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