0:00:54 > 0:00:58All living creatures on the earth and all material objects on it
0:00:58 > 0:01:02are subject to the pull of one great force.
0:01:02 > 0:01:04The force of gravity.
0:01:04 > 0:01:08Were that to be suspended, even for a moment,
0:01:08 > 0:01:11the most extraordinary things would begin happen.
0:01:11 > 0:01:15I, for example, would suddenly float into the air,
0:01:15 > 0:01:17because I at the moment...
0:01:19 > 0:01:24..am flying in an aircraft on a very special course which in effect
0:01:24 > 0:01:33cancels out the effect of gravity. So I float easily through the air.
0:01:33 > 0:01:35'Our plane is climbing and diving
0:01:35 > 0:01:38'as though it were on a giant roller coaster,
0:01:38 > 0:01:42'and as it goes over the crest of its climb, it really lifts you out
0:01:42 > 0:01:44'of your seat and keeps you there.
0:01:46 > 0:01:51'If there were no gravity on Earth, seas would rise from their beds
0:01:51 > 0:01:55'just as this water lifts out of its cup and disintegrates into droplets.
0:02:08 > 0:02:10'Nothing would remain where it was placed.
0:02:10 > 0:02:12'There would be no up and no down.
0:02:12 > 0:02:15'There would no longer be the sense of earthly order that we take
0:02:15 > 0:02:17'so much for granted.'
0:02:21 > 0:02:24Some creatures have managed to overcome the force of gravity
0:02:24 > 0:02:28sufficiently to enable them to fly, but the only ones that have been able
0:02:28 > 0:02:32to match this total freedom in the air that I have at the moment
0:02:32 > 0:02:33are those that are so small
0:02:33 > 0:02:36that they are, in effect, weightless.
0:02:38 > 0:02:42And there are more of them -
0:02:42 > 0:02:45both plant and animal -
0:02:45 > 0:02:46than you might think.
0:03:05 > 0:03:08It's the force of gravity which holds the clouds around the earth
0:03:08 > 0:03:11and the air in which they float.
0:03:11 > 0:03:13You can't touch air like a solid object -
0:03:13 > 0:03:15it's invisible and all-pervasive,
0:03:15 > 0:03:19so it's easy to forget that it has real substance.
0:03:19 > 0:03:22But it's only by exploiting the presence of air that seeds
0:03:22 > 0:03:24and insects, birds and man
0:03:24 > 0:03:28are able to overcome gravity and float above the earth's surface.
0:03:30 > 0:03:33Dandelion seeds rise because a puff of air carries them up
0:03:33 > 0:03:36and they fall slowly because their parachutes catch the air beneath.
0:03:40 > 0:03:43A tuft of fluff will serve the same purpose.
0:03:44 > 0:03:48Milkweed and cotton grass, willowherb and thistles,
0:03:48 > 0:03:51all provide their seeds with downy floats.
0:03:51 > 0:03:54These delay the fall of the seeds
0:03:54 > 0:03:57for so long that currents in the air, winds,
0:03:57 > 0:04:01can carry them for hundreds of miles from their parents.
0:04:12 > 0:04:16Seeds like these have crossed the widest oceans and landed
0:04:16 > 0:04:18on the loneliest islands.
0:04:20 > 0:04:22Pollen grains are so small,
0:04:22 > 0:04:25they don't even need fluff to keep in the air.
0:04:25 > 0:04:28The microscopic roughness of their surface is enough.
0:04:30 > 0:04:32Spores, shot out from a puffball
0:04:32 > 0:04:36and shed in tens of millions from the gills of fungi,
0:04:36 > 0:04:37are smaller still.
0:04:37 > 0:04:41The merest breath of air sweeps them away like smoke.
0:04:52 > 0:04:55The gossamer that sometimes carpets the meadows
0:04:55 > 0:04:58is the animal equivalent of downy seeds.
0:05:00 > 0:05:05It's produced by thousand upon thousand of tiny spiders.
0:05:07 > 0:05:12The young of many species of spider, soon after they hatch,
0:05:12 > 0:05:17climb to the top of grass stems or onto the tiny pinnacles of stones
0:05:17 > 0:05:19and lift their abdomens upwards.
0:05:22 > 0:05:25Then, from the spinnerets at the tip,
0:05:25 > 0:05:28they produce a thread of finest silk.
0:05:45 > 0:05:49As it lengthens and the wind catches it, the spiderling turns,
0:05:49 > 0:05:52grabs the thread with its forelegs, and away it goes.
0:06:10 > 0:06:14Only the tiniest and the lightest of animals and plants
0:06:14 > 0:06:16can defy gravity in this way.
0:06:18 > 0:06:21Many seeds are far too heavy to be lifted by the breeze,
0:06:21 > 0:06:23no matter how downy they are.
0:06:23 > 0:06:26But if they are produced at the top of a tall tree,
0:06:26 > 0:06:29they can exploit the pull of gravity.
0:06:29 > 0:06:32These, hanging in the jungle of Venezuela, grow wings.
0:06:32 > 0:06:37The wing is so shaped and weighted, with the seed at one end,
0:06:37 > 0:06:41that as it falls through the air, it spins.
0:06:58 > 0:07:00This protracted fall
0:07:00 > 0:07:04gives the breeze a chance to deflect the seeds sideways
0:07:04 > 0:07:07so that they will land some distance away from the parent tree.
0:07:13 > 0:07:17The seed is functioning like the blade of a helicopter.
0:07:17 > 0:07:20Its wing is so shaped that, as it sweeps round,
0:07:20 > 0:07:24it puts pressure on the air below and reduces pressure just above
0:07:24 > 0:07:27so that the seed hangs in the air
0:07:27 > 0:07:29much longer than it would otherwise do.
0:07:30 > 0:07:34Sycamore seeds spin and glide in the same way.
0:07:40 > 0:07:42And animals glide too.
0:07:51 > 0:07:56The flying frog of Central America has a parachute on each foot,
0:07:56 > 0:07:59formed by the web of skin between its toes.
0:07:59 > 0:08:02So one jump from a high branch is enough to carry it from
0:08:02 > 0:08:04one tree to another.
0:08:12 > 0:08:15In South-East Asia, there's a gecko
0:08:15 > 0:08:18that not only has a parachute on each foot,
0:08:18 > 0:08:20but flanges on its body and tail.
0:08:27 > 0:08:30Another lizard glides through the same forests by extending
0:08:30 > 0:08:35even bigger wings of skin from its flanks supported by elongated ribs.
0:08:41 > 0:08:45And the best glider of all - a flying squirrel.
0:08:45 > 0:08:47Its huge cloak of floppy skin
0:08:47 > 0:08:49sometimes serves as a simple parachute.
0:08:52 > 0:08:57But in horizontal flight, it does more than just trap air beneath it.
0:09:00 > 0:09:04As air passes over the front edge, it's deflected slightly upwards,
0:09:04 > 0:09:07so creating a slight reduction in the air pressure
0:09:07 > 0:09:10on the upper surface, just as happens on an aircraft wing
0:09:10 > 0:09:13or the spinning blade of a sycamore seed,
0:09:13 > 0:09:18so the squirrel creates a little lift and floats through the air.
0:09:35 > 0:09:37All those creatures are gliders.
0:09:37 > 0:09:41Some of them can control to some extent the direction
0:09:41 > 0:09:46in which they glide, but none of them can climb in the air
0:09:46 > 0:09:49except with the help of rising air currents,
0:09:49 > 0:09:53like the breezes which come sweeping up these downs in southern England,
0:09:53 > 0:09:57carrying with them whole populations of seeds and spores and spiders.
0:09:57 > 0:10:01But there are no such breezes down below the grass stems.
0:10:01 > 0:10:04Down there, if creatures want to climb into the air,
0:10:04 > 0:10:07they have to have true powered flight.
0:10:10 > 0:10:13The most demanding moment is at take-off.
0:10:15 > 0:10:17The insect has to haul itself
0:10:17 > 0:10:20into the air by sheer unaided muscle power.
0:10:20 > 0:10:24The downward sweep of the wings produces greater pressure in the air
0:10:24 > 0:10:26beneath than in that above,
0:10:26 > 0:10:28so, in a slightly different way
0:10:28 > 0:10:31from the gliding cloak of the squirrel,
0:10:31 > 0:10:36beating wings also create lift, and the insect is sucked upwards.
0:10:42 > 0:10:46Bigger insects, like grasshoppers, boost their take-off with
0:10:46 > 0:10:49a powerful spring.
0:10:49 > 0:10:51Birds are even bigger and heavier.
0:10:51 > 0:10:55For them, too, getting into the air is the most energetic and demanding
0:10:55 > 0:10:57part of flying.
0:10:59 > 0:11:01They also use their well-muscled legs
0:11:01 > 0:11:03to assist their labouring wings.
0:11:03 > 0:11:06They jump even before their wings begin their downbeat.
0:11:14 > 0:11:17But really big birds, to get airborne,
0:11:17 > 0:11:19have to generate the extra lift
0:11:19 > 0:11:23by increasing the speed of air streaming over their wings,
0:11:23 > 0:11:27so they get up quite a lot of speed on the ground or over water,
0:11:27 > 0:11:29just as an aircraft does, before they can take off.
0:11:41 > 0:11:45Once in the air, a whole new environment is open to them,
0:11:45 > 0:11:48and flying animals of all kinds exploit it to the full.
0:11:51 > 0:11:54Damsel flies catch their food in the air,
0:11:54 > 0:11:57mate in the air and even fight in the air.
0:11:57 > 0:12:02As males squabble over territory, they flutter their patterned wings
0:12:02 > 0:12:04at one another in an aggressive display.
0:12:20 > 0:12:25This hawkmoth lays its eggs on flowers while it's still flying,
0:12:25 > 0:12:27for it's too heavy to land on them.
0:12:37 > 0:12:41It feeds by hovering in front of a blossom and sucking out the nectar
0:12:41 > 0:12:44with a tube-like proboscis as thin as thread.
0:12:46 > 0:12:49One of the smallest of all birds, the bee hummingbird,
0:12:49 > 0:12:53even smaller than a hawkmoth, is equally skilled,
0:12:53 > 0:12:56beating its wings 80 times a second
0:12:56 > 0:13:00to keep itself stationary in the air as it drinks from the flowers.
0:13:10 > 0:13:13Bird wings are more versatile than those of insects,
0:13:13 > 0:13:18for their flight feathers fit so closely alongside one another
0:13:18 > 0:13:20and slide so easily past each other
0:13:20 > 0:13:23that the bird can change the shape and size of its wing
0:13:23 > 0:13:26while maintaining its continuous air-deflecting surface,
0:13:26 > 0:13:29so the wing can be spread wide on the downstroke,
0:13:29 > 0:13:32and then, on the upstroke,
0:13:32 > 0:13:34be made small to offer less resistance to the air.
0:13:37 > 0:13:41This kestrel is maintaining a steady position in the sky,
0:13:41 > 0:13:43relative to the ground,
0:13:43 > 0:13:47by facing into the wind and flying with such accuracy that it exactly
0:13:47 > 0:13:49matches the wind speed.
0:14:09 > 0:14:11The reduction of air pressure,
0:14:11 > 0:14:13creating lift on the upper surface of the wings,
0:14:13 > 0:14:17can be seen quite clearly, for it sucks up the smaller feathers.
0:14:21 > 0:14:26The albatross also habitually gets lift by gliding into the wind,
0:14:26 > 0:14:30and again, the reduction in pressure produced as the air blows over
0:14:30 > 0:14:33the bird's outstretched wings ruffles its feathers.
0:14:40 > 0:14:44When it wants to travel over the sea against the wind, it drops down
0:14:44 > 0:14:47close to the surface of the water, where the roughness of the waves
0:14:47 > 0:14:49slows down the wind blowing over them.
0:14:56 > 0:14:59Albatrosses spend most of their lives in the air.
0:14:59 > 0:15:02Occasionally, for a minute or so,
0:15:02 > 0:15:05they may alight on the water to collect food.
0:15:05 > 0:15:08And once every year or so they come down to their nesting grounds
0:15:08 > 0:15:10to meet their mates again,
0:15:10 > 0:15:13greeting one another with a charming courtship dance.
0:15:31 > 0:15:35It's difficult to appreciate just how big these magnificent birds are
0:15:35 > 0:15:38when you see them gliding over the ocean.
0:15:38 > 0:15:41It's only when you come to one of their nesting sites like this one
0:15:41 > 0:15:45in South Georgia that you really see how big they are.
0:15:45 > 0:15:50When they open these wings, they are 11 feet across,
0:15:50 > 0:15:54the biggest wingspan of any bird.
0:15:56 > 0:15:58Long, narrow wings are the most efficient shape
0:15:58 > 0:16:00for uninterrupted gliding,
0:16:00 > 0:16:03and no bird glides better than the albatross,
0:16:03 > 0:16:07but such wings are difficult to flap sufficiently fast to give take-off,
0:16:07 > 0:16:10so many species of albatross nest on the edge of cliffs,
0:16:10 > 0:16:12where they can just fall into the air.
0:16:17 > 0:16:20Cliffs are much favoured by gliders,
0:16:20 > 0:16:24for the wind from the sea striking the cliff face is deflected upwards,
0:16:24 > 0:16:26and an albatross can hang on it.
0:16:32 > 0:16:34If it wants to fly a little slower
0:16:34 > 0:16:37and prevent itself from being swept away
0:16:37 > 0:16:39or carried too high by a sudden gust,
0:16:39 > 0:16:43it uses its tail and webbed feet as air breaks,
0:16:43 > 0:16:46and reduces its lift by pulling in its wings,
0:16:46 > 0:16:49so making their surface smaller.
0:16:49 > 0:16:52With such techniques, an albatross will glide all day
0:16:52 > 0:16:54above a line of cliffs,
0:16:54 > 0:16:57travelling effortlessly along this highway in the sky.
0:17:01 > 0:17:04Land birds also exploit the air currents above cliffs
0:17:04 > 0:17:06in just the same way.
0:17:06 > 0:17:09This is the coast of Paracas in Peru.
0:17:11 > 0:17:14As the day wears on, the sun heats up these desert sands,
0:17:14 > 0:17:18causing rising air, and that in turn sucks in cold air from the sea,
0:17:18 > 0:17:21often bringing mists with it.
0:17:21 > 0:17:25And as this cold air hits the cliffs, so it's deflected upwards,
0:17:25 > 0:17:28providing just the sort of conditions that soaring birds need.
0:17:31 > 0:17:34The condor - one of the heaviest of all flying birds.
0:17:36 > 0:17:39Yet its skill in soaring is so consummate
0:17:39 > 0:17:44that it can remain in the air for hours with scarcely a wingbeat,
0:17:44 > 0:17:48sustained entirely by those air currents swept upwards
0:17:48 > 0:17:49by the cliffs.
0:18:21 > 0:18:25And something else produces columns of rising air - heat.
0:18:25 > 0:18:27When we turn on these burners,
0:18:27 > 0:18:31they will create a current of rising air so powerful
0:18:31 > 0:18:36that it'll lift this balloon, this basket and us up into the sky.
0:19:14 > 0:19:20We are in Africa, floating over the great game plains of the Serengeti.
0:19:46 > 0:19:50I'm now about 100 feet up and kept up entirely by hot air.
0:19:50 > 0:19:53But of course gas burners aren't the only things
0:19:53 > 0:19:56which produce rising currents of hot air.
0:19:56 > 0:20:00The sun does the same thing - as it rises, it heats up the landscape,
0:20:00 > 0:20:03but all parts of the landscape don't react in the same way.
0:20:03 > 0:20:08Some parts absorb the heat. Other parts, bare slopes of grass
0:20:08 > 0:20:12or patches of rock, reflect the heat, and that causes those uprising
0:20:12 > 0:20:14currents of air, the thermals.
0:20:14 > 0:20:18That's a moment those big birds down there are waiting for.
0:20:18 > 0:20:22They are vultures, and at the moment they're grounded.
0:20:22 > 0:20:24They're big birds with large wings,
0:20:24 > 0:20:28so large that beating them is a very laborious business,
0:20:28 > 0:20:31and the vultures don't do so unnecessarily.
0:20:31 > 0:20:33At this time in the morning,
0:20:33 > 0:20:35they don't try to battle against gravity
0:20:35 > 0:20:38and climb high in the sky, but limit themselves to flapping
0:20:38 > 0:20:40from one low tree to another.
0:20:40 > 0:20:44They're waiting for the land to heat up and the thermals to form.
0:21:10 > 0:21:15But we have our own thermal, created by our burner, and up we go.
0:21:19 > 0:21:22This bird begins to follow us.
0:21:22 > 0:21:24An outcrop of rock is already warming
0:21:24 > 0:21:27and providing it with the thermal it needs for effortless flight.
0:22:19 > 0:22:24And now the vultures are beginning to come up here to join me.
0:22:24 > 0:22:28They will be using the thermals to provide them with an observation
0:22:28 > 0:22:32post high in the sky from which they can scan the plains below,
0:22:32 > 0:22:36and I'm getting just about the same kind of view as they are,
0:22:36 > 0:22:38and it's a very, very exciting one.
0:22:38 > 0:22:43Below me must be the biggest concentration of meat on the hoof
0:22:43 > 0:22:45to be found anywhere in the world - wildebeest.
0:22:53 > 0:22:56Last night or in the early dawn, somewhere,
0:22:56 > 0:22:59lions or hyenas or hunting dogs will have killed.
0:23:02 > 0:23:06The soaring vultures, several thousand feet up in the sky,
0:23:06 > 0:23:09can quickly spot a kill or deduce its presence
0:23:09 > 0:23:12from the behaviour of birds in a neighbouring thermal,
0:23:12 > 0:23:14and when they do, they swiftly glide down to it.
0:23:16 > 0:23:20Once one bird finds a carcass, dozens arrive within a few minutes.
0:23:20 > 0:23:25These are tearing apart the body of a wildebeest calf.
0:23:46 > 0:23:49Most of these are medium-sized vultures -
0:23:49 > 0:23:52Ruppell's griffon and white-back.
0:23:52 > 0:23:55But among them is the biggest and most powerful of African vultures -
0:23:55 > 0:23:57the lappet-faced.
0:24:05 > 0:24:09With a heavy load of meat on board, the vultures won't fly far -
0:24:09 > 0:24:13back to the nearest tree, to perch and digest and wait for tomorrow's
0:24:13 > 0:24:18thermals to carry them effortlessly aloft once more.
0:24:26 > 0:24:30But all the sustenance has not yet been extracted from the carcass.
0:24:33 > 0:24:37In the African mountains, as well as in Asia and Europe,
0:24:37 > 0:24:42lives a species of vulture with a very specialised diet indeed.
0:24:42 > 0:24:43The lammergeier.
0:24:47 > 0:24:51It feeds, though it sounds extraordinary, not only on marrow,
0:24:51 > 0:24:53but on the bones themselves, and to do so,
0:24:53 > 0:24:56it has developed a special technique.
0:24:57 > 0:25:01First, it brings bones from a carcass to a special workshop,
0:25:01 > 0:25:03which several birds may share.
0:25:03 > 0:25:06A patch of bare rock near the top edge of a cliff.
0:25:06 > 0:25:10It chooses a cliff top so that when it takes off again
0:25:10 > 0:25:11with a heavy bone in its talons
0:25:11 > 0:25:16it has the least difficulty in getting into the air.
0:25:26 > 0:25:28Now it has to gain height.
0:25:30 > 0:25:35And this is why it chooses a patch of bare rock for its operations.
0:25:37 > 0:25:41So that the bone will land so heavily that it cracks.
0:25:44 > 0:25:48One drop, however, may not be enough.
0:26:40 > 0:26:44White-collared ravens often hang about the scene of operations.
0:27:13 > 0:27:16The ravens are starting to learn the same technique,
0:27:16 > 0:27:18but they haven't quite mastered it.
0:27:18 > 0:27:22They tend to drop their bones on grass, where they don't break.
0:27:24 > 0:27:27The lammergeier eats the splinters of bone, impossibly spiky though
0:27:27 > 0:27:29they appear to be.
0:27:33 > 0:27:37Some birds exploit the force of gravity by dropping
0:27:37 > 0:27:40not their food but themselves from the sky.
0:27:40 > 0:27:45The pied kingfisher hovers as it searches the water beneath.
0:27:58 > 0:28:02Terns dive with such speed they can strike fish several feet
0:28:02 > 0:28:05beneath the surface, pulling back their wings at the last moment
0:28:05 > 0:28:07so as to get a clean entry into the water.
0:28:32 > 0:28:35Gannets do the same thing. During the nesting season,
0:28:35 > 0:28:37when they're concentrated in their colonies,
0:28:37 > 0:28:41huge flocks of them set out on fishing trips, and when they find
0:28:41 > 0:28:45a shoal of fish near the surface, they subject it to an aerial
0:28:45 > 0:28:47bombardment of devastating intensity.
0:29:07 > 0:29:11But the ace of dive-bombers, which can reach at least
0:29:11 > 0:29:1580 miles an hour in a dive, is the peregrine falcon.
0:29:18 > 0:29:20It patrols the skies,
0:29:20 > 0:29:23often high above the flight path of other birds.
0:29:23 > 0:29:26And when it has selected its victim, it folds its wings,
0:29:26 > 0:29:30steering almost entirely with its tail, and hurtles downwards.
0:30:27 > 0:30:31Close to target, the talons are brought forward for the strike,
0:30:31 > 0:30:35and to make last-second adjustments to the accuracy of its final run.
0:30:47 > 0:30:50A hunter of the night.
0:30:50 > 0:30:54Owls - this is a barn owl - don't rely on speed like the peregrine,
0:30:54 > 0:30:56but on a slow, silent approach.
0:30:58 > 0:31:02Their flight feathers have special soft edges to them
0:31:02 > 0:31:04which serve as silencers.
0:31:04 > 0:31:08Their wings are particularly large and support the bird so easily
0:31:08 > 0:31:11that there's no need for any vigorous noisy flapping,
0:31:11 > 0:31:15and the owl can waft its way in silence through the trees.
0:31:19 > 0:31:21Although owls hunt after dark,
0:31:21 > 0:31:25they find their way with their large, highly sensitive eyes,
0:31:25 > 0:31:28and, because their flight is virtually soundless,
0:31:28 > 0:31:32they can listen acutely for the squeak of unwary voles and mice.
0:31:34 > 0:31:38But on the darkest nights, even an owl can't see,
0:31:38 > 0:31:40and it seldom ventures into the air.
0:31:40 > 0:31:42Such nights belong to bats.
0:31:44 > 0:31:49They are able to navigate without the aid of vision.
0:31:49 > 0:31:52Instead they use sonar, squeaking ultrasonically
0:31:52 > 0:31:55and guiding themselves by the reflected echoes.
0:32:10 > 0:32:13They do this so skilfully
0:32:13 > 0:32:16that they can pluck a flying moth from the air.
0:32:49 > 0:32:53It's been known for a long time that bats use high pitched sounds
0:32:53 > 0:32:54in this way,
0:32:54 > 0:32:57but it's less well known that just one or two birds have also,
0:32:57 > 0:33:00and quite independently, evolved the same technique.
0:33:02 > 0:33:06This cave in Venezuela is the home of one of them.
0:33:16 > 0:33:21These, flying all around me, are oilbirds.
0:33:21 > 0:33:23Most of the noise that they're making at the moment
0:33:23 > 0:33:26is nothing to do with navigation.
0:33:26 > 0:33:27It's their alarm calls.
0:33:27 > 0:33:31They're alarmed because of the brightness of my light.
0:33:31 > 0:33:34So what I'm going to do is to put on a deep-red filter.
0:33:34 > 0:33:39That will disturb them much less, but it will enable us to watch them
0:33:39 > 0:33:42with a special electronic device called an image intensifier.
0:33:48 > 0:33:51They're big birds - relations of the nightjars,
0:33:51 > 0:33:53and about the size of pigeons.
0:33:53 > 0:33:56Their nests are compiled from their droppings
0:33:56 > 0:33:58and bits of regurgitated food.
0:34:00 > 0:34:02When their alarm calls subside,
0:34:02 > 0:34:05you can hear the clicks by which they navigate.
0:34:05 > 0:34:07These calls are much lower
0:34:07 > 0:34:09in frequency than the sonar signals of bats,
0:34:09 > 0:34:12and although they have a longer range they're much less accurate,
0:34:12 > 0:34:15so the oilbirds can't detect objects much smaller than a foot across.
0:34:15 > 0:34:18That's quite good enough to prevent the birds crashing into
0:34:18 > 0:34:20the cave walls or one another.
0:34:40 > 0:34:43Their favourite food is the fruit of a jungle tree
0:34:43 > 0:34:46and the cave floor is covered by a soggy carpet of seeds.
0:34:46 > 0:34:51Many germinate, though in the dark they can't develop chlorophyll,
0:34:51 > 0:34:55and they remain pallid, leggy seedlings which soon die.
0:34:55 > 0:34:58The fruits themselves are too small for the oilbirds
0:34:58 > 0:35:00to locate with their echoing clicks,
0:35:00 > 0:35:03but outside in the moonlit forest, where the fruit trees grow,
0:35:03 > 0:35:06there's enough light for the birds to find them by eye.
0:35:09 > 0:35:11The mastery of the air
0:35:11 > 0:35:14and the strength to remain in flight for days on end
0:35:14 > 0:35:16has enabled birds to become
0:35:16 > 0:35:19the greatest of all animal travellers.
0:35:19 > 0:35:23In the skies above Panama every October and November,
0:35:23 > 0:35:26there is a great aerial traffic jam.
0:35:26 > 0:35:30Hawks and turkey vultures, fleeing from the approaching winter in
0:35:30 > 0:35:33North America, are on their way to spend a few months in the south.
0:35:35 > 0:35:38As the day warms up, they find the thermals in which they can
0:35:38 > 0:35:42spiral upwards, to give them the altitude they need to make the day's
0:35:42 > 0:35:44flight with the least effort.
0:35:50 > 0:35:53These long journeys require a lot of fuel.
0:35:53 > 0:35:56Big birds, like hawks, can draw it from their body tissues.
0:35:58 > 0:36:01But north-east of Panama, across the Caribbean,
0:36:01 > 0:36:05on the Atlantic coast of the United States, smaller wading birds,
0:36:05 > 0:36:09sandpipers and phalaropes, are preparing for their journey.
0:36:09 > 0:36:12They must put on a lot of fat before they start off,
0:36:12 > 0:36:14and they find the food
0:36:14 > 0:36:18in the quantities they need in the rich waters of the Bay of Fundy.
0:36:42 > 0:36:44In a few days of intensive feeding,
0:36:44 > 0:36:48each tiny bird will increase its weight by half as much again,
0:36:48 > 0:36:51and they will need all that fat, for they are about
0:36:51 > 0:36:55to travel across the open ocean, and then they can't feed at all.
0:37:16 > 0:37:18On the other side of the Atlantic,
0:37:18 > 0:37:21the migration routes also run predominantly
0:37:21 > 0:37:24north and south, as birds move back and forth to get the best
0:37:24 > 0:37:25of the changing seasons.
0:37:27 > 0:37:32In Scandinavia, every autumn, great numbers make their way south.
0:37:32 > 0:37:35Most land birds prefer to keep their flights over water
0:37:35 > 0:37:36as short as possible,
0:37:36 > 0:37:40and huge flocks assemble on the shores of the narrow straits between
0:37:40 > 0:37:44southern Sweden and Denmark to make the crossing into southern Europe.
0:37:46 > 0:37:49Small birds often fly in parties, close to the water.
0:37:59 > 0:38:02Buzzards, experts at soaring and gliding,
0:38:02 > 0:38:06use the thermals to climb so high that they eventually cover the whole
0:38:06 > 0:38:09distance in what amounts to one long, shallow glide.
0:38:11 > 0:38:16Red-breasted geese spend their summer considerably further east
0:38:16 > 0:38:18in the tundra of western Siberia.
0:38:18 > 0:38:20They too move south in the autumn.
0:38:34 > 0:38:38Their journey is almost entirely over land, so they're able
0:38:38 > 0:38:40to stop each night to refuel.
0:38:58 > 0:39:00After several weeks of travel,
0:39:00 > 0:39:06they reach their wintering grounds south of the Caspian Sea,
0:39:12 > 0:39:15Birds are not the only creatures
0:39:15 > 0:39:18to make these immense transcontinental flights.
0:39:18 > 0:39:22Almost unbelievably, a few small, seemingly frail creatures
0:39:22 > 0:39:24do so as well.
0:39:24 > 0:39:27Insects, flying with just as steadfast a purpose,
0:39:27 > 0:39:31achieve journeys as long as many migrating birds.
0:39:31 > 0:39:34Back in South America, in a high valley in Mexico,
0:39:34 > 0:39:38hundreds of thousands of monarch butterflies roost in just
0:39:38 > 0:39:40a few special trees.
0:39:46 > 0:39:50They hatched in the autumn woods of North America, and have flown
0:39:50 > 0:39:53some 2,000 miles down here to hibernate.
0:39:53 > 0:39:55They won't feed here,
0:39:55 > 0:39:57but at least they're spared the lethal frosts and snows
0:39:57 > 0:39:59further north.
0:39:59 > 0:40:03In spring they will set off back, travelling about ten miles a day,
0:40:03 > 0:40:06feeding, courting and laying eggs as they go.
0:40:06 > 0:40:10But only a few will live long enough to reach the northern woods where
0:40:10 > 0:40:12they were hatched.
0:40:14 > 0:40:18So the world is criss-crossed by the flight paths of animal migrants.
0:40:18 > 0:40:21In the Americas, nearly all pass through Panama,
0:40:21 > 0:40:24though a few hardy travellers cross the Caribbean.
0:40:24 > 0:40:27On the other side of the world, there's more land,
0:40:27 > 0:40:30and the birds and insects have greater choice of routes,
0:40:30 > 0:40:33travelling north and south, but also east and west,
0:40:33 > 0:40:34between Asia and Africa.
0:40:36 > 0:40:39Although the journeys made by these travellers may be thousands
0:40:39 > 0:40:43of miles long, the earth's wrapping of air through which they move
0:40:43 > 0:40:45is less than six miles deep.
0:40:45 > 0:40:49On rare occasions, the gases from which it's formed become visible.
0:40:49 > 0:40:53Subatomic particles from space, attracted towards the poles by the
0:40:53 > 0:40:57earth's magnetic field, energise the gases of the atmosphere so that they
0:40:57 > 0:41:02glow and form shifting veils of light - the aurora borealis.
0:41:05 > 0:41:08The atmosphere is not composed entirely of gas,
0:41:08 > 0:41:11and at certain times you can see evidence of the presence
0:41:11 > 0:41:13of other things.
0:41:13 > 0:41:15Dust particles are scattered through its lower layers,
0:41:15 > 0:41:18and when the setting sun shines obliquely across the earth,
0:41:18 > 0:41:23at dawn and sunset, they scatter its white light, turning it red.
0:41:23 > 0:41:27Minute droplets of water, being translucent, act like an infinity
0:41:27 > 0:41:29of tiny prisms and produce a rainbow,
0:41:29 > 0:41:33and at high altitudes, tiny ice crystals create a similar effect.
0:41:35 > 0:41:39As you climb up away from the earth, the gases become thinner
0:41:39 > 0:41:42and the temperature, as a result, becomes colder.
0:41:54 > 0:41:57'The balloon which is taking us to these great heights must be
0:41:57 > 0:42:00'much bigger than that we used in Africa, for, as we climb,
0:42:00 > 0:42:03'we will require a greater volume of the rarefied air
0:42:03 > 0:42:05'to give us the necessary lift.
0:42:06 > 0:42:09'A rubber bladder, sealed with a cork on the ground,
0:42:09 > 0:42:13'will gives us a rough idea of the drop in pressure as we ascend.'
0:42:20 > 0:42:26We are now at 8,000 feet, and you might think that no living creature
0:42:26 > 0:42:30would come as high as this, except perhaps some rather foolhardy men.
0:42:31 > 0:42:36But no. Some small creatures are swept up as high as this
0:42:36 > 0:42:39by the convection currents rising from the surface of the ground,
0:42:39 > 0:42:42and we're going to try and catch some
0:42:42 > 0:42:46using this rather curious machine.
0:42:47 > 0:42:49Inside, there's a fan
0:42:49 > 0:42:54which will suck in air through this end when I turn it on here,
0:42:54 > 0:42:57and I'll lower it over the side to see what we catch.
0:43:06 > 0:43:09And now we're going to go higher still,
0:43:09 > 0:43:12and it's going to get very, very cold,
0:43:12 > 0:43:15so I shall need all this warm clothing I've got,
0:43:15 > 0:43:18but, perhaps even more seriously,
0:43:18 > 0:43:21the oxygen is going to get thinner and thinner,
0:43:21 > 0:43:25and so I shall have to put on this mask in order to breathe
0:43:25 > 0:43:28oxygen as we go higher and higher.
0:43:54 > 0:43:58And now an indication of our height can come from this balloon.
0:43:58 > 0:44:01Before, it had those corners to it,
0:44:01 > 0:44:04and now it's swollen quite considerably,
0:44:04 > 0:44:08so the pressure here is really very considerably lower than it was when
0:44:08 > 0:44:10we were on the ground.
0:44:14 > 0:44:16We are now getting on
0:44:16 > 0:44:20for four miles above the surface of the earth.
0:44:20 > 0:44:23It certainly looks very far away.
0:44:23 > 0:44:27And it's shrouded beneath a pall of clouds.
0:44:28 > 0:44:34And we're getting very close to the outermost frontier of life on Earth.
0:44:35 > 0:44:40It's very cold, and I certainly wouldn't be able to talk at all
0:44:40 > 0:44:41if I hadn't got this oxygen,
0:44:41 > 0:44:47so conditions here are really very much more severe than you might
0:44:47 > 0:44:52imagine when you sit in your aircraft, flying comfortably
0:44:52 > 0:44:54from one continent to another.
0:44:54 > 0:44:57But let's see what we've caught...
0:45:00 > 0:45:02..in our apparatus.
0:45:06 > 0:45:07Turn it off.
0:45:09 > 0:45:11And...
0:45:14 > 0:45:15..take off the end.
0:45:23 > 0:45:24Well...
0:45:27 > 0:45:31We certainly haven't caught anything large.
0:45:33 > 0:45:36But if we examine this mesh,
0:45:36 > 0:45:39when we get down to earth, with a microscope,
0:45:39 > 0:45:42it's very likely that, at the very least,
0:45:42 > 0:45:46we shall have some pollen grains and spores of fungus.
0:45:48 > 0:45:51But bigger creatures are found at these heights,
0:45:51 > 0:45:54and I've some of them here, in this phial,
0:45:54 > 0:45:56that were caught here.
0:45:58 > 0:46:03I'll pour them out on a dish to get a better look at them.
0:46:10 > 0:46:14There are tiny spiders that must have sailed up
0:46:14 > 0:46:17hanging from their threads of gossamer,
0:46:17 > 0:46:19and winged aphids.
0:46:20 > 0:46:24At these altitudes they can be carried halfway around the world
0:46:24 > 0:46:27and, amazingly, be frozen solid,
0:46:27 > 0:46:31and yet revive when they fall to lower altitudes.
0:46:33 > 0:46:38But now we are very close to the top of our environment...
0:46:41 > 0:46:46..for all the weather goes on within these five brief miles -
0:46:46 > 0:46:51the envelope of atmosphere that wraps round the world.
0:46:51 > 0:46:54It's here that the weather is manufactured.
0:46:57 > 0:47:00Molecules of water, evaporating in the heat of the sun from the surface
0:47:00 > 0:47:04of the sea and from lakes, or breathed out by plants as vapour,
0:47:04 > 0:47:08rise up from the land, and as they do so, they cool and condense
0:47:08 > 0:47:10into clouds of droplets.
0:47:10 > 0:47:13Driven by the winds, the clouds evaporate and condense,
0:47:13 > 0:47:14form and re-form.
0:47:34 > 0:47:37The summit of Mount Everest
0:47:37 > 0:47:40is less than six miles above the surface of the sea,
0:47:40 > 0:47:42yet few clouds ever sail much above it.
0:47:44 > 0:47:48The earth, as it spins, creates vast eddies within the atmosphere.
0:47:48 > 0:47:52If they become intense, they will develop into hurricanes.
0:47:52 > 0:47:56From a satellite 22,500 miles away from the earth,
0:47:56 > 0:48:02the build-up and dissipation of these huge storms over 15 days
0:48:02 > 0:48:05can be seen with pictures taken every hour and run continuously.
0:48:08 > 0:48:13Away to the east of Brazil in the Atlantic, a hurricane is forming.
0:48:15 > 0:48:18As it spins, it moves west across the Caribbean.
0:48:23 > 0:48:28Northwards it goes towards Florida, while up in the north, air sweeping
0:48:28 > 0:48:32over North America moves across the Atlantic towards Europe
0:48:32 > 0:48:34in another immense, swirling storm.
0:48:42 > 0:48:46Other major disturbances in the atmosphere are caused when the sun
0:48:46 > 0:48:51builds up gigantic thermals in a sky already loaded with moisture.
0:48:51 > 0:48:54As the air is driven upwards, the tops of the towering clouds
0:48:54 > 0:48:56burgeon with fearsome speed.
0:48:57 > 0:49:00The water molecules within the clouds condense to form
0:49:00 > 0:49:04bigger and bigger droplets, but the speed of the rising air is now
0:49:04 > 0:49:07so great that it keeps them suspended within the cloud.
0:49:09 > 0:49:13Eventually, the droplets become so big that they can no longer
0:49:13 > 0:49:16be supported, and they fall as torrential rain.
0:49:16 > 0:49:20The molecules of gas and water vapour surging upwards create
0:49:20 > 0:49:23a build-up of electricity that eventually becomes so great,
0:49:23 > 0:49:24it discharges down to earth.
0:49:28 > 0:49:31The water droplets may have been carried so high
0:49:31 > 0:49:33by the great thermals that they freeze,
0:49:33 > 0:49:36and eventually tumble out of the cloud as hail.
0:49:53 > 0:49:56If the storm is really intense,
0:49:56 > 0:49:59they may rise and fall several times.
0:49:59 > 0:50:01In the lower parts of the cloud,
0:50:01 > 0:50:03the ice they accumulate forms relatively slowly
0:50:03 > 0:50:07and is clear and black. But when they get to the top again,
0:50:07 > 0:50:09it's so cold that the ice forms quickly,
0:50:09 > 0:50:12trapping tiny air bubbles, which makes the ice look white.
0:50:12 > 0:50:16So really big hailstones may be banded, like an onion,
0:50:16 > 0:50:18with alternate rings of black and white ice.
0:50:35 > 0:50:39Really big hailstones are often a sign that a truly devastating storm
0:50:39 > 0:50:41is about to strike the earth.
0:50:43 > 0:50:48A strong, high-altitude wind, linked with a severe storm such as this,
0:50:48 > 0:50:50may vacuum up lower-level air,
0:50:50 > 0:50:53increasing the updraught dramatically,
0:50:53 > 0:50:56and beginning a spiral motion in part of the storm.
0:50:56 > 0:50:59If these converging winds are powerful enough,
0:50:59 > 0:51:03the vortex at the centre of this great whirl reaches down
0:51:03 > 0:51:06to the surface of the earth as a suction funnel, a tornado.
0:51:38 > 0:51:41Winds up to 300 miles an hour
0:51:41 > 0:51:44devastate the land, tearing things apart,
0:51:44 > 0:51:49ripping the roofs from buildings, sweeping animals and trees and
0:51:49 > 0:51:53sometimes even people high into the sky and throwing them down.
0:51:54 > 0:51:58When it strikes the land, it's seldom more than 500 yards across,
0:51:58 > 0:52:02but within this area it lashes the earth with the most powerful
0:52:02 > 0:52:05and destructive of all atmospheric forces.
0:52:37 > 0:52:41Storms like that may bring death and destruction,
0:52:41 > 0:52:45but they also bring life, because the rain that comes from them,
0:52:45 > 0:52:49distilled by the sun from the surface of the ocean, is fresh water,
0:52:49 > 0:52:56salt-free, and that is something that all life on land must have.
0:52:56 > 0:52:59And when that rain, that sweet, fresh water,
0:52:59 > 0:53:01accumulates in rivers and lakes,
0:53:01 > 0:53:06then it supports a community of plants and animals all of its own,
0:53:06 > 0:53:08and it's those communities that we're going to be looking at
0:53:08 > 0:53:10in the next programme.
0:54:08 > 0:54:10BBC Two is going on a journey
0:54:10 > 0:54:14through the clouds for television's most ambitious experiment.
0:54:14 > 0:54:18With this, we're going to be able to see the weather from the inside.
0:54:18 > 0:54:20So that small cloud weighs
0:54:20 > 0:54:21four tonnes? Yes.
0:54:21 > 0:54:25There might be microscopic life in cloud vapour.
0:54:25 > 0:54:27Right at the cutting edge of science.
0:54:27 > 0:54:30Join us on the ground-breaking voyage as we examine