1 00:00:56,300 --> 00:01:00,350 All living creatures on the earth and all material objects on it 2 00:01:00,560 --> 00:01:05,810 are subject to the pull of one great force: The force of gravity. 3 00:01:06,390 --> 00:01:13,240 Were that to be suspended, even for a moment, the most extraordinary things would happen. 4 00:01:13,440 --> 00:01:20,160 I, for example, would suddenly float into the air because I at the moment... 5 00:01:21,870 --> 00:01:31,000 ...am flying in an aircraft on a very special course which in effect cancels out the effect of gravity. 6 00:01:31,290 --> 00:01:36,050 So I float easily through the air. 7 00:01:37,050 --> 00:01:41,430 Our plane is climbing and diving as though it were on a giant roller coaster, 8 00:01:41,720 --> 00:01:47,890 and as it goes over the crest of its climb, it really lifts you out of your seat and keeps you there. 9 00:01:50,520 --> 00:01:54,860 If there were no gravity on earth, seas would rise from their beds 10 00:01:55,110 --> 00:01:59,820 just as this water lifts out of its cup and disintegrates into droplets. 11 00:02:12,250 --> 00:02:16,760 Nothing would remain where it was placed. There would be no up and no down. 12 00:02:17,010 --> 00:02:22,140 There would no longer be the sense of earthly order that we take so much for granted. 13 00:02:26,390 --> 00:02:31,940 Some creatures have overcome the force of gravity sufficiently to enable them to fly, 14 00:02:32,110 --> 00:02:37,030 but the only ones that match this total freedom in the air that I have now 15 00:02:37,190 --> 00:02:41,240 are those that are so small that they are, in effect, weightless. 16 00:02:45,240 --> 00:02:49,920 And there are more of them... both plant and animal... 17 00:02:51,330 --> 00:02:52,920 ...than you might think. 18 00:03:12,270 --> 00:03:17,610 The force of gravity holds the clouds around the earth and the air in which they float. 19 00:03:17,900 --> 00:03:22,360 You can't touch air, it's invisible and all-pervasive, 20 00:03:22,610 --> 00:03:25,950 so it's easy to forget that it has real substance. 21 00:03:26,240 --> 00:03:31,790 But it's only by exploiting the presence of air that seeds, insects, birds and man 22 00:03:31,960 --> 00:03:36,500 are able to overcome gravity and float above the earth's surface. 23 00:03:37,920 --> 00:03:41,050 Dandelion seeds rise because a puff of air carries them up 24 00:03:41,260 --> 00:03:45,430 and they fall slowly because their parachutes catch the air beneath. 25 00:03:48,680 --> 00:03:51,730 A tuft of fluff will serve the same purpose. 26 00:03:52,980 --> 00:03:56,310 Milkweed and cotton grass, willowherb and thistles, 27 00:03:56,480 --> 00:03:59,820 all provide their seeds with downy floats. 28 00:04:00,650 --> 00:04:06,200 These delay the fall of the seeds for so long that currents in the air, winds, 29 00:04:06,370 --> 00:04:10,290 can carry them for hundreds of miles from their parents. 30 00:04:22,380 --> 00:04:28,810 Seeds like these have crossed the widest oceans and landed on the loneliest islands. 31 00:04:30,770 --> 00:04:35,100 Pollen grains are so small, they don't even need fluff to keep in the air. 32 00:04:35,400 --> 00:04:38,820 The microscopic roughness of their surface is enough. 33 00:04:40,360 --> 00:04:42,940 Spores, shot out from a puffball 34 00:04:43,150 --> 00:04:47,490 and shed in tens of millions from the gills of fungi, are smaller still. 35 00:04:48,160 --> 00:04:52,080 The merest breath of air sweeps them away like smoke. 36 00:05:03,760 --> 00:05:06,510 The gossamer, that sometimes carpets the meadows, 37 00:05:06,680 --> 00:05:09,970 is the animal equivalent of downy seeds. 38 00:05:12,180 --> 00:05:16,690 It's produced by thousand upon thousand of tiny spiders. 39 00:05:20,570 --> 00:05:24,030 The young of many species of spider, soon after they hatch, 40 00:05:24,280 --> 00:05:28,660 climb to the top of grass stems or onto the tiny pinnacles of stones 41 00:05:28,870 --> 00:05:31,580 and lift their abdomens upwards. 42 00:05:35,620 --> 00:05:41,090 Then, from the spinnerets at the tip, they produce a thread of finest silk. 43 00:05:58,600 --> 00:06:02,940 As it lengthens and the wind catches it, the spiderling turns, 44 00:06:03,110 --> 00:06:06,740 grabs the thread with its forelegs and away it goes. 45 00:06:25,880 --> 00:06:31,550 Only the tiniest and the lightest of animals and plants can defy gravity in this way. 46 00:06:33,050 --> 00:06:38,640 Many seeds are far too heavy to be lifted by the breeze, no matter how downy they are. 47 00:06:38,980 --> 00:06:44,400 But if they are produced at the top of a tall tree they can exploit the pull of gravity. 48 00:06:45,070 --> 00:06:49,110 These, hanging in the jungle of Venezuela, grow wings. 49 00:06:49,780 --> 00:06:53,740 The wing is so shaped and weighted, with the seed at one end, 50 00:06:53,910 --> 00:06:57,580 that as it falls through the air, it spins. 51 00:07:15,850 --> 00:07:20,890 This protracted fall gives the breeze a chance to deflect the seeds sideways 52 00:07:21,100 --> 00:07:24,730 so that they will land some distance away from the parent tree. 53 00:07:31,200 --> 00:07:34,320 The seed is functioning like the blade of a helicopter. 54 00:07:34,870 --> 00:07:37,330 Its wing is so shaped that as it sweeps round, 55 00:07:37,490 --> 00:07:41,960 it puts pressure on the air below and reduces pressure just above 56 00:07:42,210 --> 00:07:46,880 so that the seed hangs in the air much longer than it would otherwise do. 57 00:07:48,300 --> 00:07:51,800 Sycamore seeds spin and glide in the same way. 58 00:07:59,350 --> 00:08:01,480 And animals glide too. 59 00:08:10,780 --> 00:08:14,870 The flying frog of Central America has a parachute on each foot, 60 00:08:15,070 --> 00:08:18,330 formed by the web of skin between its toes. 61 00:08:18,830 --> 00:08:23,500 So one jump from a high branch is enough to carry it from one tree to another. 62 00:08:33,180 --> 00:08:37,760 In South-East Asia lives a gecko that not only has a parachute on each foot, 63 00:08:38,050 --> 00:08:40,720 but flanges on its body and tail. 64 00:08:47,650 --> 00:08:50,070 Another lizard glides through the forests 65 00:08:50,230 --> 00:08:55,700 by extending even bigger wings of skin from its flanks supported by elongated ribs. 66 00:09:02,500 --> 00:09:06,040 And the best glider of all: A flying squirrel. 67 00:09:06,670 --> 00:09:11,630 Its huge cloak of floppy skin sometimes serves as a simple parachute. 68 00:09:14,380 --> 00:09:19,430 But in horizontal flight it does more than just trap air beneath it. 69 00:09:22,060 --> 00:09:26,390 As air passes over the front edge, it's deflected slightly upwards, 70 00:09:26,600 --> 00:09:30,480 creating a slight reduction in the air pressure on the upper surface, 71 00:09:30,650 --> 00:09:35,400 like on an aircraft wing or the spinning blade of a sycamore seed, 72 00:09:35,820 --> 00:09:40,700 so the squirrel creates a little lift and floats through the air. 73 00:09:58,680 --> 00:10:01,470 All those creatures are gliders. 74 00:10:01,850 --> 00:10:06,600 Some can control to some extent the direction in which they glide, 75 00:10:06,810 --> 00:10:12,440 but none of them can climb in the air except with the help of rising air currents, 76 00:10:12,650 --> 00:10:16,700 like the breezes which sweep up these downs in southern England, 77 00:10:16,950 --> 00:10:21,580 carrying with them whole populations of seeds and spores and spiders. 78 00:10:21,870 --> 00:10:25,700 But there are no such breezes down below the grass stems. 79 00:10:25,950 --> 00:10:31,630 Down there, if creatures want to climb into the air they have to have true powered flight. 80 00:10:36,210 --> 00:10:38,800 The most demanding moment is at take-off. 81 00:10:41,050 --> 00:10:45,720 The insect has to haul itself into the air by sheer unaided muscle power. 82 00:10:46,350 --> 00:10:52,230 The downward sweep of the wings produces greater pressure in the air beneath than above, 83 00:10:52,480 --> 00:10:56,230 so, in a slightly different way from the cloak of the squirrel, 84 00:10:56,440 --> 00:11:01,660 beating wings also create lift, and the insect is sucked upwards. 85 00:11:08,750 --> 00:11:13,920 Bigger insects, like grasshoppers, boost their take-off with a powerful spring. 86 00:11:16,090 --> 00:11:18,220 Birds are even bigger and heavier. 87 00:11:18,550 --> 00:11:24,100 For them, too, getting into the air is the most energetic and demanding part of flying. 88 00:11:25,810 --> 00:11:30,100 They also use their well-muscled legs to assist their labouring wings. 89 00:11:30,350 --> 00:11:34,440 They jump even before their wings begin their downbeat. 90 00:11:42,530 --> 00:11:47,240 But really big birds, to get airborne, have to generate the extra lift 91 00:11:47,410 --> 00:11:50,830 by increasing the speed of air streaming over their wings, 92 00:11:51,160 --> 00:11:54,670 so they get up a lot of speed on the ground or over water, 93 00:11:54,840 --> 00:11:58,090 just as an aircraft does, before they can take off 94 00:12:09,810 --> 00:12:13,770 Once in the air, a whole new environment is open to them, 95 00:12:14,020 --> 00:12:18,360 and flying animals of all kinds exploit it to the full. 96 00:12:21,150 --> 00:12:26,870 Damsel flies catch their food in the air, mate in the air and even fight in the air. 97 00:12:27,120 --> 00:12:34,460 As males squabble over territory, they flutter their patterned wings in an aggressive display. 98 00:12:50,720 --> 00:12:56,190 This hawkmoth lays its eggs on flowers while it's still flying, 99 00:12:56,400 --> 00:12:58,400 for it's too heavy to land on them. 100 00:13:08,240 --> 00:13:12,790 It feeds by hovering in front of a blossom and sucking out the nectar 101 00:13:12,950 --> 00:13:15,750 with a tube-like proboscis as thin as thread. 102 00:13:18,090 --> 00:13:23,170 One of the smallest of all birds, the bee hummingbird, even smaller than a hawkmoth, 103 00:13:23,340 --> 00:13:27,550 is equally skilled, beating its wings 80 times a second 104 00:13:27,720 --> 00:13:31,930 to keep itself stationary in the air as it drinks from the flowers. 105 00:13:43,570 --> 00:13:46,530 Bird wings are more versatile than those of insects, 106 00:13:46,820 --> 00:13:52,990 for their feathers fit so closely alongside one another and slide so easily past each other 107 00:13:53,200 --> 00:13:56,370 that the bird can change the shape and size of its wing 108 00:13:56,580 --> 00:14:00,170 while maintaining its air-deflecting surface, 109 00:14:00,460 --> 00:14:03,380 so the wing can be spread wide on the downstroke, 110 00:14:03,710 --> 00:14:08,550 and then, on the upstroke, be made small to offer less resistance to the air. 111 00:14:11,850 --> 00:14:16,480 This kestrel is maintaining a steady position in the sky, relative to the ground, 112 00:14:16,770 --> 00:14:23,780 by facing into the wind and flying with such accuracy that it exactly matches the wind speed. 113 00:14:44,800 --> 00:14:48,550 The reduction of air pressure, creating lift on the surface of the wings, 114 00:14:48,720 --> 00:14:52,970 can be seen quite clearly, for it sucks up the smaller feathers. 115 00:14:57,850 --> 00:15:02,610 The albatross also habitually gets lift by gliding into the wind, 116 00:15:03,150 --> 00:15:09,950 and the reduction in pressure produced as the air blows over the wings ruffles its feathers. 117 00:15:16,750 --> 00:15:22,630 When it wants to travel against the wind, it drops down close to the surface of the water, 118 00:15:22,880 --> 00:15:26,760 where the roughness of the waves slows down the wind blowing over them. 119 00:15:33,850 --> 00:15:37,020 Albatrosses spend most of their lives in the air. 120 00:15:37,350 --> 00:15:41,850 Occasionally, for a minute or so, they alight on the water to collect food. 121 00:15:42,100 --> 00:15:47,780 Once every year or so they come to their nesting grounds to meet their mates again, 122 00:15:48,110 --> 00:15:51,570 greeting one another with a charming courtship dance. 123 00:16:10,300 --> 00:16:17,060 It's difficult to appreciate how big these birds a when you see them gliding over the ocean. 124 00:16:17,390 --> 00:16:24,900 It's only when you come to one of their nesting sites that you really see how big they are. 125 00:16:25,150 --> 00:16:30,530 When they open these wings, they are 11 feet across, 126 00:16:30,780 --> 00:16:33,990 the biggest wingspan of any bird. 127 00:16:35,660 --> 00:16:40,000 Long, narrow wings are the most efficient shape for uninterrupted gliding, 128 00:16:40,200 --> 00:16:43,000 and no bird glides better than the albatross, 129 00:16:43,170 --> 00:16:47,290 but such wings are hard to flap fast enough to give take-off, 130 00:16:47,590 --> 00:16:53,300 so many species of albatross nest on the edge of cliffs, where they can just fall into the air. 131 00:16:57,800 --> 00:17:00,390 Cliffs are much favoured by gliders, 132 00:17:00,890 --> 00:17:05,440 for the wind from the sea striking the cliff face is deflected upwards, 133 00:17:05,690 --> 00:17:08,150 and an albatross can hang on it. 134 00:17:14,490 --> 00:17:18,490 If it wants to fly slower and prevent itself from being swept away 135 00:17:18,660 --> 00:17:21,290 or carried too high by a sudden gust, 136 00:17:21,540 --> 00:17:24,660 it uses its tail and webbed feet as air breaks, 137 00:17:24,830 --> 00:17:30,130 and reduces its lift by pulling in its wings, so making their surface smaller. 138 00:17:30,800 --> 00:17:35,590 With such techniques, an albatross will glide all day above a line of cl 139 00:17:35,760 --> 00:17:39,970 travelling effortlessly along this highway in the sky. 140 00:17:44,140 --> 00:17:48,770 Land birds also exploit the air currents above cliffs in the same way. 141 00:17:49,060 --> 00:17:52,480 This is the coast of Paracas in Peru. 142 00:17:53,190 --> 00:17:58,780 As the day wears on, the sun heats up these desert sands, causing rising air, 143 00:17:59,030 --> 00:18:04,080 and that in turn sucks in cold air from the sea, often bringing mists with it. 144 00:18:04,330 --> 00:18:08,630 As this cold air hits the cliffs, so it's deflected upwards, 145 00:18:08,880 --> 00:18:12,090 providing just the sort of conditions that soaring birds need. 146 00:18:14,590 --> 00:18:19,090 The condor, one of the heaviest of all flying bird 147 00:18:21,140 --> 00:18:23,680 Yet its skill in soaring is so consummate 148 00:18:23,890 --> 00:18:28,690 that it can remain in the air for hours with scarcely a wingbeat, 149 00:18:28,900 --> 00:18:33,940 sustained entirely by those air currents swept upwards by the cliffs. 150 00:19:07,230 --> 00:19:11,730 And something else produces columns of rising air: Heat. 151 00:19:11,980 --> 00:19:17,280 When we turn on these burners, they will create a current of rising air so powerful 152 00:19:17,440 --> 00:19:23,490 that it'll lift this balloon, this basket and us up into the sky. 153 00:20:02,860 --> 00:20:08,620 We're in Africa, floating over the great game plains of the Serengeti. 154 00:20:35,520 --> 00:20:40,940 I'm now about 100 feet up and kept up entirely by hot air. 155 00:20:41,320 --> 00:20:46,070 But gas burners aren't the only things which produce rising currents of hot air. 156 00:20:46,280 --> 00:20:50,040 The sun, as it rises, heats up the landscape, 157 00:20:50,330 --> 00:20:53,670 but all parts of the landscape don't react in the same way. 158 00:20:53,870 --> 00:20:59,210 Some parts absorb the heat. Other parts, bare slopes of grass or patches of rock, 159 00:20:59,460 --> 00:21:04,630 reflect the heat, and that causes those uprising currents of air, the thermals. 160 00:21:04,890 --> 00:21:09,180 That's a moment those big birds down there are waiting for. 161 00:21:09,390 --> 00:21:13,140 They are vultures, and at the moment they're grounded. 162 00:21:13,810 --> 00:21:19,270 They're big birds with large wings, so large that beating them is a very laborious business, 163 00:21:19,480 --> 00:21:22,650 and the vultures don't do so unnecessarily. 164 00:21:22,900 --> 00:21:28,410 At this time in the morning, they don't try to battle against gravity and climb high, 165 00:21:28,660 --> 00:21:31,790 but flap from one low tree to another. 166 00:21:32,160 --> 00:21:36,040 They're waiting for the land to heat up and the thermals to form. 167 00:22:03,190 --> 00:22:08,740 But we have our own thermal, created by our burner, and up we go. 168 00:22:12,990 --> 00:22:15,080 This bird begins to follow us. 169 00:22:15,540 --> 00:22:17,420 An outcrop of rock is already warming 170 00:22:17,620 --> 00:22:21,500 and providing it with the thermal it needs for effortless flight. 171 00:23:16,520 --> 00:23:20,520 And now the vultures are beginning to come up here to join me. 172 00:23:20,770 --> 00:23:25,940 They will use the thermals to provide them with an observation post in the sky 173 00:23:26,110 --> 00:23:28,360 from which they can scan the plains below, 174 00:23:28,570 --> 00:23:35,370 and I'm getting the same kind of view as they are, and it's a very, very exciting one. 175 00:23:35,790 --> 00:23:39,410 Below me must be the biggest concentration of meat on the hoof 176 00:23:39,620 --> 00:23:43,380 to be found anywhere in the world: Wildebeest. 177 00:23:51,380 --> 00:23:54,300 Last night or in the early dawn, somewhere, 178 00:23:54,600 --> 00:23:58,020 lions or hyenas or hunting dogs will have killed. 179 00:24:00,600 --> 00:24:06,860 The vultures, several thousand feet up in the sky, quickly spot a kill or deduce its presence 180 00:24:07,030 --> 00:24:10,150 from the behaviour of birds in a neighbouring thermal, 181 00:24:10,360 --> 00:24:13,200 and when they do, they swiftly glide down to it. 182 00:24:15,240 --> 00:24:19,330 Once one bird finds a carcass, dozens arrive within a few minutes. 183 00:24:19,910 --> 00:24:23,960 These are tearing apart the body of a wildebeest calf. 184 00:24:46,650 --> 00:24:51,860 Most of these are medium-sized vultures: Ruppell's griffon and white-back. 185 00:24:52,110 --> 00:24:57,830 But among them is the biggest and most powerful of African vultures: The lappet-faced. 186 00:25:06,630 --> 00:25:11,720 With a heavy load of meat, the vultures won't fly far, to a nearby tree, 187 00:25:11,920 --> 00:25:18,010 to perch and digest and wait for tomorrow's thermals to carry them effortlessly aloft again. 188 00:25:28,110 --> 00:25:31,690 But all the sustenance has not yet been extracted from the carcass. 189 00:25:35,570 --> 00:25:38,700 In the African mountains, as well as in Asia and Europe, 190 00:25:38,950 --> 00:25:45,210 lives a species of vulture with a very specialised diet indeed: The lammergeier. 191 00:25:49,500 --> 00:25:54,970 It feeds, though it sounds extraordinary, not only on marrow but on the bones themselves, 192 00:25:55,220 --> 00:25:58,800 and to do so, it has developed a special technique. 193 00:26:00,100 --> 00:26:06,100 First it brings bones from a carcass to a special workshop which several birds may share. 194 00:26:06,390 --> 00:26:09,560 A patch of bare rock near the top edge of a cliff. 195 00:26:09,940 --> 00:26:14,190 It chooses a cliff top so that when it takes off again with a heavy bone, 196 00:26:14,400 --> 00:26:18,450 it has the least difficulty in getting into the air. 197 00:26:30,540 --> 00:26:32,550 Now it has to gain height. 198 00:26:35,050 --> 00:26:39,800 And this is why it chooses a patch of bare rock for its operations. 199 00:26:42,930 --> 00:26:46,520 So that the bone will land so heavily that it cracks. 200 00:26:50,190 --> 00:26:52,610 One drop, however, may not be enough. 201 00:27:47,910 --> 00:27:51,710 White-collared ravens often hang about the scene of operations. 202 00:28:22,780 --> 00:28:27,080 The ravens are starting to learn the technique but haven't mastered it. 203 00:28:27,290 --> 00:28:31,160 They tend to drop their bones on grass, where they don't break. 204 00:28:32,960 --> 00:28:38,670 The lammergeier eats the splinters of bone, impossibly spiky though they appear to be. 205 00:28:44,260 --> 00:28:50,770 Some birds exploit the force of gravity by dropping not their food but themselves from the sky. 206 00:28:51,270 --> 00:28:55,480 The pied kingfisher hovers as it searches the water beneath. 207 00:29:08,620 --> 00:29:13,620 Terns dive with such speed, they can strike fish several feet beneath the surface, 208 00:29:13,830 --> 00:29:19,000 pulling back their wings at the last moment so as to get a clean entry into the water. 209 00:29:44,200 --> 00:29:45,990 Gannets do the same thing. 210 00:29:46,280 --> 00:29:49,790 During the nesting season, concentrated in their colonies, 211 00:29:49,950 --> 00:29:55,920 huge flocks set out on fishing trips, and when they find a shoal of fish near the surface, 212 00:29:56,080 --> 00:30:00,750 they subject it to an aerial bombardment of devastating intensity. 213 00:30:21,610 --> 00:30:26,570 But the ace of dive-bombers, which can reach at least 80 miles an hour in a dive, 214 00:30:26,860 --> 00:30:28,570 is the peregrine falcon. 215 00:30:32,740 --> 00:30:36,670 It patrols the skies, high above the flight path of other birds. 216 00:30:36,920 --> 00:30:40,000 When it has selected its victim, it folds its wing 217 00:30:40,210 --> 00:30:45,010 steering almost entirely with its tail, and hurtles downwards. 218 00:31:44,980 --> 00:31:48,030 The talons are brought forward for the strike 219 00:31:48,200 --> 00:31:52,530 and to make last-second adjustments to the accuracy of its final run. 220 00:32:06,050 --> 00:32:07,760 A hunter of the night. 221 00:32:08,170 --> 00:32:15,220 Owls, this is a barn owl, don't rely on speed like the peregrine, but on a slow, silent approach 222 00:32:17,810 --> 00:32:22,270 Their flight feathers have special soft edges to them which serve as silencers. 223 00:32:22,810 --> 00:32:26,820 Their wings are large and support the bird so easily 224 00:32:26,980 --> 00:32:29,820 that there's no need for any noisy flapping, 225 00:32:30,030 --> 00:32:34,240 and the owl can waft its way in silence through the trees. 226 00:32:38,910 --> 00:32:44,170 Although owls hunt after dark, they find their way with their large, sensitive eyes, 227 00:32:44,330 --> 00:32:52,180 and, because their flight is virtually soundless, they can listen for the squeak of voles and mice. 228 00:32:54,510 --> 00:33:00,180 But on the darkest nights, even an owl can't see, and it seldom ventures into the air. 229 00:33:00,430 --> 00:33:03,150 Such nights belong to bats. 230 00:33:05,480 --> 00:33:09,030 They are able to navigate without the aid of vision. 231 00:33:09,230 --> 00:33:15,950 Instead they use sonar, squeaking ultrasonically and guiding themselves by the reflected echoes. 232 00:33:32,800 --> 00:33:38,350 They do this so skilfully that they can pluck a flying moth from the air. 233 00:34:12,670 --> 00:34:16,930 It's been known for a long time that bats use sounds in this way, 234 00:34:17,140 --> 00:34:24,180 but it's less well known that one or two birds have, independently, evolved the same technique. 235 00:34:26,020 --> 00:34:29,650 This cave in Venezuela is the home of one of them. 236 00:34:41,370 --> 00:34:45,410 These, flying all around me, are oilbirds. 237 00:34:45,910 --> 00:34:50,790 Most of the noise that they're making is nothing to do with navigation. 238 00:34:51,000 --> 00:34:55,420 It's their alarm calls. They're alarmed by the brightness of my light. 239 00:34:55,840 --> 00:34:59,140 So what I'm going to do is to put on a deep-red filter. 240 00:34:59,390 --> 00:35:03,850 That will disturb them less, but it will enable us to watch them 241 00:35:04,020 --> 00:35:08,730 with a special electronic device called an image intensifier. 242 00:35:14,280 --> 00:35:19,030 They're big, relations of the nightjars, and about the size of pigeons. 243 00:35:19,320 --> 00:35:24,750 Their nests are compiled from their droppings and bits of regurgitated food. 244 00:35:26,460 --> 00:35:30,880 When their alarm calls subside, you can hear the clicks by which they navigate. 245 00:35:31,840 --> 00:35:38,590 These calls are lower in frequency than the signals of bats, and they're less accurate, 246 00:35:38,800 --> 00:35:42,470 so the oilbirds can't detect objects much smaller than a foot across. 247 00:35:43,010 --> 00:35:47,430 That's quite good enough to prevent the birds crashing into the cave walls or one another. 248 00:36:08,080 --> 00:36:10,710 Their favourite food is the fruit of a jungle tree 249 00:36:10,960 --> 00:36:14,630 and the cave floor is covered by a soggy carpet of seeds. 250 00:36:14,920 --> 00:36:19,340 Many germinate, though in the dark they can't develop chlorophyll, 251 00:36:19,550 --> 00:36:23,470 and they remain pallid, leggy seedlings which soon die. 252 00:36:23,930 --> 00:36:28,480 The fruits are too small for the oilbirds to locate with their clicks, 253 00:36:28,770 --> 00:36:32,480 but out in the moonlit forest, where the trees grow 254 00:36:32,730 --> 00:36:35,320 there's enough light for the birds to find them by eye. 255 00:36:39,360 --> 00:36:42,950 The mastery of the air and the strength to remain in flight for days 256 00:36:43,160 --> 00:36:48,080 has enabled birds to become the greatest of all animal travellers. 257 00:36:49,620 --> 00:36:52,790 In the skies above Panama every October and November, 258 00:36:53,000 --> 00:36:55,670 there is a great aerial traffic jam. 259 00:36:55,920 --> 00:37:00,720 Hawks and turkey vultures, fleeing from the winter in North America, 260 00:37:00,970 --> 00:37:04,180 are on their way to spend a few months in the south. 261 00:37:05,470 --> 00:37:09,930 As the day warms up, they find the thermals in which they can spiral upwards, 262 00:37:10,140 --> 00:37:14,770 to give them the altitude they need to make the day's flight with the least effort. 263 00:37:20,780 --> 00:37:23,450 These long journeys require a lot of fuel. 264 00:37:23,740 --> 00:37:27,490 Big birds, like hawks, can draw it from their body tissues. 265 00:37:29,330 --> 00:37:35,000 But north-east of Panama, across the Caribbean, on the Atlantic coast of the United States, 266 00:37:35,250 --> 00:37:40,920 smaller wading birds, sandpipers and phalaropes, are preparing for their journey. 267 00:37:41,470 --> 00:37:44,260 They must put on fat before they start off, 268 00:37:44,510 --> 00:37:50,350 and they find food in the quantities they need in the rich waters of the Bay of Fundy. 269 00:38:15,250 --> 00:38:21,510 In a few days of intensive feeding, each tiny bird will increase its weight by half as much again, 270 00:38:21,880 --> 00:38:26,300 and they need all that fat, for they are about to travel across the ocean, 271 00:38:26,470 --> 00:38:29,100 and then they can't feed at all. 272 00:38:50,780 --> 00:38:56,540 On the other side of the Atlantic, migration route also run predominantly north and south, 273 00:38:56,750 --> 00:39:00,670 as birds move back and forth to get the best of the changing seasons. 274 00:39:02,130 --> 00:39:06,470 In Scandinavia, every autumn great numbers make their way south. 275 00:39:07,180 --> 00:39:11,350 Most land birds prefer to keep their flights over water short, 276 00:39:11,600 --> 00:39:17,100 and huge flocks assemble on the shores of the narrow straits between Sweden and Denmark 277 00:39:17,270 --> 00:39:19,480 to make the crossing into southern Europe. 278 00:39:22,480 --> 00:39:25,690 Small birds often fly in parties, close to the water. 279 00:39:35,660 --> 00:39:38,710 Buzzards, experts at soaring and gliding, 280 00:39:38,920 --> 00:39:43,040 use the thermals to climb so high that they cover the distance 281 00:39:43,210 --> 00:39:46,340 in what amounts to one long, shallow glide. 282 00:39:49,010 --> 00:39:55,270 Red-breasted geese spend their summer much farther east in the tundra of western Siberia 283 00:39:55,560 --> 00:39:57,680 They too move south in the autumn. 284 00:40:12,700 --> 00:40:18,830 Their journey is almost entirely over land, so they're able to stop each night to refuel. 285 00:40:37,680 --> 00:40:42,730 After several weeks, they reach their wintering grounds south of the Caspian Sea, 286 00:40:42,900 --> 00:40:46,190 many of them on the marshes of the Danube delta. 287 00:40:52,450 --> 00:40:57,580 Birds are not the only creatures to make these immense transcontinental flights. 288 00:40:57,790 --> 00:41:03,080 Almost unbelievably, a few small, seemingly frail creatures do so as well. 289 00:41:03,830 --> 00:41:11,220 Insects, flying with just as steadfast a purpose, achieve journeys as long as many migrating birds. 290 00:41:11,550 --> 00:41:14,970 In South America, in a high valley in Mexico, 291 00:41:15,140 --> 00:41:20,600 hundreds of thousands of monarch butterflies roost in just a few special trees. 292 00:41:28,020 --> 00:41:34,410 They hatched in the autumn woods of North America and have flown 2,000 miles to hibernate. 293 00:41:34,700 --> 00:41:40,290 They won't feed here, but they're spared the lethal frosts and snows farther north. 294 00:41:40,580 --> 00:41:44,460 In spring they will set off back, travelling ten miles a day, 295 00:41:44,620 --> 00:41:47,750 feeding, courting and laying eggs as they go. 296 00:41:48,130 --> 00:41:53,260 But only a few will live long enough to reach the northern woods where they were hatched. 297 00:41:56,390 --> 00:42:00,100 The world is criss-crossed by the flight paths of animal migrants. 298 00:42:00,350 --> 00:42:06,150 In the Americas, nearly all pass through Panama. A few hardy travellers cross the Caribbean. 299 00:42:07,060 --> 00:42:12,650 On the other side of the world there's more land, and birds and insects have more routes, 300 00:42:12,820 --> 00:42:17,490 travelling north and south but also east and west between Asia and Africa. 301 00:42:19,370 --> 00:42:23,160 Although the journeys may be thousands of miles long, 302 00:42:23,460 --> 00:42:28,380 the earth's wrapping of air is less than six miles deep. 303 00:42:28,960 --> 00:42:32,460 On rare occasions the gases from which it's formed become visible. 304 00:42:32,630 --> 00:42:38,350 Subatomic particles from space, attracted to the poles by the earth's magnetic field, 305 00:42:38,510 --> 00:42:44,060 energise the gases of the atmosphere so that they glow and form shifting veils of light 306 00:42:44,230 --> 00:42:46,350 the aurora borealis. 307 00:42:49,940 --> 00:42:52,480 The atmosphere is not composed entirely of gas 308 00:42:52,650 --> 00:42:56,610 and at certain times you can see evidence of other things. 309 00:42:57,450 --> 00:43:04,160 Dust particles are scattered through its lower layers, and when the sun shines across the earth, 310 00:43:04,330 --> 00:43:07,080 they scatter its white light, turning it red. 311 00:43:08,040 --> 00:43:14,260 Minute droplets of water, being translucent, act like tiny prisms and produce a rainbow, 312 00:43:14,550 --> 00:43:19,010 and at high altitudes tiny ice crystals create a similar effect. 313 00:43:21,220 --> 00:43:27,730 Up away from the earth, the gases become thinner and the temperature becomes colder. 314 00:43:40,700 --> 00:43:45,660 The balloon taking us to these heights must be bigger than that we used in Africa 315 00:43:45,830 --> 00:43:51,710 for, as we climb, we will require a greater volume of the rarefied air to give us the necessary lift. 316 00:43:52,920 --> 00:43:59,130 A rubber bladder, sealed with a cork, gives us a rough idea of the drop in pressure as we ascend. 317 00:44:07,930 --> 00:44:16,230 We are now at 8,000 feet, and you might think that no living creature would come as high as this 318 00:44:16,490 --> 00:44:19,030 except perhaps some rather foolhardy men. 319 00:44:19,280 --> 00:44:24,370 But no. Some small creatures are swept up as high as this 320 00:44:24,580 --> 00:44:28,040 by the convection currents rising from the surface of the ground, 321 00:44:28,290 --> 00:44:35,250 and we're going to try and catch some using this rather curious machine. 322 00:44:36,090 --> 00:44:43,970 Inside there's a fan which will suck in air through this end when I turn it on here, 323 00:44:44,300 --> 00:44:47,100 and I'll lower it over the side to see what we catch. 324 00:44:55,820 --> 00:45:01,950 And now we're going to go higher still and it's going to get very, very cold, 325 00:45:02,160 --> 00:45:05,280 so I shall need all this warm clothing I've got, 326 00:45:05,490 --> 00:45:10,960 but, perhaps even more seriously, the oxygen is going to get thinner and thinner, 327 00:45:11,160 --> 00:45:18,920 and so I shall have to put on this mask in order to breathe oxygen as we go higher and higher. 328 00:45:45,240 --> 00:45:51,040 And now an indication of our height can come from this balloon. 329 00:45:51,250 --> 00:45:55,920 Before it had those corners to it and now it's swollen quite considerably, 330 00:45:56,080 --> 00:46:01,760 so the pressure here is really considerably lower than it was when we were on the ground. 331 00:46:07,550 --> 00:46:12,640 We are now getting on for four miles above the surface of the earth. 332 00:46:13,230 --> 00:46:20,070 It certainly looks very far away. And it's shrouded beneath a pall of clouds. 333 00:46:20,530 --> 00:46:27,490 And we're getting very close to the outermost frontier of life on earth. 334 00:46:28,330 --> 00:46:35,000 It's very cold and I certainly wouldn't be able to talk at all if I hadn't got this oxygen, 335 00:46:35,250 --> 00:46:42,130 so conditions here are really very much more severe than you might imagine 336 00:46:42,300 --> 00:46:48,260 when you sit in your aircraft flying comfortably from one continent to another. 337 00:46:48,680 --> 00:46:52,680 But let's see what we've caught... 338 00:46:53,770 --> 00:46:56,730 in our apparatus. 339 00:47:00,860 --> 00:47:01,940 Turn it off. 340 00:47:04,150 --> 00:47:05,360 And... 341 00:47:08,370 --> 00:47:10,160 ...take off the end. 342 00:47:18,420 --> 00:47:19,540 Well... 343 00:47:22,090 --> 00:47:27,510 We certainly haven't caught anything large. 344 00:47:29,720 --> 00:47:35,060 But if we examine this mesh, when we get down to earth, with a microscope, 345 00:47:35,350 --> 00:47:43,360 it's very likely that, at the very least, we shall have some pollen grains and spores of fungus. 346 00:47:44,650 --> 00:47:48,240 But bigger creatures are found at these heights 347 00:47:48,740 --> 00:47:53,660 and I've some of them here, in this phial, that were caught here. 348 00:47:55,500 --> 00:48:01,590 I'll pour them out on a dish to get a better look at them. 349 00:48:08,010 --> 00:48:14,350 There are tiny spiders that must have sailed up hanging from their threads of gossamer. 350 00:48:15,140 --> 00:48:22,480 And winged aphids. At these altitudes they can be carried halfway around the world 351 00:48:22,730 --> 00:48:25,280 and, amazingly, be frozen solid, 352 00:48:25,440 --> 00:48:29,610 and yet revive when they fall to lower altitudes. 353 00:48:30,950 --> 00:48:37,700 But now we are very close to the top of our environment, 354 00:48:39,040 --> 00:48:45,380 for all the weather goes on within these five brief miles, 355 00:48:45,590 --> 00:48:50,550 the envelope of atmosphere that wraps round the world. 356 00:48:50,800 --> 00:48:54,390 It's here that the weather is manufactured. 357 00:48:56,010 --> 00:49:01,100 Molecules of water, evaporating in the heat of the sun from the surface of the sea and lakes, 358 00:49:01,190 --> 00:49:03,480 or breathed out by plants as vapour, 359 00:49:03,650 --> 00:49:09,030 rise up from the land and cool and condense into clouds of droplets. 360 00:49:09,860 --> 00:49:15,200 Driven by the winds, the clouds evaporate and condense, form and re-form. 361 00:49:36,260 --> 00:49:41,140 The summit of Mount Everest is less than six miles above the sea, 362 00:49:41,350 --> 00:49:43,810 yet few clouds ever sail much above it. 363 00:49:45,560 --> 00:49:50,530 The earth, as it spins, creates vast eddies within the atmosphere. 364 00:49:50,990 --> 00:49:54,570 If they become intense, they will develop into hurricanes. 365 00:49:54,870 --> 00:49:58,950 From a satellite 22,500 miles away from the earth, 366 00:49:59,200 --> 00:50:03,710 the build-up and dissipation of these huge storms over 15 days 367 00:50:03,870 --> 00:50:08,000 can be seen with pictures taken every hour and run continuously. 368 00:50:11,300 --> 00:50:15,760 Away to the east of Brazil in the Atlantic, a hurricane is forming. 369 00:50:17,600 --> 00:50:21,350 As it spins, it moves west across the Caribbean. 370 00:50:26,480 --> 00:50:32,150 Northwards it goes towards Florida, while up in the north, air sweeping over North America 371 00:50:32,320 --> 00:50:37,990 moves across the Atlantic towards Europe in another immense, swirling storm. 372 00:50:46,210 --> 00:50:51,670 Other disturbances in the atmosphere are caused when the sun builds up gigantic thermals 373 00:50:51,880 --> 00:50:54,510 in a sky already loaded with moisture. 374 00:50:54,880 --> 00:51:00,640 As the air is driven upwards, the tops of the towering clouds burgeon with fearsome speed. 375 00:51:01,560 --> 00:51:06,310 The water molecules within the clouds condense to form bigger and bigger droplets, 376 00:51:06,690 --> 00:51:11,980 but the speed of the rising air is now so great that it keeps them suspended within the cloud. 377 00:51:14,360 --> 00:51:18,530 Eventually, the droplets become so big that they cannot be supported, 378 00:51:18,740 --> 00:51:20,870 and they fall as torrential rain. 379 00:51:21,280 --> 00:51:26,420 The molecules of gas surging upwards create a build-up of electricity 380 00:51:26,580 --> 00:51:30,420 that eventually becomes so great, it discharges down to earth. 381 00:51:34,460 --> 00:51:38,640 The water droplets may have been carried so high that they freeze 382 00:51:38,840 --> 00:51:42,010 and eventually tumble out of the cloud as hail. 383 00:52:01,200 --> 00:52:05,910 If the storm is really intense, they may rise and fall several times. 384 00:52:06,160 --> 00:52:12,040 In the lower parts of the cloud, the ice forms relatively slowly and is clear and black. 385 00:52:12,250 --> 00:52:16,380 But when they get to the top again, the ice forms quickly, 386 00:52:16,550 --> 00:52:19,430 trapping air bubbles, which makes the ice look white. 387 00:52:19,680 --> 00:52:26,270 So big hailstones may be banded, like an onion, with alternate rings of black and white ice. 388 00:52:43,910 --> 00:52:50,370 Really big hailstones are often a sign that a truly devastating storm is about to strike the earth. 389 00:52:52,670 --> 00:52:57,010 A strong, high-altitude wind, linked with a severe storm such as this, 390 00:52:57,170 --> 00:53:01,930 may vacuum up lower-level air, increasing the updraught dramatically, 391 00:53:02,180 --> 00:53:05,720 and beginning a spiral motion in part of the storm. 392 00:53:06,180 --> 00:53:11,440 If these converging winds are powerful enough, the vortex at the centre of this great whirl 393 00:53:11,600 --> 00:53:17,440 reaches down to the surface of the earth as a suction funnel, a tornado. 394 00:53:50,520 --> 00:53:55,560 Winds up to 300 miles an hour devastate the land, tearing things apart, 395 00:53:55,730 --> 00:54:01,700 ripping the roofs from buildings, sweeping animals and trees and sometimes even people 396 00:54:01,860 --> 00:54:04,910 high into the sky and throwing them down. 397 00:54:05,870 --> 00:54:09,910 When it strikes the land, it's seldom more than 500 yards across, 398 00:54:10,080 --> 00:54:17,750 but it lashes the earth with the most powerful and destructive of all atmospheric forces. 399 00:54:50,740 --> 00:54:54,660 Storms like that may bring death and destruction, 400 00:54:54,870 --> 00:54:58,590 but they also bring life, because the rain that comes from them, 401 00:54:58,790 --> 00:55:05,260 distilled by the sun from the surface of the ocean is fresh water, salt-free, 402 00:55:05,470 --> 00:55:10,760 and that is something that all life on land must have.