NINE

The Problems of Parenthood

Sometimes, an egg speaks to another egg. That of a ground-living bird such as a quail, lying snugly in its nest beneath a bush, begins to make clicking noises as it approaches hatching. The little chick within has poked its beak into the air-filled space between the shell and the internal membrane at the blunt end of the egg and is taking its first breaths. The clicks it makes are very rapid – over a hundred per second – and they can be heard by the chicks within the dozen or so eggs lying alongside it. Those that are more advanced and on the verge of hatching will now slow down. They themselves are also clicking but much less rapidly – sixty times per second or less – and these noises have the effect of encouraging other chicks to speed up their development. As such messages circulate among the clutch, so the eggs adjust their development to one another and eventually, although the female quail laid her eggs at 24-hour intervals over a period of two weeks, they all approach the moment of emergence simultaneously. That is important for their survival.

The nest in which they lie may have become increasingly conspicuous. The visits back and forth by the female may have worn a faint path through the surrounding vegetation. The activities of hatching and the white newly-exposed insides of the vacated broken shells, even though the assiduous parent will have removed them as soon as possible, will inevitably cause a certain amount of commotion and that could have caught the attention of a sharp-eyed predator. If the hatching process were to be extended over many days, the whole clutch could be imperilled. It is much better that they should all hatch together so that the parents can lead the whole family away as soon as possible from the tell-tale nest.

Escaping from the confines of a shell is not easy. The newly formed beak of the imprisoned chick is still soft and not strong enough to be used as a hammer. In any case, the chick does not have enough room in the egg to draw its head back and strike a blow. The best it can do is to press hard on the inside of the shell. To concentrate its effort on one particular place, its beak is armed with a tiny white spike, the egg tooth. This is sometimes placed on the end of the beak, but more usually it is on the top of the upper mandible, a little way back from the tip. The chick also has a special muscle at the back of the neck to give it added strength as it pulls its head backwards and exerts as much pressure as it can manage on the shell. In this way and sometimes by also kicking convulsively with its feet, the shell is first cracked, then broken apart so that the chick can struggle free.

Quail eggs, like those of all ground-nesting birds, contain very generous quantities of yolk and this nutriment enables the chicks to develop very fully while they are still in the shell. To give time for that to happen, the adults have incubated the eggs for a particularly long period. As a result, the infants that now stagger to their feet are far from helpless. Their eyes are open, they have downy coats which, as soon as they are dry, will keep them warm and they soon develop the ability to control their own body temperature. They do not yet have either the breast muscles to beat their wings, nor blade-like feathers that will sustain them in the air. So they are flightless. But their legs are strong enough and sufficiently well-muscled to enable them to run. Their tiny stomachs still contain a considerable quantity of yolk – in some species as much as a third of the amount that was in the egg when it was first laid – and this will sustain them for a day or so (which explains why day-old chicks of the domestic hen can survive being packed in boxes and despatched on long journeys without being fed). Quail chicks, within an hour or so of the first eggs hatching, are ready to move and their mother leads the whole brood away from the nest to look for food.

Precocious though such chicks of ground-nesting birds are, they still need protection from being chilled in the rain or overheated in the sun, so when necessary their parents continue to care for them, gathering them together beneath their wings, close to the warmth of their body, brooding them in much the same way as they brooded the eggs. If danger threatens, a parent will give a warning call and the young will either sit totally motionless or alternatively rush to safety beneath their parents’ wings.

The mother quail, like a mother chicken, helps her young to find food, scratching away with her feet and, when she finds an edible particle, putting it down in front of one of her chicks and pointing at it with her beak. Ostriches and many pheasants behave in the same way. Baby cranes and guans are marginally less able to look after themselves and have to be fed, their mothers offering morsels to them directly. Newly hatched kiwis and many shorebirds, however, are able to find their own food without any help of any kind. One way or another, all chicks of ground-nesting birds soon learn to feed themselves.

Water birds also vacate their nests with their families as soon as they can. Jacanas pick up their newly hatched youngsters from the floating rafts that are their nests, tuck them beneath their wings and with the tiny legs dangling untidily down their flanks, march away across the floating leaves to find a place where there are lots of insects for the young to eat. Grebes, which make similar floating nests, provide a more dignified ferry service. The chicks climb up on to the back of one of their parents. Once they are on board, the adult raises its wings slightly to prevent them falling off and sails away across the lake with the youngsters peering inquisitively over the sides. The parent grebe feeds its passengers by twisting its head round and offering them morsels in its beak. But the first objects the chicks are given are not food. They are small feathers either picked up by the adult from the surface of the water or plucked from its own breast. Each little chick will swallow a great number. They may be indigestible but they are very valuable, nonetheless. They accumulate in the chick’s stomach. Some form a felted plug in the opening that leads to the intestine. This prevents any sharp fish bones or indigestible parts of insects that the chick will soon be eating from passing through the stomach and damaging the delicate walls of the gut. Others collect into balls with which the fish bones become entangled and held until they are largely dissolved. This practice of feather-eating will continue throughout their lives, but it must be a specially valuable precaution at this early stage.

The sungrebe, which is not a grebe at all but occupies a family entirely of its own, has the most extraordinary of any method used by birds to transport their infants. The male has pouches in his skin like saddle-bags, one on either side of his body beneath the wing. His two young hatch at an astonishingly early stage, after only ten or eleven days of incubation and they are so underdeveloped that, unlike those of any other water bird, they are blind and virtually naked. Somehow or other – no one is sure exactly how – these tiny helpless young arrive in the male’s pouches, one on each side. The male is even able to take to the air with them on board. No other bird is known to carry its young with it on flights like this.

Duckling and goslings are even more independent than the young of chickens and quails. On the island of Greenland in the Arctic, barnacle geese nest on the face of high inland cliffs where they are safe from raids by foxes. When the young hatch, the parents fly down to the foot of the cliffs and from there call to them. Down the young come, sliding and somersaulting between boulders and tumbling over sheer drops until, miraculously, they arrive at the bottom and scamper after their parents down to a stream. The departure of the torrent ducklings from their nests beside rivers in the high Andes is so dramatic it seems almost suicidal. The tiny bundles of fluff tumble out and dive straight into the rushing waters. They bob up within seconds, their little webbed feet paddling energetically, their water-repellent down making them almost unsinkable. Miraculously they manage to avoid being swept downstream. They are so light in weight that they can ride on top of a raft of spume. They exploit the eddies, take refuge in the swirling water in the lee of a boulder and somehow or other stay on the limited stretch of river that constitutes their parents’ territory.

Such independent young must be able to recognise almost immediately what is edible and what is not. They cannot, however, defend themselves so their mother must keep a close watch over them and do what she can to protect them from predators. It is therefore very important that, as far as possible, they stay close beside her and come to her when she calls. This attachment is formed early and irrevocably, with consequences that may extend throughout their lives. It may even have begun before hatching, for since the chick within the egg can assuredly hear sounds made in the outside world, it may be that even before it breaks out of the shell it has learned the distinctive sound of its mother’s voice. Then, immediately after hatching, for a period which may be as brief as a few hours but in no case is longer than a few days, the ducklings and goslings learn to recognise the appearance of their parents and the sound of their voice. Those memories will stay with them for the rest of their lives and greatly influence in due course their choice of mate.

The link is tested immediately. Their parents, having been guarding and incubating the eggs, are now hungry and anxious to get to their feeding grounds. So they set off on what may be a long journey. Since the young cannot yet fly, it has necessarily to be on foot. The babies trail after their parents, led by the sound of their voice and the vision of their legs. Shelduck families, which nest in disused rabbit burrows or hollow trees, waddle determinedly down to the nearest stretch of water that can provide them with food. There they assemble in groups. As more and more families emerge, a large flock forms. The young are vulnerable to gulls which swoop down and snatch one if they can. The adult shelducks on guard react ferociously and drive off the gulls with vigour. Indeed, they are so competent at doing so that one or two adults can effectively protect a whole raft of ducklings. So as more adults arrive, others who may have been guarding the creche for a day or so, leave to feed in more distant waters.

Young shelduck, with such protection, can fend for themselves. The young of terns, flamingos and king penguins, all of which are also left in creches, cannot. So the parents of these birds have to return regularly with food and it is then that the imprinted sound of the parental voice becomes essential to the chick. In the incessant din of a king penguin colony, it seems impossible to a human ear that the voice of any one bird could be disentangled let alone recognised individually. Yet it has been shown incontrovertibly by marking chicks and adults, that both parent and offspring have no difficulty in recognising one another. This does not mean, of course, that a young penguin will not try its luck. An adult returning to the creche with a full crop will be importuned by a crowd of youngsters, desperate for food, shrieking and prodding with their beaks to try to persuade the adult to disgorge a fish or two. But the adult knows its own young and nearly always manages to deliver the food where it should belong.

Ostrich chicks are also herded into groups by adults, but for a very different reason. The adults seem positively eager to take on the responsibility of caring for them. If two pairs meet, each with its own brood of a dozen or so, there is often a dispute. One pair drives the other away and the victors chivvy the losers’ chicks into their own flock. This may happen several times in the season so that a particularly vigorous and assertive pair may end up with a troop of youngsters several hundred strong. The different families of chicks are likely to be of different ages, so in spite of the fact that the pair’s own clutch hatched almost simultaneously, the group they now shepherd about will contain individuals of very varied ages and sizes. This behaviour may well be to the advantage of the true chicks of the guarding pair, for they are likely to be safer surrounded by other youngsters than they would be on their own.

Of all ground-nesting birds, the most swiftly independent is the infant megapode. Its incubation period is particularly long – sixty to eighty days. The egg, being buried in a mound, does not have to withstand being rolled around and sat on, as eggs in nests must be able to do. Accordingly, it has one of the thinnest of shells and the chick is able to break its way out without much effort. Indeed, although some three weeks before it hatches, a little egg tooth begins to develop on its beak, this comes to nothing and soon disappears unused. Megapode chicks are the only birds that manage to break out of its shell without the assistance of such a tool. But the task that the megapode chick next faces is a very exhausting one. Above it lies a foot or so of earth. Lying on its back, it kicks out with its feet which, like those of its parents, are disproportionately large. As it loosens the earth, it humps its back and wriggles so that dislodged soil is pushed beneath it and it slowly moves upwards. Short bouts of frenzied activity are interspersed with long periods of rest during which it recovers its strength. All this activity is fuelled entirely by the remains of the huge yolk which lies within its infant stomach. It will take several days to dig its way up to the surface, but by the time it does arrive there, it is a totally independent individual with a full complement of feathers that are so well developed that it is able to fly immediately.

Birds that nest in trees or holes do not need to vacate their nests as speedily as those that lay their eggs on the ground for their nurseries are much more secure. Accordingly, they can rear their young in a different way. Their chicks hatch at a much earlier stage in their development. They are naked and their eyes are closed. Their legs are only feebly developed and they are unable to stand. In fact they hardly look like birds at all, more like some kind of grub. Their best-developed organs are their gut, gizzard and liver, for their priority at this stage is simply to take in and process food. Hatching early made it unnecessary for the female to provision her eggs with large yolks and indeed, such chicks as these, unlike those that left the nest early, have by this stage very little yolk left in their stomachs But now their parents can deliver food directly to them and they grow much more quickly than independent ground-living chicks manage to do.

Some adults provide their offspring with a special pre-digested food. The pelican produces a kind of fish soup, and shearwater parents turn their meals of plankton and small fish into a rich oil which they regurgitate for their young. Pigeons produce a very special secretion from their crop which is rich in protein and fat and is known as ‘pigeon’s milk’. Unlike mammalian milk, it is produced by both male and female. Most birds, however, provide the same kind of food for their young, offered in suitably small portions, as they themselves eat, though among those that have a mixed diet, it will be biased towards proteins which are so important for physical growth.

The chicks beg for their food. Young herring gulls, sitting in the scrape on the ground that passes for a nest, point their mouths at a red spot on their parents’ beak. Even thrushes so young that their eyes are not yet open will suddenly crane their heads upwards and open their beaks at the slightest vibration that might suggest that one of their parents has arrived with food. Their gapes, at this stage in their lives, are a bright yellow with swollen flanges and doubtless indicate to the parents where the beakfuls of food should be delivered. The flanges are extremely sensitive, so that if for some reason a chick has ceased to gape the lightest touch of its beak flange will stimulate it to rear up and open its beak. Such a signal is more necessary for those parents whose chicks sit deep in the recesses of a hole. Gouldian finches nest in this way. They are the most colourful of their family and so presumably they are particularly sensitive to colour. Their chicks certainly exploit it. They have large knobs on each side in the corners of their mouths which are an opalescent green and blue and reflect light filtering down into the depths of their nest hole in such an effective way that they look almost luminous.

Colourful gapes can tell parents more than just the location of their chicks. They can also indicate which chicks among their brood have just been fed and which are still in need of a meal. The gapes of young linnets are red, due to the blood in the vessels just beneath the skin of the throat, but when the chicks are given a meal, much of that blood is diverted to their stomach to collect the digested nourishment. The gapes of those chicks that are still hungry are thus the reddest and it has been shown experimentally that the parents use this difference in colour to determine which of their brood will receive the next delivery of food.

The chicks of birds that, like the cuckoos, dump their eggs in the nests of other species have to take account of this signalling system if they are not to starve. A newly hatched African whydah whose mother sneaked it into the nest of a waxbill seems an obvious interloper for whereas the young waxbills are naked and pink, it is covered with a mauve down. But the waxbill parents never see it like that. As soon as one of them alights at the nest, the young reach up with open mouths – and then they all appear to be virtually identical, for the young whydah has developed mouth spots that closely mimic those of the young waxbills. The screaming cowbird in Argentina enhances its mimicry even more profitably. It leaves its eggs in the nests of several other species, including those of other non-parasitic cowbirds. All its victims, however, have chicks with plain red gapes. But the gape of young screaming cowbirds is a far more intense red and permanently so. In consequence they are always fed before their foster parents’ own brood.

Linnets do not start incubating their eggs until their clutch is complete and as a result all hatch within a relatively short time. Their parents then rely on the colour of the chicks’ gapes to ensure that the youngsters are fed in turn. Rosella parrots in Australia, however, have a more difficult problem. They start incubation immediately the first of their clutch of half a dozen or so is laid. Their eggs therefore hatch over a period of at least eight days and to begin with there is a great difference in size between the chicks, with the oldest sometimes weighing five times as much as the youngest. But rosella parents distribute food to their offspring so equally that by the time the chicks are ready to fly, they are all the same size.

The young of European coots are also treated even-handedly. If one of them becomes too greedy, squawking loudly and pushing the others out of the way, its parent will pick it up and chastise it by shaking it vigorously. It may even be given a thorough ducking in the water and become so distressed that it takes refuge in the reeds while its parent feeds its siblings. It may never return. This happens with such regularity that most coot broods are quickly reduced from seven to three.

Coots in America, which belong to a different species, behave somewhat differently. Their chicks have a brilliant orange tuft of down on their heads beside a bald patch which is bright scarlet. When they beg for food they flaunt this brilliant head gear, nodding frantically in front of their parents. The brightest are fed first. These are likely to be the strongest and most vigorous of the brood and therefore the ones most likely to survive the trials that lie ahead. So it is better that they should get as much food as they need rather than to spread what food there is more thinly and perhaps inadequately among them all. The result is that American coot families also reduce quickly in size. Almost a third of the chicks that hatch each year die from starvation.

Such a strategy of differential feeding is practised by several different groups of birds. All boobies adopt it to varying degrees. The blue-footed booby lays two or three eggs which hatch about four days apart. There is consequently a considerable difference in size between the chicks. In years when there are plenty of fish around, the parents may be able to feed all three, but if fish are hard to find, the youngest chick will not be able to compete with its elders. As they grow stronger, it gets weaker. Eventually it will be pushed out of the nest and die. The system seems unnecessarily harsh, but the boobies at the time they are laying eggs, have no means of predicting how abundant fish will be in the weeks ahead. If they lay only a single egg and there proves to be an abundance of fish, they will have missed a precious breeding opportunity. A second or third egg is a small stake in a gamble that could double or even triple the number of offspring a pair raises in a season.

Most birds of prey also lay more eggs than they can raise, feed the eldest preferentially and allow it to harry its younger sibling so unrelentingly that it dies. The winner will then usually eat the loser, so the nutriment invested by the parents in the extra egg and the food they gave to the nestling it produced, is not wasted. The macaroni penguin has a strange variant of this practice. It also lays two eggs, but the first is smaller than the second, hatches later and seldom survives. Why there should be this reversal in the fate of the first-laid egg is uncertain. It may be that macaroni eggs and chicks are at the greatest risk early in the breeding season when the colony is just establishing itself. At that time, adults are quarrelling viciously among themselves and skuas lurking around the outskirts of the colony have the best chance of grabbing an egg or a newly hatched chick. Indeed about half the first eggs laid by the macaronis are lost. So in the unsentimental practice of natural selection, it is better for these gamblers to place a heavier bet on the second runner.

The chicks of other birds, even though they hatch more or less simultaneously, will nonetheless fight among themselves for survival. Young Asian blue-throated bee-eaters even have a special weapon for doing so. To begin with, there may be seventeen of them in a clutch. Each has a sharp downward-pointing hook on the tip of its bill with which it slashes at its still-naked siblings in the darkness of the nest hole. Two or three are likely to be so badly wounded that they die. The mayhem will continue for about two weeks, by which time those still in contention have started to grow feathers. These protect them from further injury. The hooks on their beaks by now have either worn away or dropped off. The survivors – and there are very seldom more than three – then settle down amicably together.

The dangers facing most chicks, however, come not from their siblings but from predators. In Europe, magpies regularly raid the nests of blackbirds and other garden birds. Toucans, in South America, will eagerly vary their diet of fruit with a small defenceless bird if they get the chance. The African harrier-hawk or gymnogene has such a taste for chicks that it has evolved special adaptations to obtain them. Its legs are unusually long and double-jointed. It is thus able to reach into a nest hole, or up into a suspended weaver bird nest, even one with a long protective entrance tube, and drag out the struggling chick.

Many snakes are very agile climbers, using the scales on their underside to catch on the bark, winding themselves through the twigs to slide into a nest and grab a chick. On the ground, rats and stoats, cats and squirrels, foxes and raccoons, all find small chicks excellent meals, packed with nourishment. Concealment of a nest, wherever possible, is therefore of great importance to birds.

Parents may take steps to avoid leaving clues to their nest’s position. Many chicks, in their first days, extrude their faeces enclosed in gelatinous sacs. The adults often swallow these and some seem almost to relish doing so. Perhaps they contain some residual nourishment that an adult’s more robust digestion is able to extract. At any rate, a parent bird, having fed a chick, will often prod its baby’s rear, encouraging it to raise its rump and defecate, and when it does so, its parent collects the little white sac as it emerges. As the chick grows older and its digestion becomes more efficient, its faeces are less likely to be eaten. Instead they are carried some distance away and dumped. The superb lyrebird takes no chances. If there is a stream within reasonable distance, it will drop its load in the water. If there is not, it buries it. A potoo chick is just as well-concealed as its parents but it would soon be detected if it whitewashed its perch. It is, however, possessed of a phenomenally powerful anal squirt. Its faeces are not so much droppings as missiles which land several metres away from where it sits. Birds that nest in holes are also usually faecal squirters. Kingfishers and hornbills will back up to the entrance of their holes, lift their tails and eject a stream with sufficient power to ensure that, as far as possible, the tell-tale streak of white falls clear of the tree trunk or riverbank. This removal of faeces from the nest is not only protective, it also keeps down the risk of disease and infection by parasites.

As chicks grow, they make greater and greater demands on their parents. In the tropics, young cranes and storks, sitting on their stick platforms out in the open beneath the blazing sun, are in real danger of getting seriously overheated, so some parents bring stomachfuls of water and regurgitate it over the young as a cooling shower. Herons will provide the same relief but in a less sanitary way and defecate over their babies. But the greatest, the most insistent of the demands of the young is for food.

The appetite of chicks can be gargantuan and seemingly insatiable. A great tit, which feeds its young with beakfuls of insects, may deliver food to its nest nine hundred times a day. The foster parents of a young European cuckoo, driven into a frenzy of food-collecting by the super-stimulus of the chick’s enormous yellow gape, thrust food into it with such assiduity that in three weeks the chick increases its weight fifty times. Nevertheless, if both parents share the task, the labour of feeding and protecting a young family is not overwhelming. Where conditions are at their most favourable, the whole job can even be done by a single adult. That usually proves to be the female. The males then pay no attention whatever to such offspring they have fathered and devote their lives largely to display, as do birds of paradise, pheasants, manakins and other polygamous birds.

But not all birds live surrounded by such abundance. Some birds live in territories that are so poor in food that they can only succeed in rearing a family if they get help. And that often comes from previous generations of young.

Such assistance is not a demonstration of unselfish altruism on the part of the young birds. There may be no room in the surrounding area for new territories. If a youngster tries to establish one, other residents will fight to repel it. Better to stay where it is welcome, as it most certainly is when its parents are desperately searching for food for their new family. Furthermore, by doing this, it is at least aiding the survival of those genes that it shares with its younger siblings and that, while not as good as parenting its own brood, is better than not breeding at all. Eventually one of the old birds will die, and then one of the helpers will be well placed to mate with the survivor and inherit the family property. This habit, which was recognised only recently, is now known to be widespread.

Jays in the arid, impoverished scrublands of Florida behave in this way. So do babblers in the deserts of Arabia and white-fronted bee-eaters in Africa where conditions are so variable from year to year that food can unexpectedly become desperately difficult to find. Almost half the bird species that live in the eucalypt woodland of southeastern Australia also behave in this way. At first sight, this seems odd, for the bush here does not appear to be particularly impoverished. The cause, however, may lie in the equability of the climate. Here there is no sudden spring to bring a surge of new leaves and a flush of insects to feed on them, as there is in the woodlands of the northern hemisphere. There is therefore no sudden superabundance of food at any time. The birds in residence have no difficulty in feeding themselves, but when they begin to breed and suddenly have four or five additional mouths to supply, finding sufficient food becomes very difficult indeed. Kookaburras live in these woodlands, subsisting on snakes, lizards and small rodents. Their fledged young not only stay with their parents to help with gathering food for the new broods, but also assist them in defending their feeding territory. They do so, primarily, by sound – that hysterical laugh for which the kookaburra is famous. The louder the laugh, the better guarded the territory, so young birds join their parents in laughing choruses.

White-winged choughs also live in these Australian woodlands. They do not become sexually mature until they are four years old and until that time many of the immature birds stay with their parents. Their staple diet is grubs and finding them is a very labour-intensive business. They have to dig holes in the ground that may need to be as much as 20 centimetres deep. Older birds are very skilled at this and can quickly assess which places are likely to yield grubs. Young birds take some time to learn this and do not therefore hunt with such success. As a consequence, they continue to rely on their parents for food until they are about eight months old.

With food so scarce and hard to collect, the breeding season inevitably precipitates a crisis for the choughs. A pair usually lays four eggs. Their only chance of raising any young at all is with help. With two assistants, they may be able to gather enough food to rear a single chick. To raise all four, they will need at least eight helpers. Such a powerful group may then be able to expand its territory. A family gang – a pair with up to a dozen of their immature young – will raid the territory of a neighbour and, if they can, destroy their nest, tipping out the eggs or young and pecking to pieces the beautifully crafted mud bowl. And the warfare does not end there. If at a later stage when the nestlings have fledged, two families encounter one another, there will be great displays of aggression. But while some adults are squabbling, others may be engaged in kidnapping. They approach a newly fledged nestling of their rivals’ group and entice it away with offers of food. If a youngster is lured across, it will within half an hour or so accept its erstwhile rivals as its own family. It may be that at this early stage in its life, it has not properly learned the identity of its own family, for a month later such kidnappings never happen. But for the family which has recruited an extra member, there will be more helpers to rear the next generation.

As nestlings approach the end of their infancy, they begin to fledge. Proper feathers begin to appear, displacing any down that the chick may have had initially. Each develops from a small pocket in the skin. At first it is a soft rod encased in a sheath. It grows very rapidly indeed and soon bursts free and expands. The core of blood vessels and living cells is then absorbed back into the skin and the quill, now hollow, together with the barbs on either side, hardens. It is now as dead as a toe-nail.

Most youngsters at this stage have still not achieved their adult weight and will not do so until several weeks after they have left the nest. About a fifth of all species, however, are heavier than their parents. A newly fledged great white pelican chick has in fact reached full size but it weighs nearly 40 per cent more than an adult. This is because growing tissues contain more water than mature ones. A few – among them albatrosses, parrots, owls and kingfishers – owe their extra weight to an additional cause. They have built up large food reserves, thanks to the lavish feeding of specially attentive parents. Those of the sooty shearwater of New Zealand weigh twice as much as an adult and it is these generous fat reserves beneath their skin that cause them to be called mutton birds.

The young bird, in its new but untried coat of flight feathers, sitting on a ledge, peering out of its nest hole, or squatting precariously on a nest high in a tree, now faces its first real move to independence. Many need little encouragement to leave the nest which by now has become seriously overcrowded. Those such as thrushes, blackbirds and robins that, as adults, will collect their food from the soil or the leaf litter, flutter clumsily to the ground or perch unsteadily in bushes, pleading with their parents to continue supplying them with food. They are still not skilled at getting into the air and are easily caught by ground predators such as cats. Many will certainly perish.

Those that sit in nests high in trees or on the ledges of cliffs, if they are to avoid a catastrophic crash, have to be reasonably competent in the air from the very moment that they launch themselves into it. Young eagles spend hours every day beating their wings and bouncing up and down on their nest, strengthening their wing muscles and doubtless generally getting the feel of what it is like to be airborne. Young hummingbirds also practise but, to begin with, do so more cautiously, holding on to the rim of their tiny nest-cup with their feet to prevent themselves being swept upwards by their whirring wings before they are quite ready for it.

Some youngsters are so hesitant that they have to be persuaded to take to the air. The parent shearwaters, having fed their young in their nest burrows with such generosity for sixty days, simply stop doing so. After six days without eating anything, the chicks, now living on their fat reserves, come to the burrow mouth for short periods and exercise their wings, something that they cannot do in the restricted space of the nest chamber. It is only after twelve days of starvation that they feel confident enough – or hungry enough – to spread their wings and launch themselves into the air.

Peregrine parents use the carrot rather than the stick, the promise of food rather than the threat of starvation. It is usually the female who takes this responsibility. Instead of bringing newly caught prey back to the nest, she settles in a tree nearby and, with her catch still in her talons, calls to her three or four young. Having attracted their attention, she flies from tree to tree carrying the meal that they seek, calling as she does so. And then, at last, one of the young daringly launches itself into the air. Flapping rapidly and clumsily, the young bird lumbers through the air towards its meal and eventually claims it.

But this is only the first lesson. Each chick now takes up its own perch where it will be fed. As they grow in strength and confidence in the air, the female makes the collection of food a little more difficult. She holds on to the prey and persuades the young to try to take it in mid-air. When the youngster comes towards her, she rolls over on her back so that it can take the prey from her talons. The next lesson, which requires still greater aeronautical skills, is how to catch things in mid-air. She flies ahead of one of her young and drops the food so that the youngster has the chance to swoop and grab it. If it misses, she or perhaps her mate who may be keeping a watchful eye on these tutorials and is flying below, will catch it before it hits the ground and then they give the youngster a second chance.

So at last, young birds of all kinds achieve their independence. The labours of their parents in selecting the best possible mate, placing the eggs in as safe a position as can be found, and working unceasingly for days on end to provide food for extra mouths has, at last, been rewarded. A new generation is in the air.