping.
0:09:19 > 0:09:25Aerodynamically, it's better for a bird to time its flaps
0:09:25 > 0:09:27with those of the bird ahead.
0:09:27 > 0:09:33So it is, that pelicans give amazing displays of synchronised flying.
0:10:13 > 0:10:16The most economical way of flying
0:10:16 > 0:10:21is to draw almost all the energy you need directly from the sun.
0:10:21 > 0:10:26As it warms the ground in the morning, the rocks reflect its heat
0:10:26 > 0:10:29and shimmering columns of air - thermals - begin to rise.
0:10:32 > 0:10:38Griffon vultures in Spain leave the ledges where they've spent the night
0:10:38 > 0:10:40and launch themselves into the air.
0:10:43 > 0:10:46As the thermals rise beneath their wings,
0:10:46 > 0:10:49they sail effortlessly upwards.
0:11:00 > 0:11:06All they have to do is ensure that they stay within the warm air column.
0:11:08 > 0:11:12So, dozens of them spiral together in tight circles,
0:11:12 > 0:11:17adjusting their flight with tiny movements of their wings and tail.
0:11:17 > 0:11:22There can be no more economical flight than this.
0:11:26 > 0:11:31The vultures' ability to read the air conditions above their landscape
0:11:31 > 0:11:37and detect where the thermals are at their most powerful, seems uncanny.
0:11:38 > 0:11:42But human beings have also mastered it.
0:11:42 > 0:11:47When you hit a thermal in the glider, you really feel it.
0:11:47 > 0:11:50Your stomach drops beneath your feet.
0:11:50 > 0:11:55Oooh! Ride away! We're gonna roll into the thermal. Wow!
0:11:59 > 0:12:04Glider pilots go round in circles a lot,
0:12:04 > 0:12:11as birds of prey do, because a thermal's a rising column of air and to stay in it, you have to turn.
0:12:11 > 0:12:16There's nothing to see, though, apart from what's on the ground.
0:12:16 > 0:12:24There's nothing in the air to show a thermal. There's no cumulus cloud over this one, but often there is.
0:12:24 > 0:12:29Under most of those cumulus clouds there, there would be a thermal.
0:12:29 > 0:12:35That's one thing we look for and I'm sure birds look for it, too.
0:12:35 > 0:12:40Look, feel that! There's a big, rocky outcropping
0:12:40 > 0:12:45and there's our lift. Look at that.
0:12:45 > 0:12:47The altimeter's winding up. Look.
0:12:47 > 0:12:51How high could we go with this?
0:12:51 > 0:12:55Probably to about 14-15,000 feet with no problem.
0:12:55 > 0:12:58And do the birds go as high as that?
0:12:58 > 0:13:01I've seen them up to 16-18,000 feet,
0:13:01 > 0:13:06looking like they're flying for fun. How do you know?
0:13:06 > 0:13:10Because how can you see a mouse from 18,000 feet?
0:13:10 > 0:13:16And they do tricks and aerobatics. Look, we're really going up now.
0:13:19 > 0:13:25The pressure on the wings is actually bending them, isn't it?
0:13:25 > 0:13:30The spar is flexible, so as you develop lift, it bends upwards -
0:13:30 > 0:13:33very similar to a bird.
0:13:35 > 0:13:38Every flight has to end in a landing.
0:13:38 > 0:13:44That requires less energy, but more skill if disaster is to be avoided -
0:13:44 > 0:13:48particularly if you're a big bird like a pelican.
0:13:58 > 0:14:02A swan, one of the heaviest of flying birds,
0:14:02 > 0:14:08can only come down on water's smooth and forgiving surface.
0:14:08 > 0:14:11There, you can use your feet as brakes.
0:14:22 > 0:14:27Albatross are not so lucky. They have to alight on the ground.
0:14:30 > 0:14:35Indeed, THEIR landings seem scarcely better than controlled accidents.
0:14:43 > 0:14:48Most birds have to come down with much greater precision than that.
0:14:48 > 0:14:54They may have to land, after all, on a narrow ledge or thin branch.
0:14:54 > 0:14:59To do that, they have to put down the undercarriage and brake
0:14:59 > 0:15:05so that they lose speed the moment they come alongside their perch.
0:15:05 > 0:15:08That requires judgment and co-ordination.
0:15:11 > 0:15:16A griffon vulture is able to exploit the position of its nest -
0:15:16 > 0:15:19usually on the ledge of a cliff.
0:15:20 > 0:15:23It descends towards it at speed.
0:15:25 > 0:15:28It aims for a point BELOW its nest,
0:15:28 > 0:15:31then brakes by swooping upwards
0:15:31 > 0:15:36so that as it arrives at its ledge, its forward speed is zero.
0:15:38 > 0:15:43Landing into the wind helps any bird. It keeps air flowing over the wings
0:15:43 > 0:15:46and maintains lift until the last moment.
0:15:50 > 0:15:53So, birds can complete an operation
0:15:53 > 0:15:58fraught with danger, with virtually total success.
0:15:59 > 0:16:06Anyone who has paid for excess baggage knows that if you fly, it's important to keep your weight down.
0:16:06 > 0:16:12And this magnificent golden eagle manages to do that beautifully.
0:16:12 > 0:16:17It's the same size as a bulldog, but I couldn't hold THAT on my wrist.
0:16:17 > 0:16:22But this bird only weighs about a quarter as much.
0:16:22 > 0:16:26How do birds manage to keep so light?
0:16:26 > 0:16:32Well, the beak is not so heavy as the bony jaws and teeth of a mammal
0:16:32 > 0:16:35or the birds' reptilian ancestors.
0:16:35 > 0:16:38Its disadvantage is that a bird can't chew -
0:16:38 > 0:16:42just pluck, crush or, like an eagle, tear and rip.
0:16:42 > 0:16:48They also have weight-saving features INSIDE their bodies -
0:16:48 > 0:16:55a skeleton with fewer bones than a mammal's, no tailbones, one wingbone instead of five fingers
0:16:55 > 0:16:58and a pelvis fused to the backbone.
0:16:58 > 0:17:03And the bones themselves are not solid like a mammal's, but hollow
0:17:03 > 0:17:07with interior cross-struts strengthening them.
0:17:07 > 0:17:11But the most remarkable weight-saving features
0:17:11 > 0:17:15are the things only birds possess - feathers.
0:17:15 > 0:17:20They look simple but they have a very complex structure.
0:17:20 > 0:17:26The quills are hollow and light, yet resilient and strong.
0:17:26 > 0:17:31The filaments attached to the quills are fringed with microscopic hooks.
0:17:31 > 0:17:36They link, latching together to form a continuous surface.
0:17:36 > 0:17:41That means that if a feather gets damaged or overstrained,
0:17:41 > 0:17:47it can be repaired instantly - it can be zipped up.
0:17:47 > 0:17:53Not surprisingly, all birds lavish a lot of attention on their feathers.
0:17:53 > 0:17:56After all, their lives depend on them.
0:17:56 > 0:18:01And since birds have no hands, they have little alternative
0:18:01 > 0:18:05but to care for them with their beaks.
0:18:32 > 0:18:36But one bird, uniquely, can't do that.
0:18:36 > 0:18:41The swordbill hummingbird's beak is so long,
0:18:41 > 0:18:44that its tip can't touch its feathers.
0:18:44 > 0:18:51It has to comb its plumage with one foot, while balancing on the other.
0:18:51 > 0:18:53Not easy!
0:18:57 > 0:19:01A good bath is also important
0:19:01 > 0:19:05to keep feathers clean and in first-class condition.
0:19:05 > 0:19:08Most birds take one every day.
0:19:08 > 0:19:16Watching them, it's difficult to avoid coming to the conclusion that they enjoy it just as much as we do.
0:20:12 > 0:20:18Not all birds, of course, can get to water deep enough for bathing.
0:20:18 > 0:20:20Then - like the quail
0:20:20 > 0:20:25that lives on dry plains and in the summer seldom finds even a puddle -
0:20:25 > 0:20:28they may have to use dust.
0:20:33 > 0:20:39This may not exactly make them cleaner, but it dislodges parasites
0:20:39 > 0:20:47such as lice that nibble their feathers, mites that scavenge dead skin and blood-sucking ticks.
0:20:55 > 0:21:03Many parrots and cockatoos grow special feathers that fray at the end into a fine powder.
0:21:03 > 0:21:07These are scattered throughout the plumage
0:21:07 > 0:21:13and when a bird scratches after its toilet, the powder is dislodged
0:21:13 > 0:21:16and caught in its ruffled feathers.
0:21:26 > 0:21:31Exactly HOW this powder improves the feathers is not really certain -
0:21:31 > 0:21:36it probably helps with waterproofing and discourages parasites.
0:21:46 > 0:21:49Parasites are such a problem
0:21:49 > 0:21:54that some birds may recruit assistants to help get rid of them...
0:21:55 > 0:21:58..ants.
0:22:02 > 0:22:08Crows and jays deliberately land on an ants' nest and stir up the colony
0:22:08 > 0:22:11so that they swarm all over them.
0:22:11 > 0:22:16In their irritation, the ants discharge formic acid -
0:22:16 > 0:22:19a particularly powerful insecticide.
0:22:21 > 0:22:26But we don't really understand this behaviour.
0:22:26 > 0:22:32Maybe the birds are stimulating the ants to get rid of their formic acid
0:22:32 > 0:22:35so that they are more digestible.
0:22:41 > 0:22:44Regular, meticulous maintenance
0:22:44 > 0:22:48is essential for safety in the air -
0:22:48 > 0:22:51for everything that flies.
0:23:27 > 0:23:29That was about 500 miles an hour
0:23:29 > 0:23:35and birds can't equal that. But before planes were invented,
0:23:35 > 0:23:39a bird was the fastest living thing in the air.
0:23:39 > 0:23:43The peregrine holds the record.
0:23:46 > 0:23:51Diving on its prey, it can exceed 200 miles an hour.
0:23:56 > 0:24:03It achieves maximum aerodynamic efficiency by sweeping back its wings like the jet fighter.
0:24:03 > 0:24:06Then, it accelerates by beating them.
0:24:31 > 0:24:39The barn owl, on the other hand, owes its success as a hunter to its ability to fly extremely slowly.
0:24:39 > 0:24:43It hunts voles and mice
0:24:43 > 0:24:48and to find them in grass, it has to search intently - that takes time.
0:24:51 > 0:24:56Its wings, therefore, are shaped very differently from a peregrine's.
0:24:56 > 0:25:01They're rounded and much broader - giving maximum lift at slow speeds.
0:25:01 > 0:25:06The barn owl also has a very special adaptation for this kind of hunting.
0:25:07 > 0:25:12The rodents it seeks are often invisible from the air -
0:25:12 > 0:25:15hidden beneath the matted grass.
0:25:15 > 0:25:20The barn owl detects them by the rustling sounds they make.
0:25:20 > 0:25:26It has acute hearing - sounds are focused by hair-like feathers
0:25:26 > 0:25:29on discs on the sides of its head.
0:25:29 > 0:25:34But if it is to hear them, it has to fly very quietly indeed
0:25:34 > 0:25:39so its wings have silencers - fluffy margins to its wing feathers.
0:25:39 > 0:25:46So, in the evenings, a barn owl can waft over the countryside as silent as a moth.
0:26:21 > 0:26:25This little dot, suspended in the sky,
0:26:25 > 0:26:29might seem to be the slowest flyer of all.
0:26:29 > 0:26:33It's a kestrel. It's not, in fact, truly stationary.
0:26:33 > 0:26:39It's facing a gentle wind, so an air current passes over its wings -
0:26:39 > 0:26:42giving it all the lift it requires.
0:26:42 > 0:26:49Silence is not as important for the kestrel as it is for the barn owl, for IT hunts by sight.
0:26:54 > 0:26:57The wind has dropped a little.
0:26:57 > 0:27:02Now, to keep its position relative to the ground
0:27:02 > 0:27:06it must flap to keep air moving over its wings.
0:27:17 > 0:27:22It has spotted something - a quick turn into the wind and a drop...
0:27:26 > 0:27:31A turn back to face the wind for a stationary check...
0:27:32 > 0:27:35Another quick look...
0:27:36 > 0:27:40But whatever it was has gone.
0:27:41 > 0:27:46Only one group of birds can manage to hover for any length of time
0:27:46 > 0:27:50without the help of a headwind - the hummingbirds.
0:27:57 > 0:28:00HUMMING
0:28:03 > 0:28:08Their wings work in a way quite unlike that used by any other bird.
0:28:08 > 0:28:14They beat routinely 25 times a second - so fast that they hum,
0:28:14 > 0:28:16hence their name.
0:28:16 > 0:28:22It's impossible to see how they operate without slowing the camera.
0:28:22 > 0:28:27The wings have become twirling blades that create downdraughts -
0:28:27 > 0:28:33rather like those that man produces with HIS hovering machines.
0:28:37 > 0:28:41Helicopters, however, have a special device -
0:28:41 > 0:28:46a wheel revolving continuously on an axle.
0:28:52 > 0:28:57No bird or other animal has evolved a mechanism that can parallel this.
0:28:58 > 0:29:03But hummingbirds have the next best thing -
0:29:03 > 0:29:08wings which beat in a figure eight and flick over on the backstroke.
0:29:08 > 0:29:16Uniquely, their wings have symmetry in cross section and work equally well with either surface uppermost.
0:29:20 > 0:29:23By changing the angle of the beat,
0:29:23 > 0:29:29the thrust can be directed forwards or backwards as well as downwards.
0:29:29 > 0:29:34So, a hummingbird, steering with its tail, can move in any direction.
0:29:37 > 0:29:43Beating wings at such speed, however, uses a lot of fuel.
0:29:43 > 0:29:47Even at rest, hummers need lots just to keep their bodies ticking over.
0:29:47 > 0:29:50They have to refuel very frequently.
0:29:50 > 0:29:55But their fuel STATIONS - flowers - close at night. So, what do they do?
0:29:55 > 0:30:01It's a problem in the Andes, where the nights can be very cold indeed.
0:30:01 > 0:30:06As evening comes on, the hillstar hummingbird
0:30:06 > 0:30:11makes its way to its regular roosting place in a cave.
0:30:22 > 0:30:28After its regular toilet, it settles down for the night and, in effect, turns off all its motors.
0:30:30 > 0:30:35Its heart, that in flight contracted 1,000 times a minute,
0:30:35 > 0:30:40slows until its beat is virtually undetectable.
0:30:41 > 0:30:46Its body temperature falls and its breathing seems to cease.
0:30:46 > 0:30:49Like a hedgehog in winter,
0:30:49 > 0:30:51it's hibernating.
0:30:51 > 0:30:56But for a hummingbird, winter comes 365 times a year.
0:31:02 > 0:31:06The sun returns and the temperature rises.
0:31:10 > 0:31:13The hillstar starts up its motors.
0:31:13 > 0:31:18Its heartbeat quickens, its muscles slowly warm to flying temperature.
0:31:23 > 0:31:26A quick pre-flight check...
0:31:30 > 0:31:32..and it's off again!
0:31:32 > 0:31:37At higher altitudes, it seldom gets warm - even at mid-day.
0:31:37 > 0:31:42This is the territory of the giant hummingbird.
0:31:43 > 0:31:46It's as big as a thrush.
0:31:46 > 0:31:51Its size helps retain body heat, but this is as big as a hummingbird gets.
0:31:51 > 0:31:56Any larger and it couldn't beat its wings fast enough for flight.
0:32:00 > 0:32:04And this is one of the smallest of all birds -
0:32:04 > 0:32:09a purple-collared woodstar from Ecuador with a two-inch wingspan.
0:32:09 > 0:32:14Small wings are easier to flap, but they must move faster
0:32:14 > 0:32:18to produce sufficient downward thrust.
0:32:18 > 0:32:23And this hummingbird beats its wings at an astonishing 75 times a second.
0:32:23 > 0:32:26It's barely bigger than a moth.
0:32:29 > 0:32:33This moth looks so like a tiny hummingbird
0:32:33 > 0:32:41that people in the south of England, where it often appears, think that they have seen a real hummer.
0:32:49 > 0:32:54GEESE HONK
0:32:59 > 0:33:04The ability to fly gave birds the freedom of the planet.
0:33:04 > 0:33:09Rivers, deserts, seas, even mountain ranges are no obstacle to them
0:33:09 > 0:33:14as they are to land-bound creatures such as ourselves.
0:33:14 > 0:33:19They can fly easily and quickly to collect a sudden glut of food.
0:33:19 > 0:33:22And that's what has happened here.
0:33:22 > 0:33:28I'm in northern Canada. It's June - the start of the short arctic summer.
0:33:28 > 0:33:33Rising temperatures have caused the plants to put out leaves and roots,
0:33:33 > 0:33:38and tens of thousands of snow geese have come here to graze.
0:33:40 > 0:33:45They nested almost as soon as they arrived,
0:33:45 > 0:33:48and many have already got families.
0:33:54 > 0:33:58CHICKS CHIRRUP
0:34:07 > 0:34:13Even hummingbirds have come to the far north to collect nectar
0:34:13 > 0:34:18from the bushes that are now briefly blooming within sight of glaciers.
0:34:28 > 0:34:32On the Arctic coast, little waders -
0:34:32 > 0:34:39Western sandpipers - are collecting a rich harvest of small worms that are swarming in the mud.
0:34:44 > 0:34:48In the middle of the continent, on the prairies,
0:34:48 > 0:34:53grain crops are ripening in the summer sun.
0:34:53 > 0:34:58Dickcissels, relatives of the common sparrow, are here for their share.
0:35:16 > 0:35:21The warm weather has caused swarms of insects to hatch.
0:35:21 > 0:35:26They provide young dickcissels with essential protein.
0:35:37 > 0:35:41Hawks are also breeding here in the north -
0:35:41 > 0:35:45attracted by the seasonal abundance of small mammals,
0:35:45 > 0:35:51finches and songbirds that they need to feed their young.
0:36:03 > 0:36:07But the superabundance of summer is brief.
0:36:11 > 0:36:14By the end of July, days shorten.
0:36:14 > 0:36:18Many trees are preparing to shed their leaves.
0:36:22 > 0:36:27The birds that flew up for the summer banquet can no longer stay.
0:36:30 > 0:36:34Across the northern hemisphere, the story is the same.
0:36:34 > 0:36:40From Siberia, across Asia and Europe to the tundra of North America,
0:36:40 > 0:36:43birds are starting to fly south.
0:36:48 > 0:36:54The sandpipers are stocking up for the 6,000-mile journey ahead.
0:36:54 > 0:36:59They eat so voraciously that they will nearly double their weight -
0:36:59 > 0:37:02adding layers of fat to their flanks.
0:37:02 > 0:37:09They even shrink their internal organs, partially absorbing them as though they were food reserves
0:37:09 > 0:37:13and replacing them with fat.
0:37:13 > 0:37:16They must wait for the right weather conditions.
0:37:16 > 0:37:21Then, when the wind blows strongly from the north, they set off.
0:37:47 > 0:37:51Hawks and vultures are now finding it harder to discover any food, too.
0:37:51 > 0:37:54They, too, must prepare to leave.
0:37:54 > 0:37:58But the weather THEY require is rather different.
0:37:58 > 0:38:06They need a hot day, when thermals shimmer from the rocks that are still warming in the late summer sun.
0:38:09 > 0:38:14As the last thermals of summer start to rise,
0:38:14 > 0:38:19the birds circle up to great heights - 10,000 feet or more -
0:38:19 > 0:38:23to get a good start for their long journey.
0:38:23 > 0:38:27As they glide southwards, slowly losing height,
0:38:27 > 0:38:34they will make for another thermal's base, so that again they will be lifted high enough to reach the next.
0:38:36 > 0:38:39The snow geese are already on their way -
0:38:39 > 0:38:44thanks to shortening days and dropping temperatures.
0:38:44 > 0:38:49They will rely on straightforward muscle power.
0:38:53 > 0:38:59They will travel continuously for great lengths of time,
0:38:59 > 0:39:02both through the day and the night.
0:39:28 > 0:39:33The raptors, however, have had to stop, to overnight in a roost.
0:39:33 > 0:39:36Without thermals they can't go far.
0:39:43 > 0:39:46But the snow geese fly on.
0:39:46 > 0:39:52The exertion of continuously beating their wings creates body heat,
0:39:52 > 0:39:57so travelling in the cool of the night does, in fact, suit them.
0:39:57 > 0:40:00They navigate by the stars.
0:40:00 > 0:40:05If the skies are overcast, they may get lost - but that is exceptional.
0:40:07 > 0:40:11Day returns and the stars fade.
0:40:11 > 0:40:14Now, they steer by the sun.
0:40:14 > 0:40:19But the sun, of course, moves from east to west during the day,
0:40:19 > 0:40:26so for navigation, they must have internal clocks and know fairly exactly what the time is.
0:40:26 > 0:40:30Members of the same family travel together,
0:40:30 > 0:40:34calling to one another as they go.
0:40:34 > 0:40:38GEESE HONK
0:41:21 > 0:41:24And the geese have made it.
0:41:24 > 0:41:30One has been recorded as covering 1,700 miles in a mere 70 hours!
0:41:30 > 0:41:34These Californian fields are now home
0:41:34 > 0:41:38until they return north on their spring migration.
0:41:45 > 0:41:51The sandpipers have gone even further - they have reached Mexico.
0:42:01 > 0:42:05They will spend only a few days here,
0:42:05 > 0:42:07for this is merely a refuelling stop.
0:42:07 > 0:42:12They feed intensively, replacing fat reserves that they have lost.
0:42:20 > 0:42:27The raptors, so conscious of the nature of the land beneath them that generates essential thermals,
0:42:27 > 0:42:30also look to it for their signposts.
0:42:30 > 0:42:36They are passing Mexico's highest mountain - the Pico de Orizaba.
0:42:47 > 0:42:53There are no thermals over the sea, so they are tied to the land
0:42:53 > 0:42:59and have to go right round the western side of the Gulf of Mexico.
0:42:59 > 0:43:04There is, of course, a short cut - directly south, across the sea.
0:43:04 > 0:43:10The ruby-throat hummingbird tackles that 500-mile journey.
0:43:10 > 0:43:15It must be non-stop, for a hummingbird cannot land on water.
0:43:15 > 0:43:21A feeder in Texas provides a final top-up of nectar for a ruby-throat.
0:43:27 > 0:43:31Its cruising speed is about 27 miles an hour.
0:43:31 > 0:43:38So, if conditions are good, it could make the crossing by flying for a little over 18 hours.
0:43:38 > 0:43:41That's the limit of its endurance -
0:43:41 > 0:43:45if there's even a light headwind, it will perish at sea.
0:43:51 > 0:43:56Delphiniums blooming on the Mexican shore await with life-saving nectar.
0:44:02 > 0:44:06A ruby-throat arrives after its epic journey
0:44:06 > 0:44:11and feeds urgently, before it runs out of fuel and is fatally grounded.
0:44:15 > 0:44:19But, even now, its journey is not finished.
0:44:19 > 0:44:22It still has several hundred miles to go
0:44:22 > 0:44:26and may go as far as the Panama border.
0:44:34 > 0:44:39The hawks and vultures, travelling round the western side of the Gulf,
0:44:39 > 0:44:44have now reached Panama City. They came from all over North America,
0:44:44 > 0:44:49converged on the isthmus and flew together down that strip of land
0:44:49 > 0:44:54so that now, for the only time each year, they form dense flocks.
0:45:04 > 0:45:12Below, on the mud of Panama Bay, the sandpipers are feeding. This, at last, is the end of their journey.
0:45:12 > 0:45:16The mud here never freezes, the sea enriches it
0:45:16 > 0:45:21and each bird returns every year to exactly the same patch.
0:45:23 > 0:45:27The raptors rise once more in an immense vortex.
0:45:27 > 0:45:32They will take their separate ways all over South America -
0:45:32 > 0:45:35some going as far south as Argentina.
0:45:35 > 0:45:39Only by dispersing widely will each bird find enough prey to survive.
0:45:43 > 0:45:48The dickcissels have also travelled down the isthmus of Panama.
0:45:48 > 0:45:52They, too, have come from all over North America
0:45:52 > 0:45:57and have now been funnelled together into gigantic, dense flocks.
0:46:02 > 0:46:07This, surely, is the very acme of flying skill.
0:46:07 > 0:46:12How they co-ordinate their flight in those extraordinary concentrations,
0:46:12 > 0:46:17changing direction as if with one mind, is a mystery of ornithology.
0:46:25 > 0:46:31Years ago, they, like the hawks and eagles, would have gone on south
0:46:31 > 0:46:36and spread over the American plains to feed on the seeds of wild grasses.
0:46:38 > 0:46:43But here in Venezuela, they find great fields of cultivated grain
0:46:43 > 0:46:47exactly like they found in the north.
0:46:47 > 0:46:54So, they have no need to disperse, but remain together and devastate the crops wherever they settle.
0:47:09 > 0:47:14It seems they positively prefer one another's company -
0:47:14 > 0:47:17flocks may be half a million strong.
0:47:17 > 0:47:21And Man's practice of intensive cultivation
0:47:21 > 0:47:25allows them to stay and feed together.
0:47:32 > 0:47:37At night, they select a relatively small patch within a huge field,
0:47:37 > 0:47:41where the whole half-million roost -
0:47:41 > 0:47:44half a dozen birds to a single stem.
0:47:53 > 0:47:58Flying, when all is said and done, takes a great deal of energy.
0:47:58 > 0:48:01So, birds have huge appetites
0:48:01 > 0:48:06and have to spend much of their lives in an unending search for food
0:48:06 > 0:48:09to fuel their expensive lifestyle.
0:48:09 > 0:48:16Just how they find it, we will be looking at in the next programme in The Life Of Birds.
0:48:47 > 0:48:51Subtitles by Alison Haggart BBC Scotland - 1998