TWO

The Mastery of Flight

In the night, on a small offshore island in Japan, birds wait in line to clamber up a tree trunk. They are streaked shearwaters, seabirds the size of pigeons. They nest in holes in the soft ground and if you are not careful, as you pick your way down through the steeply sloping wood, you either step in one of their holes and trip, or you stumble over a newly emerged bird, in which case it will flounder off in alarm and crash through the blackness into the undergrowth. Like all shearwaters, their legs are placed far back on their bodies. That is the most effective position for propelling them through water. It does, however, mean that they cannot stand upright. The only way they can get about on land is to shuffle forward in an ungainly fashion with their breasts close to the ground. They make their way down the slope towards a chestnut tree that is particularly suited to their purpose. There they wait their turn to scramble up on to its sloping trunk which is as thick as a telegraph pole. Nose to tail, they inch up it, pushing with their legs, scrabbling with the elbows of their closed wings. The tree has clearly been used in this way for a long time, for its bark is heavily scratched, down to its red underbark.

The trunk is hollow. Unluckily for the shearwaters, about 3 metres up there is a hole in it. Occasionally, one of the climbers blunders into the hole and drops down the interior. Undeterred by this mishap, it emerges at the bottom and rejoins the queue to start all over again. But most of them manage to avoid this setback and continue to inch higher and higher up the trunk until, some 6 metres above the ground, it bends down towards the horizontal. Having reached that point the birds stop and look about them. They are above the canopy of the surrounding trees and can see out through a gap in the branches towards the moonlit sea. One by one, they open their long narrow wings, lean forward into space and launch themselves into the air. Instantly they are transformed from clumsy halting climbers into superbly competent aeronauts and away they sail into the black night, off to the sea to fish.

It is the shape of their wings that sustains them aloft. In section, each is thick and rounded at the front edge and tapers, curving slightly downwards, towards the back edge where it has only the thickness of a feather. This aerofoil shape, somewhat like a comma lying on its side, is the almost magical device that keeps a bird in the air. As it glides forward, the air flowing over the upper surface of the wing is deflected upwards so reducing the air pressure just above. The air passing beneath the wing is somewhat impeded by the wing’s downward curve, so there the air pressure is increased. With less pressure above and greater pressure below, the wing tends to rise upwards. As a shearwater launches itself off the branch, gravity pulls it downwards and gives it the speed necessary for its wings to function as aerofoils and so keep it in the air. Its laborious climb up the tree has brought its reward.

Most members of the shearwater family nest on cliffs where they can easily shuffle to the edge and fall into space. Usually, there is an onshore wind which also helps their flight. The Japanese species is unusual in having developed a talent for tree climbing and this has enabled it to extend its nesting sites farther away from the sea than most other species have been able to do. The wandering albatross, the largest of all seabirds, breeds on oceanic islands where there are seldom either cliffs or trees, so it has to create an air current over its wings in some other way. It uses the same method as mankind’s aeroplanes. Its colonies, like airports, have long wide runways that are oriented along the path of the prevailing wind and run right through the colony with nests clustered closely together on either side. To take off, an albatross runs down this clear path into the wind, smacking the ground with its large, webbed feet. As it runs, it beats its wings. Each forward-reaching downstroke increases the speed of the air passing over its wings and therefore the strength of the lift they generate, until at last it lifts off the ground. But it can only achieve this if there is at least some headwind blowing over its wings. On the rare occasions when there is a flat calm on their islands, the albatrosses are grounded.

One of the heaviest of all flying birds is the swan. A full-grown one may weigh close to 16 kilos. To get into the air a swan requires the most level and smoothest runway in the whole of nature – the still surface of a lake. Even so, taking off demands an explosion of activity. Like the albatross, the swan runs as fast as it can across the surface on its short legs and beats its wings frantically. Spray flies everywhere. As the wings begin to generate lift, so it rises higher above the water. Still it runs, its webbed feet leaving distinct and separate puddles on the surface until eventually they lose contact with the water. The bird tucks them beneath its tail, like an aeroplane retracting its undercarriage, and at last it rises into the air.

The lift created by an aerofoil is directly proportionate to the speed of the air that passes over it, so the speed that a bird can run on take-off governs the height that it can attain. The swan, taking off in still air, must sprint across the water at a speed of 15 metres per second. Watching one do so, it is difficult to imagine how any large bird could manage to run faster. So it may be that any bird heavier than a swan simply would not be able to take off.

Heavy a swan may be, but it is astonishingly light compared with a mammal. A similar-sized mammal, say a bulldog, weighs about four times as much. Birds owe this lightness to the many adaptations they have acquired since Archaeopteryx first took to the air a hundred and fifty million years ago – hollow bones supported by internal struts; a fan of stout-quilled feathers instead of a bony tail; and a horny beak instead of jawbones laden with a battery of teeth. But they also have one other feature that is not apparent from skeletal remains. A substantial proportion of their bodies is occupied by air. This is held in sacs. Most birds have nine of them. They lie in the neck, the fore-part of the chest and towards the back of the abdomen. Some even extend into the bones of the wings and legs. These are not only weight-saving devices. Flying demands so much energy that a bird needs a very large supply of oxygen. The air sacs are an essential part of its breathing system and enable it to extract far more oxygen from each breath than a similar-sized mammal is able to get.

A mammal’s lungs are, in effect, bags. Air taken in with the breath has to return along the same passage, the windpipe, as it entered. And the lung does not completely empty with each breath. The net result is that only about 20 per cent of the oxygen in a breath of air is absorbed by the mammalian lung. A bird’s breathing is much more efficient. When it inhales, the air passes first into its lungs. They are relatively small and lie beneath its spine, moulded against its ribs. It then continues through a number of small tubes into those sacs that lie towards the rear of the body. When the bird breathes out, the air in the rear sacs moves back along another set of tubes to a different part of the lungs. With the next breath, this inhaled air moves on yet again, out of the lungs and into another group of air sacs towards the front of the body. Then, with the next exhalation, that air leaves the bird’s body through its nostrils and goes back into the atmosphere. In this way, the airflow in all of the many passageways and sacs of the bird’s respiratory system is always in the same direction and the absorption of oxygen from each intake of breath is ultimately almost total.

All flying birds share these weight-saving features. Species smaller than a swan and with stronger legs than a shearwater are light enough to get into the air from a standing start. A dove begins by bending its legs and, at the same time, opening its wings and lifting them above its back. As it straightens its legs and starts its jump, it brings down its open wings with such force against the resisting air that the bird is lifted sufficiently far upwards for the tips of its wings to clear the ground. Now it must raise its wings for the second beat. It twists them at the wrist so that they partly fold, thus reducing their surface area, and the long wing feathers separate, allowing air to stream through them. It continues raising its wings until they are above its back. They are now fully open once again and they clap together. This disperses the air between them, lowering its pressure so sucking the bird upwards and reinforcing the upward push created by the wings as they begin their third downstroke. By the time the dove completes that, it is well above the ground.

Now the great versatility of a bird’s wing becomes apparent. Thanks to the manner in which the feathers slide over one another, the surface of the wing remains perfectly smooth whether the wing is closed or opened or at any position in between. The dove, as it beats its wings, not only pushes itself upwards through the air, counteracting the downward pull of gravity, but at the same time reaches forward with a rowing motion so that it maintains its height and at the same time advances through the air. At no stage during the whole action is the smooth flow of air across the wing surface interrupted by any irregularity that might cause it to break up into turbulent eddies. Were that to happen, the lift created by the aerofoil would be greatly diminished or even lost altogether.

The feathers on a bird’s body are also crucial in minimising turbulence. They contour it so that the curve from head to neck to shoulder, back and tail is so gentle that the air flows smoothly over it. The importance of this streamlining is easily demonstrated. Watch an osprey fishing. As it flies above a lake, it is the epitome of grace and aerodynamic efficiency, beating its wings in an almost leisurely fashion. It spots a fish and dives down to grab it. As it rises with its catch in its talons, it greatly increases the rate of its wing beats. It has to make this extra effort not only because of the weight of the fish but because of the drag the fish creates as the bird carries it through the air. To keep this to a minimum, the osprey adjusts its grip so that the fish’s head points forward, and its streamlined shape which created the least resistance in water, continues to do exactly that now that it is in the air. But the osprey can no longer draw its feet upwards and tuck them away close to its body. These two handicaps cause so much turbulence and drag that the osprey’s flight becomes heavy and laboured and it has to beat its wings very much faster simply to stay in the air.

Boobies and gannets, which also fish, have a way of avoiding this problem. They store their catch within their bodies in a crop, a bag that branches internally from their throat. That way even a large catch creates no more than a smooth swelling which, in terms of drag, hinders them hardly at all.

Beating wings demands such energy that it is clearly valuable to do it in as economical a way as possible. One simple method of achieving that is to stop every now and then. A woodpecker in flight regularly interrupts its rapid wing beats for a few moments by holding its wings closed tightly against its body. Its forward momentum is such that, without the drag created by its open wings, it continues to shoot forward through the air. But it cannot do this for long. Deprived of the lift created by its wing beats, it loses height and after a few seconds it has to resume flapping. This gives it an undulating, bounding flight.

Only a small bird can use this energy-saving trick. If a bigger heavier bird tried to do so, it would drop like a stone. Nonetheless, even a big bird can economise on its wing beats. If it stops flapping with its wings not closed but open, their surface area is big enough to impede its fall and it will glide. Pelicans regularly do this. How long they can glide depends on how high above the ground they are, how much height they can afford to lose, and how fast they are travelling.

American white pelicans have a special additional way of saving energy. They take advantage of the turbulence in the air created by their companions. The high pressure air created below a wing by its aerofoil shape leaks round the tip of the wing into the low pressure area on the upper surface. This slight upward current in the air remains briefly in its wake. A pelican flying in a group takes advantage of this by flying behind the wing tip of the bird ahead rather than directly behind its tail. The wing tip station also gives it a better view of what lies ahead. So groups of pelicans often take up a V-formation. Furthermore, because the effect of the wing tip turbulence is at its greatest immediately after the downstroke of the wing and rapidly fades, the pelicans not only fly in formation but beat their wings with the near perfect unison of a well-drilled corps de ballet. The only bird of the group that does not benefit from this order of flight is the leader of the squadron and after having done its share, it will fall behind and allow another to take on the job. Geese also fly in this way to form those huge and unforgettable skeins across the sky.

All birds eventually must come down from the skies, if only to lay their eggs and rear their young. To do so, they must first lose speed. That is done most simply by lowering the tail and the rear edge of the wings. Water birds with webbed feet stretch them out to serve as additional brakes. The swan is so large that it cannot fly slowly without stalling and falling out of control. It hardly ever attempts a dry landing but comes down on an unimpeded stretch of water. Even so, when its webbed feet, thrust forward, hit the surface it is still moving at such speed that it almost disappears in a cloud of spray. As the waves subside, the swan folds its wings, shuffles them a little to get them neatly into position and then swims away with proper swan-like dignity.

Albatrosses, coming down to one of their great island colonies, land on the same runways they used for take-off. As they approach the ground, they lower their feet as air brakes and start paddling in anticipation. Only too often, their speed through the air is faster than they can run along the ground. The result, inevitably, is a minor crash-landing as the bird tips forward on to its chest, but it quickly recovers and staggers away to its nest unharmed.

Ducks and geese have a special technique that allows them to drop sharply through the air and land on quite small ponds. It is called whiffling. They tip from side to side with the long wing feathers widely spread so that they separate and air spills through them making a tearing noise. They may even twist on their backs, so that the aerofoil effect of their wings is totally nullified but their momentum is increased. Only when they are a few metres above their chosen patch of water do they straighten out so that their wings can serve as air brakes and slow them sufficiently for a safe landing.

Perching birds have to alight with much greater accuracy than those that come down on flat ground or water. This, perhaps, puts a curb on their size. Certainly none are quite as big. The harpy of tropical America, one of the biggest of eagles, is only half as heavy as a swan. Nonetheless it is a very big bird indeed and it is very difficult for it to slow down without stalling. Yet it has to reduce its speed to zero at the exact moment that it arrives at its perch. This requires the most accurate control. If the bird decelerates too rapidly, it will drop beneath its perch. If it does not do so sufficiently, it will overbalance as it reaches the branch and topple forward. As the eagle comes in, it fans its tail and pulls it down at an angle to the body to reduce speed and control direction. It lowers the trailing edges of its wings to act as additional brakes. As it loses speed, there is a danger that turbulence will develop on the upper surface of the wings and cause it to stall. It prevents that by raising its alulas. These are tufts of three or four feathers developed by many flying birds on the leading edge of the wings that are attached to the stunted relics of the thumb bones. They let in a stream of air across the wing surface and so maintain a smooth flow over the aerofoil. The bird has now lost nearly all its air speed. It reaches forward with its huge talons, grabs the branch ahead and finally brings itself to a full stop.

Once landed, a bird must service the feathers on which its life in the air depends. Almost all birds, if they have the chance, will take a daily bath to rid their feathers of dirt, ruffling up their feathers, ducking their heads and beating the water into a drenching spray. Thoroughly soused, they then comb themselves. Their long wing feathers on which they rely for flight are given particular attention. Each may be carefully passed through the beak so that it is cleaned and any separated filaments zipped back together. The sword-billed hummingbird has a unique problem. It alone among birds has a beak that is longer than its body. As a consequence, it cannot be used as a comb and the bird has no alternative but to preen itself with one foot while standing precariously on the other. Happily, the processes of evolution which led to the development of its extraordinary beak, thus enabling it to reach nectar in the depths of deep trumpet-shaped flowers, also responded to its need to reach its head feathers by equipping it with disproportionately long legs.

If water is not available, some birds, such as guans, larks, wrens and sparrows will use dust. That, if followed by a vigorous shake, helps to get rid of parasites of which they may have plenty – chewing lice that nibble their feathers, louse flies, bugs, mites, fleas and ticks that seek to suck their blood.

Herons and parrots produce a powder for their toiletry. It comes from the fraying ends of specialised feathers. In some species, such as pigeons and parrots, these are scattered throughout the plumage. In others, notably herons, they are clumped together in small patches. Exactly what function this powder plays is not fully understood, but it probably helps with waterproofing. Egrets, pelicans and other water birds anoint themselves with oil that they squeeze from a gland in the skin at the base of the tail. Washing, dusting and powdering completed, the feathers are finally put back into their proper positions.

Even with the best care, however, feathers wear out and all birds have to replace them. For most species, moulting takes place over a longish period. Chaffinches take ten or eleven weeks to do so. A few flight feathers are shed and replaced, followed by others so that at no time is the bird unable to fly. But some birds that can find a safe refuge, as ducks and seabirds can do on water, change all their feathers quickly over three or four weeks, during which time they are completely flightless.

Different habitats and different ways of collecting food demand different styles of flight and specialist aeronautical equipment. The wandering albatross spends most of its life above the open ocean. There the wind blows almost continuously. Simply facing into the wind with its wings outstretched is enough to keep the albatross in the air. Since the lift created by any wing is at its weakest at the very tip, where the high pressure beneath escapes on the upper surface, it is better, aerodynamically, to keep the tip as far away as possible from its body. That can be done by making the wing very long and the wandering albatross has the greatest wingspan of any bird – up to 3.5 metres. If the wind is reasonably strong, these huge wings will generate enough lift to enable the bird to travel at an angle to it. Furthermore, wind whips the surface of the sea into waves. As it blows across them, it is deflected upwards. The albatross is able to exploit these upward currents and tack, zig-zagging across the face of the wind, from one wave crest to the next. So skilled is it that it can sail for hours on end without a single wing beat. Holding wings outstretched with tensed muscles would, in itself, require energy for most birds. But the albatross does not have to use its muscles. It has a catch-like mechanism in its wing bones that sets its wings in the open position. So it flies continuously for days, weeks, even months with the minimum expenditure of energy. It has no need to drink for it extracts all the water it needs from its food, the dead squid and fish which it finds floating on the surface of the sea.

The carrion to be found on the surface of the land is collected by other extraordinarily accomplished aeronauts, vultures. They do not have the advantage of an almost ever-present wind. Instead, they exploit rising currents of warm air. Such currents, called thermals, are generated by the way in which different surfaces of the land react to the sun’s heat. A tract of lush grass absorbs a great deal of heat. A patch of bare rock, however, reflects heat into the surrounding air. So a column of warm air rises from rock patches and continues upwards, high into the sky. A vulture, once it manages to reach such a thermal, circles within it and spirals upwards until, perhaps 300 metres above the ground, the thermal has lost so much of its heat into the surrounding air that it no longer has any strength.

Once the vulture has gained sufficient height, it can circle for hours, scanning the plains below for a carcass that might provide it with food. Its wings are not long and slim like those of a high-speed glider such as an albatross, which rarely has to dodge obstacles above the open ocean or has the need to make accurate landings. Instead they are very broad with a large surface area that enables the bird to take full advantage of air rising from beneath, while being sufficiently short in span to allow them to dodge trees and bushes every time they have to make a precision landing.

Flying at high speed requires yet another wing shape. The fastest of all birds – indeed the fastest creature in the air, apart from a human being in a machine – is the peregrine. When that dives on its prey, which is nearly always another bird, it first increases its velocity by beating its wings and then, in the last stage of its stoop, draws them back so that it assumes a silhouette like that of a supersonic jet aircraft and reaches a speed over 300 kilometres per hour.

The kestrel, a close relative of the peregrine, has a very different hunting tactic. It hangs in the air, apparently stationary, while it searches the ground beneath. In fact, the bird is not motionless in relation to the air around it. It is facing into the wind so that it gets enough lift to remain airborne. It spreads its broad tail to supplement the air-catching effect of its spread wings. It also raises its alulas which further reduce the danger of stalling because of turbulence. It separates the feathers at the broad ends of its wings so that little upward jets of air are generated which dispel any turbulent eddies on its upper surface. Carefully adjusting these various controls, it manages to match exactly its forward motion through the air with the speed of the wind and hangs directly above the patch of ground it is scanning for prey.

Only one family of birds can truly hover in still air for any length of time. They are the hummingbirds. They need to do so in order to hang in front of a flower while they perform the delicate task of inserting their slim, sharp bills into its depths to drink nectar. Their thin wings are not contoured into the shape of aerofoils and do not generate lift in this way. Their flying technique differs from that of most other birds as radically as helicopters do from fixed-wing aircraft.

The long bones of their wings have been greatly reduced in length, and the joints at the wrist and the elbow have lost nearly all their mobility. Their paddle-shaped wings are, in effect, hands that swivel at the shoulder. They beat them in such a way that the tip of each wing follows the line of a figure-of-eight, lying on its side. The wing moves forward and downwards into the front loop of the eight, creating lift. As it begins to come up and goes back, the wing twists through 180 degrees so that once again it creates a downward thrust. If the lift produced on each loop of the figure-of-eight is equally strong, the bird will remain stationary in the air. A slight alteration in the twist, changing the angle at which the wing moves downward, is enough to move the bird forwards – or even backwards.

This manner of flying demands a great deal of energy. Nectar, the hummingbird’s food, is the biological equivalent of high-octane fuel but even so a hummingbird consumes such quantities that it may need to refuel as many as two thousand times a day. Even at rest its body needs a great deal of fuel simply to keep ticking over. Part of this is used to keep its flying muscles at a high temperature and ready for instant take-off. But when night comes and it is unable to see to fly, those muscles are not used and that heat is not needed. So in the evening when a hummingbird arrives on its roost, it deliberately ruffles its feathers and allows its body to cool. During the day, its heart beats between five hundred and twelve hundred times a second. Now it slows down so much that its throb is virtually undetectable. Nor does the bird appear to be breathing. In effect, it is doing what a hedgehog does when winter approaches. It is hibernating. A hummingbird, however, has to hibernate three hundred and sixty-five times every year.

The hummingbird’s method of flying does have a major limitation. The smaller a wing, the faster it has to beat in order to produce sufficient downward thrust. An average-sized hummer beats them 25 times a second. The bee hummingbird from Cuba which, being only 5 centimetres long, is indeed scarcely bigger than a bumble bee, has to do so at an astonishing two hundred times a second. There is a limit however to the speed at which an electric signal can pass down a nerve in order to trigger a muscle. The bee hummingbird’s wing beat is on the edge of that limit. There can be no smaller flying bird.

There are, of course, smaller flying insects. They, however, have had to adopt a fundamentally different mechanism for beating their wings at high speed. Their bodies are encased in a horny external skeleton to which the wings are fixed. They are able to make this vibrate with a relatively simple contraction of a muscle in the same sort of way that a sharp blow will make a tuning fork vibrate. They therefore have no need to send separate nerve pulses to initiate every beat.

The power of flight, demanding though it is, has made birds the swiftest moving creatures on the planet. The fastest animal on earth is a cheetah which recent research suggests cannot exceed 80 kilometres per hour even in short sprints. The fastest fish in the sea, the sailfish can, exceptionally and over a short distance, achieve a speed of 100 kilometres per hour. But the spine-tailed swift in level flight has been credited with a speed of 160 kilometres per hour. Flight has also enabled birds to overcome all the physical obstacles that restrict the movements of land-living animals. They, to a degree unequalled by any other kind of animal, are able to travel anywhere to find the conditions that best suit them at any particular time, avoiding seasonal bad weather and visiting places where food is suddenly but only briefly available.

The high Arctic is just such a place. For at least six months of the year, conditions are crushingly hostile to life. For much of that time, the sun does not rise above the horizon. Even when it does, its rays are so low and glancing that they give little heat to the snow-covered ground. Without light, plants cannot grow; and without plants, herbivorous animals cannot feed and predatory animals have no prey. Nonetheless, a few animals and plants manage to live here permanently. They do so by reducing their life processes to the bare minimum during the dark winter months and concentrating all their activity into the short summer. As the sun begins to clear the horizon in spring, temperatures begin to rise. The snow melts and reveals stunted heather, willows, saxifrages and poppies. The ice on the bog pools disappears and cotton grass begins to flower. Lemmings that have been hibernating deep in burrows beneath the snow, venture out and start ravenously to crop the leaves and make good the deficits of winter. And from the pools, insects haul themselves from over-wintering pupae and swarm in such numbers that the air hums with their droning. By high summer, the Arctic animals are more continuously active than any below the Arctic circle, for now it is light twenty-four hours a day.

There is food to spare for birds. Poverty has become plenty and birds fly up from the south to benefit from the glut. Among them are snow geese. They establish colonies of a hundred thousand pairs or more among the clumps of coarse tussock grass. Had they done that farther south, they would have attracted stoats and weasels, foxes, wolverines and wolves and other predators. But such four-footed hunters do not appear because they cannot make the long journey northwards as swiftly and easily as birds. The only mammalian predators that birds have to fear up here is a small permanent population of Arctic foxes. With so many geese nesting, these foxes are faced with far more eggs than they can possibly consume. Those they cannot eat immediately they bury to be devoured later, but even so, the foxes are so few that they have little effect on the huge assembly of snow geese.

By late summer most geese have four or five young which are already fledging. Unlike their parents, which are pure white except for the black tips to their wings, the young have grey upper parts. All over the tundra, these goose families are busy feeding, the adults digging for roots of bulrushes and the young nibbling the tender tips of the marsh grass leaves. They all put on weight fast. They need to. They cannot stay much longer in the face of the approaching winter. They have a long journey ahead of them.

Three thousand kilometres farther south, in the Canadian woodlands, the ruby-throated hummingbird, almost as different in size and flying skills from the snow goose as it is possible for a bird to be, is behaving in exactly the same way. It travelled up from the south to take advantage of the summer flush of flowers. The supply of nectar was so lavish that it was able to feed not only itself, but its brood of young. But now that the plants have been pollinated, their flowers are withering and falling. So the hummingbirds and their young must also embark on the long journey south.

Raptors – hawks, buzzards and eagles – also came to these woods, hunting for voles and other rodents as well as preying on smaller birds and their newly hatched young. But the voles are beginning to disappear below ground to hibernate and most of the small birds are starting to depart southwards. The raptors must follow. Six hundred and fifty species of birds feasted and nested in the lands of North America during the summer. Five hundred and twenty of them, come the autumn, prepare to retreat in the face of the increasing cold.

Different birds, with different flying abilities, have developed different travel strategies. The snow geese, since they are large comparatively heavy birds, have to fly fast to keep airborne. That, in itself requires them to carry substantial food supplies as fuel on even short flights. Even at the best of times, they have little reserves of power to spare. They will have to make frequent stops on the way to refuel, sometimes spending several days feeding intensively before they have taken on enough to resume their journey.

The raptors are more fortunate. Gliding uses only a twentieth of the energy required for beating wings. They will rely on thermals to take them to altitude and then they cover as much distance as they can in long shallow glides that are almost effortless. So they will wait for a good warm day before they set off. They also know particular places where the thermals are powerful and reliable. Hawk Mountain in Pennsylvania is just such a place and in September, just before the woods begin to flush red with autumn colours, thousands of raptors begin to assemble.

Waders look small and frail, but they are among the most competent and accomplished of travellers. They do not have to rely on warm weather as hawks do and, unlike geese, they can carry significant reserves of fuel. They accumulate it in the form of fat and feed so voraciously on the mudflats of the coast that in a few weeks they almost double their summer weight. These reserves are even bigger than that statistic might suggest, for many of their internal organs, including their brain and their guts, shrink in size to accommodate this additional fuel and save weight.

So all over North America, the mass migration begins. Flying during the day under a blazing sun can bring a risk of serious overheating for birds that beat their wings non-stop. Geese are able to avoid the danger by travelling at night. The families keep together by calling to one another as they go. Smaller birds – thrushes, flycatchers and many others – are also night flyers. Raptors, however, have no option. They must travel by day when the thermals are strong but the problem for them is not so severe since so much of their time in the air is spent gliding. The ruby-throated hummingbird also flies during the day, for it is only then that its fuelling stations, the flowers, are open. It follows traditional routes with extraordinary precision. The same ringed bird will appear from the north in the autumn, year after year, to drink from the same flowering bush.

Waders – sandpipers, sanderlings and knots – find much of their food on the seashore and mudflats, so those that leave the Arctic tundra and travel down the east side of the continent, stick to the coast of Hudson’s Bay as long as they can. They then make a long overland trip across southern Canada down to the coast of New England. There they pause, restock their fat reserves and, in many cases moult so that they have a new set of feathers with which to tackle the ocean crossing to South America. Although they spend their lives close to water, they are not able to settle on its surface or to swim. Their 3,000-kilometre sea passage will have to be non-stop.

Those birds that have been travelling overland down the centre of the continent, many following the broad path of the Mississippi valley, eventually reach the Gulf of Mexico. The shortest crossing, from Texas to the Yucatan peninsula, is a journey of 800 kilometres. To go round the west coast, by way of Texas and northern Mexico, is three times farther. To take the eastern route, down Florida and onwards by way of Cuba, is also a much longer journey and involves several major sea crossings from one Caribbean island to another.

The raptors once more have no alternative. There are no thermals over the sea so they have to take the long western route, overland. Ducks, plovers, nightjars and swallows opt for the eastern route, along the islands of the Caribbean. The tiny ruby-throated hummingbird, almost unbelievably, tackles the sea crossing directly. Its cruising speed is about 43 kilometres an hour, so if conditions are favourable, it can make the transit, non-stop, in around 18 hours. But the passage is a formidable one and it taxes the hummingbird to the very limit of its endurance. A headwind, even a mild one, may hamper it so severely that it will never reach the far shore and perish at sea.

How do birds find their way on these immense journeys? There is no single answer. Each species almost certainly uses several techniques. Some follow major geographical features – the Appalachian Mountains, the Mississippi valley, the coastline of Hudson’s Bay. Night flyers navigate by the stars, perhaps by recognising the point of the night sky around which the constellations revolve which is conveniently marked by the pole star. As a consequence, on cloudy nights they may go astray and sometimes get quite lost. Day flyers use the sun, though this is more difficult since the sun moves so swiftly. If they do use it, they must also have an internal clock to give them an accurate idea of the time. More mysteriously, many if not all birds are able to sense the magnetic field of the earth. That can be demonstrated by fitting a group of them with slender rods of iron. Some are given rods magnetised in a way that obscures the earth’s magnetism. Birds with these get lost. Others are given unmagnetised bars, and these find their way perfectly well. Microscopic grains of an iron oxide, magnetite, have been discovered in the brains of some birds. It may be that they are part of the mechanism of this sensing system. Some birds, like the geese which travel in families, undoubtedly learn their traditional routes from their parents and pass on that information by example to their own young. Young European cuckoos, however, are always abandoned by their parents, yet they are able to find their way to wintering sites that are mainly in and around the Congo rainforest of central Africa totally unaided. They must have inherited their route-finding skill genetically.

By October, the great journeying is coming to an end. The snow geese have not tackled the sea passage across the Mexican Gulf. They have settled for the winter around the Mississippi delta and farther to the west in the northern part of Mexico. The ruby-throated hummingbirds have reached southern Mexico and Panama. Hawks, warblers and nightjars, ducks, plovers and terns are all settled in their warm South American quarters for the winter. Bobolinks, relatives of the oriole, have made one of the longest journeys of all North American birds. They bred in northern Canada and not only crossed the Caribbean but continued south and after a journey of 8,000 kilometres, reached their final destination on the pampas of Argentina. Alaskan bar-tailed godwits undertake an 11,000-kilometre migration to wintering grounds in New Zealand, a journey made in an incredible non-stop flight, over nine days. All these migrants will remain in the south while their summer homes in the north are covered by snow and ice. Then, after a few months, their internal clocks and the changing season will tell them that the time has arrived for them to return once more to feast in the lands of the north.

The pattern of migration is particularly clear in the Americas, but the phenomenon occurs all over the world. European swallows having nested in England fly across Europe, over the Mediterranean either by way of the Straits of Gibraltar or down the long peninsula of Italy and on to the island of Sicily, then across the ferociously hot sands of the Sahara and down to the grasslands of South Africa. In Asia, bar-headed geese that nested on the high plateau of Tibet fly right across the Himalayas, sometimes travelling as high as 7 kilometres, to winter in India and red knots from the high Arctic fly south along the coasts of Japan and Vietnam and may even cross the South China Sea to winter on the southern and western coasts of Australia. In the southern hemisphere, the autumn journeys, of course, are in the opposite direction. The biggest of all hummingbirds, the Patagonian giant hummingbird, flies up from the bleak mountains of Chile to the lush forests of Ecuador and Pacific long-tailed cuckoos travel from New Zealand across the Tasman Sea to tropical Australia.

The rewards for these arduous journeys are clear, even if they are hard won. By making them, the birds are able to reach resources of food in parts of the world where they could not live permanently. How they came to know of the existence of these resources so far away and across such immense obstacles is less clear. It lies in the past.

There have been two Ice Ages within the last one hundred and fifty thousand years. During those times, the areas where birds could live were limited to a band of lands on either side of the equator. But as the earth warmed again, those regions began to expand to the north and to the south. As they did so, the birds followed. Each year, the journey to reach the summer feeding grounds became longer, but only marginally so and the birds were able to keep pace with the changes, learning the way and developing the techniques of navigation. If the world continues to warm in coming centuries, as it seems possible that it may, then snow geese and bobolinks may eventually make even longer journeys than they do today.

The power of flight has enabled birds to exploit every part of the planet in a way that cannot be paralleled by any other group of animals. No physical obstacle has totally defeated them. They have crossed the highest mountain range. They have traversed the widest ocean. The longest of all annual journeys is made by Arctic terns. Every August, they leave their summer quarters in the north and start to fly south. Those in Arctic Canada and Greenland cross the Atlantic and meet others that have come from Arctic Russia around the coasts of western Europe. They travel onwards skirting the coast of West Africa. Some do so all the way to the Cape of Good Hope. Others cross the Atlantic again to the eastern coast of South America. A third group travels down the eastern margin of the Pacific from the northwest coast of Canada, down across the seas off California, Peru and Chile to Cape Horn. All three groups then cross the Great Southern Ocean to Antarctica. At the height of the northern summer they experienced daylight for twenty-four hours of the day. Now they do the same in the Antarctic so they see more of the sun in a year than any other animal. The annual round trip could be as much as 40,000 kilometres.

Terns can come down briefly on the sea or perch on icebergs, so they are able to get some rest. The European swift, however, does not even do that. Its legs are of even less use to it than those of the shearwater, for they are reduced to little more than clusters of four curved needle-thin claws. If a swift comes down to the ground, it finds it almost impossible to take off again. It nests in holes in cliffs and, predominantly these days, in the attics and lofts of houses which it manages to enter through grilles and other spaces below the eaves. To make its rudimentary cup-like nest, it snatches feathers and grasses that drift in the air and cements them together with its sticky spittle. It drinks on the wing, dipping down to make a shallow dive through the surface of a pond. It feeds in mid-air entirely on flying insects. It sleeps in mid-air too, rising in the evening to heights of 2 kilometres and drifting in the wind with only occasional flickers of its outstretched wings. It even mates in mid-air. The male clings on the female’s back and united the two descend for a few seconds in a shallow glide. When a swift, young or adult, leaves its nest in early August, bound for Africa, it may not touch down again until it returns to its nest site nine months later.

An individual swift is known to have lived for as long as eighteen years. In its lifetime, it must have flown more than six million kilometres. That is the equivalent of flying to the moon and back eight times. The swift, truly, is the most aerial of animals.