SIX

Signals and Songs

Few animals, other than ourselves, use their voices as eloquently as birds. Ours are created by the larynx, the lump high in our throats that we call the Adam’s apple. Birds have a larynx too, but it does not produce sound. It serves instead as a valve which prevents food and water from entering a bird’s windpipe and getting into its lungs. Birds’ voices come from a different structure that lies much deeper within their body, one that is possessed by no other creatures, called the syrinx.

This box-shaped organ, strengthened around the outside with hoops of cartilage, lies at the bottom end of the bird’s windpipe where that divides into the two tubes which lead to its pair of lungs. Within the syrinx, each of these tubes can be closed completely or partially by a pair of fleshy lips. When the bird contracts its lungs, a jet of air is blown through each pair, creating a musical sound. Muscles within the syrinx enable a bird to vibrate each pair of lips independently and so vary the pitch and quality of the note that comes from each.

The windpipe up which the notes then travel also modifies them. As with the pipe of an organ, the longer the windpipe, the deeper the sounds it emits as the air in it resonates. Cranes, which produce trombone-like calls, have windpipes that are so long they curl into loops alongside the keel of their breastbones and in some species actually pass right through it. One of the plainer birds of paradise, the trumpet manucode, has one that runs down across its chest just beneath the surface of the skin and forms a loop. As a trumpet manucode gets older so it develops more loops. Only the males have them and only the males produce the organ-like trumpeting calls that give the species its name – though considering the lavish looping of its windpipe, it might have been more appropriate to call it the French horn bird.

Some birds are able at will to shorten their windpipes. Like the syrinx, the windpipe is encircled by strengthening rings of cartilage. These are connected to one another by muscles which enable the bird to pull the rings closer to one another like the pleats of a closing concertina. As that happens so the resonances from the windpipe rise in pitch. Penguins have a partition running along the length of their windpipe, dividing it into two unequal halves, so when a jackass penguin brays – it can hardly be said to sing – it produces a two-note chord.

By taking very shallow mini-breaths, accurately coordinated with the notes of its call, a bird can sing without a noticeable break in the sound for minutes on end, far longer than even the most highly trained human singer is able to do. Each pair of lips within the syrinx may produce its own sound. Canaries, when trilling, create 90 per cent of the sound with the left tube and use the right mainly for breathing. Cardinals, whistling a glissando, start the lower section with one tube and mix through imperceptibly to the other tube for the higher half. The two sounds from the two tubes may also combine and interact with one another to create a quite different sound. This explains how some birds, such as parrots and mynahs, are able to produce eerily close imitations of human speech, even though they lack the lips and variable mobile tongue with which humans make such sounds. All in all, in duration, variety and complexity, no other vocalisations produced by any other animal can match the song of a bird.

Birds also communicate visually. Their feathers can, with comparatively little cost to the bird, be coloured and patterned. Muscles in the skin enable a bird to fan them out or erect them to display any message they may carry or, if it is a dangerous time to do that, to depress them and conceal it. They can be given different messages at particular times of the year and at different stages in a bird’s life, for the feathers have, in any case, to be regularly changed as they become worn by flight.

Some of the colours of feathers come from pigments. The commonest of these is melanin, a pigment that human beings produce in their skin when exposed to the sun. This creates the black of a blackbird’s feathers but also, in different varieties and strengths, browns and yellows. Reds and oranges are created by other pigments called carotenoids. These, however, a bird cannot make for itself. It has to obtain them, directly or indirectly, from plants. Flamingos and scarlet ibis derive them from small crustaceans which in turn get them from the blue-green algae on which they feed. Such birds in zoos, deprived of their natural food, will lose their red colour unless they are given carotenoids in some other form. Turacos, handsome pigeon-sized fruit-eaters from African forests, have a pigment based on copper that is found in no other animal and is accordingly called turacin. It is responsible for the dramatic flash of magenta that appears when a turaco opens its wings.

Other colours are created not by chemical pigments but physical structures within the feathers. The blue of a jay’s wing feathers is created by microscopic bubbles in the keratin of the feathers which refract the light. So such a feather will not look blue but brown if light shines not on it but through it.

These two sources of colour, chemical and physical, are sometimes combined in the same feather. The green of the wild budgerigar comes from a yellow pigment overlying a blue created by an internal structure. It has thus been comparatively easy for breeders of pet budgerigars to produce strains which lack the pigmented yellow and so expose the structural blue.

The shimmering iridescence of a male mallard’s head in spring, the ‘eyes’ in a peacock’s tail, the almost metallic glint on a Himalayan monal pheasant’s plumage and the glittering bib of many hummingbirds, all come from a particular modification in the shape of the filaments on the feather. They are flattened and twisted so that their broad side faces outwards. The filaments can only develop in this way if they are detached from one another, so they lack the tiny hooks which in other feathers link these into a continuous vane. Thus iridescence of this kind is never found in flight feathers. Microscopic layers of melanin particles within the filaments split the light in the same way as a film of oil on the surface of water can do and like such a film, the colour varies according to the angle from which you view them.

Birds use both sound and vision, either separately or together, to communicate with their own particular mates, with strangers of their own kind, with birds of quite different species, and even with utterly different animals. They issue battle cries, send warnings, summon mates, declare war and even, on occasion, tell lies.

Communicating with an animal of a fundamentally different kind, which expresses itself in ways that you cannot parallel and which perceives the world in a quite different manner from you, must present huge problems. Nonetheless some birds, such as honeyguides, do so with such success that they rely on that ability to get their food. Honeyguides are small thrush-like birds that are distantly related to woodpeckers. The species are found in Asia and Africa. They eat a variety of insect food but all have a taste, not so much for honey as for bee grubs and bees’ wax.

Wild bees choose a variety of sites for their nests. Those that live in the Himalayas suspend them beneath overhanging rocks, like stalactites hanging from the ceiling of a cave. There they are unprotected and Asiatic honeyguides have little difficulty in raiding them. As with all members of their family, the feet of these birds resemble those of a woodpecker with two toes pointing forwards and two backwards so that their owners can cling with ease to a vertical surface, whether it is a tree trunk or a bees’ comb. African bees, however, favour more protected sites for their nests – holes in trees or deep in crevices between rocks – and to reach those, honeyguides need help.

The ratel is a kind of badger and a powerful digger. It will eat all kinds of small animals – frogs, mice, termites, fish and lizards – but it too has a particular passion for honey and bee grubs. At certain times of the year it lives on little else and so earns its alternative name of honey badger. It is a nomadic creature that wanders widely through the bush, searching for food. As it enters the territory of a honeyguide, the bird will fly down to perch in a bush close to it and call with a distinctive chattering cry. The ratel answers with grunts and ambles towards it. The bird flies off and the ratel follows. Every now and then, the bird stops and calls again, flirting its tail which has distinctive white feathers on either side, as it waits for the ratel to catch up. Eventually the bird flutters up to a higher perch and gives a different call. That indicates that a bees’ nest is close by, perhaps in a tree hole. When the ratel finds it, it starts excavations.

The bees retaliate. The ratel is by no means impervious to their stings. It turns, sticks its rear into the hole it has created and releases a foul-smelling secretion from glands beneath its tail. Travellers who have disturbed a ratel at work have said that the stench it creates is unendurable. The bees apparently also find it so, for many leave the nest and the animal, now far less troubled by clouds of angry insects buzzing around its head, rips away the wood to enlarge the entrance hole. Eventually it claws out the combs, dripping with rich deep-brown honey and takes them away to eat. The honeyguide now has its chance and flutters down to feast on what is left of the wrecked nest.

The benefits of the partnership to both animals are clear. The bird, flying every day around its territory, knows the ground intimately and becomes aware of every bees’ nest it contains just as soon as one is established. The ratel, wandering into that territory perhaps for the first time, lacks that knowledge but has the strength to excavate nests that the bird could never reach unaided. The partnership can only exist, however, because of the bird’s skill as a communicator.

When this relationship was first established we cannot know, but the bird has now evolved such a specialised digestive system that it is likely to have been very long ago. Nor do we know whether the ratel was the bird’s original partner. It is certainly not its only one today. The Boran people in northern Kenya are only too glad to enlist its help. A skilled Boran honey-hunter knows just how to call to the bird. He whistles, by blowing across a hollow seed, a shell or the thumbs of his clasped hands. The same little procession then takes place, the bird giving directions both with its calls and its behaviour, the man following. And when the prize has at last been claimed by the man, he by tribal tradition will always leave a section of the comb in a prominent place so that the bird is able to take its share.

Mankind and other mammals, however, are more often the enemies of birds than their partners and the messages birds send to their enemies are simpler and more forthright. The sun bittern uses visual signals. It builds its nest in swampy regions of the South American rainforest. When it is sitting, it is difficult to detect for its brown plumage, barred with thin wavy stripes of grey, white and olive, blends in closely with its background. But if you do see it and walk slowly towards it, the bird, most unexpectedly, will fan its tail and spread its wings, revealing on each a bright chestnut patch, edged on the upper side with black and heightened by a surround of glowing gold. The two patches glare at you like a pair of huge eyes. If you stand your ground, the bird will rise and stalk towards you with wings outstretched. The effect is so startling and the bird behaves with such confidence that anyone unfamiliar with it might be forgiven for thinking that the creature in front of them was actually dangerous. That, doubtless, is the intended message. It is not, of course, true. The bird is telling a lie.

Owl chicks send similar signals and are similarly mendacious. Climb up to look at a barn owl sitting on its nest on a ledge high in a farm building. The bird opens its wings but holds them low. It fluffs out its body feathers, with its beak held wide open. Every ten seconds or so, it looks directly downwards and shakes its head. This is a most surprising thing to do, for it involves the bird taking its eyes off the intruder and thus, presumably, making itself vulnerable to attack. Nonetheless, the effect, like that of the sun bittern’s display, is to alarm an intruder and to give the impression that the bird is more dangerous than it actually is.

The most convincing of all avian lies, however, are not those that say ‘I am here and dangerous’ but ‘I am not here – I am something else.’ A great number of birds, like a sitting sun bittern, are camouflaged by their plumage. A woodcock on its nest on the ground in an English woodland, surrounded by the dead brown bracken, sits tight and, seemingly, is as invisible to a prowling fox as to a human being. Its camouflage depends on it remaining perfectly motionless. The nacunda nighthawk, a kind of nightjar, lives on open grassland in Brazil and spends much of its day resting on the ground. There is, of course, little there to provide cover, but the bird manages to hide by crouching and, helped by its appropriate coloration, disguising itself, it seems, as a pile of cow dung. The resemblance is certainly remarkably close. But there is one difficulty about this explanation. Cows did not graze the Brazilian grasslands until they were introduced from Europe a few centuries ago. The nacunda can hardly have evolved its remarkable imitation in such a short time and there are no other large grazing animals on the plains to provide a model of the bird to imitate. But once there were. A thousand or so years ago, giant sloths and great armadillos the size of small cars trundled across the plains. They are now all extinct, but maybe the nacunda is giving us an accurate picture of what giant sloth droppings looked like when once they littered these grasslands.

The potoos reinforce their impersonations with actions. They are big birds, giant relations of the nightjars from the American tropics which stand 45 centimetres high. Potoos perch, habitually, on a tree stump. The mottled plumage is much the same colour as bark so they are not easily noticed at any time. But as you approach, one moves to make its imitation of a tree stump even more perfect. Very slowly, it lowers its tail and presses it against the bark of the stump so the junction between the two is invisible. Then, equally slowly, it raises its head until its beak is pointing vertically upwards and shuts its eyes. This might seem just as risky an action as the barn owl lowering its head, since the bird has now such confidence in the efficacy of its disguise that it will stay rigid and motionless even if you come to within a metre of it. But the bird can see you, even though its eyelids are closed, for each has two tiny vertical slits in it. They let through enough light into the bird’s supersensitive eyes for it to keep a check on what you are doing and how close you are getting. Only at the very last moment will one lose courage and flap away. The effect can be very startling indeed for it is quite likely that until that very moment you were totally unaware that a bird was there at all.

Up in the far north, in the tundra of the Arctic, it is rather more difficult for a bird to disguise itself as part of the landscape, since that changes so radically with the seasons. Nonetheless, the ptarmigan has to do so for it lives there throughout the year. In winter, it is almost invisible. Its plumage is as white as the snow across which it wanders looking for the exposed tufts of vegetation that might provide it with a meagre meal. Only its black eyes and beak are visible to the cursory glance and they might easily be mistaken for fragments of rock projecting through the snow.

But as winter retreats, giving way to spring, the snow melts. If the white birds do not change colour, they will become very conspicuous indeed. The long-term solution is to change their feathers for ones of a different colour, but that takes some time and while it is happening, the birds must cluster closer and closer together on the shrinking snow patches. The females complete their moult first. As soon as they have done so, they fly off into the low bushes of the tundra, confident in the effectiveness of their new brown camouflage, and start to make a nest. The males, however, are still competing among themselves for dominance. A male who has a female already sitting needs to be able to drive his rivals away. It seems that he cannot do this effectively if he is wearing his brown summer plumage. At any rate, the white-clad males start their squabbles at the end of winter and continue them, as the temperature rises, on the dwindling patches of snow. They cannot therefore start their moult until some time after the females, by which time spring is already advanced. Suddenly, their need for disguise becomes urgent. Swift though their moult might be, it still takes three to four weeks and that could be fatally long. The males, however, have found a way of changing from white to brown in a matter of minutes. They fly to a dust-bowl and there they wallow, turning their white feathers brown, matching themselves to their new background.

A bird may also need to communicate with other species of bird. That, perhaps is not as remarkable as communicating with a mammal, but it is still a notable achievement. Each species, after all, has its own range of sounds and gestures. But just as human beings in an emergency once used SOS and Mayday signals that were understood by all people, no matter what their native language, so birds also have calls that are universally understood between themselves.

The arrival of a sparrowhawk hunting along a hedgerow is just such an emergency, as far as the hedgerow birds are concerned. The first to spot it sounds the alarm. Doing so risks drawing attention to itself but the call it utters is short, soft and high-pitched. Such a sound is very difficult to locate. It is often written, accurately, as ‘seet’. Having made such a call once, the sentinel then keeps quiet and hides in the foliage. All the other hedgerow birds do likewise. Tits, thrushes, and finches each have their own seet calls but all the versions have the same characteristics and all are understood by them all. The mixed community has evolved an international language.

On the floor of the Amazonian rainforest, insect-feeding birds form a similar mixed community and hunt together, rummaging through the leaf litter and overturning bits of bark, to dislodge insects which, as they run for safety, may escape one beak but will be snapped up by another. Two birds that join such groups habitually act as sentinels – shrike-tanagers which perch in the forest canopy and antshrikes which stay in the understorey. If either sees danger, it will make a seet call and all the birds in the feeding flock will immediately take cover. The sentinels’ task requires them to keep looking around and so prevents them from searching in the leaf litter with the same dedication as others in the flock, but their reward is that they are able to collect insects disturbed by others. But they do not always tell the truth. It has been reliably reported that if a particularly large and succulent insect is unearthed by one of the birds in the flock below, a sentinel may quickly utter a seet call, even though no danger has appeared. The flock immediately dashes for cover – and the sentinel swoops down to steal the prize.

A seet is not the only way of sounding the alarm. A very different call is sounded if the danger comes from a hunting mammal on the ground. If it is a cat rather than a hawk that threatens the birds in a hedgerow, the sentinels, instead of remaining unobtrusive and hard to locate when they warn, create a huge hullabaloo, screaming angrily and continuously. Their scolding, of course, warns others and to that extent has the same function as the seet call. But if that was its only message, you might expect that the birds would then fly off to keep out of harm’s way. But they do not do that. Instead, they fly towards their enemy, perch in the branches close to it and squall in angry chorus. Unlike the seet call, their shrieks are loud, long and relatively low in pitch, so the position of the callers is immediately obvious.

They are in no real danger. A cat cannot approach and pounce with the speed of a hawk. Its technique is to creep stealthily towards its prey until it is close enough to grab it. Once it has been observed, it cannot win. The screams of the birds tell it that its presence is now known to every bird in the hedgerow and it realises that there is no point in trying to creep up on one of them. At that stage, the cat usually abandons the hunt and stalks away with an air which, in a human being, would be construed as one of injured innocence.

Fieldfares, one of the larger members of the thrush family, take even more vigorous action against their enemies. They are very gregarious birds and often nest in colonies. Should a nest-robber and chick-stealer such as a magpie appear near one of their colonies, the first fieldfare to notice it will scream a warning. Its call is quickly taken up by others. Their cries certainly convey their anger to the magpie and are likely to unnerve it, but they also summon the rest of the colony. These are battle cries, summoning the troops. Still screaming, the fieldfares take to the air. Then they begin to dive on the intruder. As each makes its attack, it shrieks until it is within a few feet of its target and then releases a bomb of faeces. Many of these missiles are so well aimed that they actually hit the magpie. Soon its plumage is badly soiled. Sometimes it is so thoroughly plastered that it tumbles to the ground and has to hop away to try to clean itself.

If birds of the same species are to have any form of social life, they have to exchange not only alarms and war cries but many other messages. One of the most important concerns identity. An individual male needs to know whether another bird he encounters belongs to the same species as he does. If it is not, then providing it is not an enemy, it may be ignored. But if it is, then the stranger may be a rival for the same meal, the same nest site, the same territory or the same female. Action has to be taken accordingly. So it is that many birds carry patterns on their plumage which declare exactly what kind of bird they are.

Human beings, who also like to be able to identify bird species, have worked out just which colours and patterns on a bird enable them to do that. Any page in a field guide will show you the features they use. A European robin can be recognised immediately by its red breast, a male blackcap by the black feathers on his head. Finches have bodies that are very similar in shape, but each has a different patch of colour on the head – the chaffinch has a grey-blue one, the hawfinch a brown one, the greenfinch a yellowish face with a golden yellow edge to the wings, and the bullfinch red on its cheeks that extends down its breast. Confirmation that these colour patches are the features that birds themselves also use for identification can be seen if you watch a male bird establishing his territory. The cock robin thrusts out his red chest; the blackcap erects his black crest; and every male finch chooses to demonstrate his proprietorship by alighting on a prominent perch where his splendid uniform in all its detail can be seen to the best advantage.

The comparison with a uniform is indeed apt, for in the days when wars were fought hand-to-hand and face-to-face it was imperative that a soldier knew immediately whether the man who suddenly appeared beside him was a friend or foe. So opposing armies assumed spectacular uniforms with widely differing colours. It was an arrangement between enemies that suited both sides. But within an army, it is also necessary for individuals to recognise something else about another soldier. Given that the overall character of a stranger’s uniform indicates that he is fighting on the same side, is he senior or junior? Does he take orders or will he give them? The insignia that convey that information are not so obvious and require closer inspection. How many stripes does he have on his arm? Is there a red tab on his lapel? And so it is with birds.

A male house sparrow has a black bib, but look closely and you will see that it varies in size. A male great tit has a black stripe running down his chest, but some have a broader one than others. A collared flycatcher has a white patch on his forehead but different males have them in different sizes. In all these instances and many more, the dimensions of the colour patches indicate seniority. The bigger it is, the more senior the bird that carries it. If and when two birds begin to dispute, over a morsel of food or the possession of a territory, the junior bird will usually defer to the senior.

Why do they not make a fight of it? The dimensions of these insignia are partly determined by genetics – a vigorous male is likely to pass on his strength to his offspring – and partly by the way that those offspring are cared for. A well-fed young male house sparrow, strong and lusty, will have a bigger bib than a weakly ill-fed one, and a great tit a wider breast stripe. Young collared flycatchers, as they fledge in their nests in Europe and prepare to migrate south for the first time, do not have that white forehead patch. It only appears when they moult on their wintering grounds in Africa. Its size depends on how well they feed there. So when they return to Europe to breed, they carry accurate indications of their strength. If it came to a physical fight, a well-fed male with a big patch would be likely to beat an opponent who has a smaller forehead patch and is therefore not as strong. So it saves energy all round if the weaker bird concedes straightaway.

These badges of rank need not be on permanent display. The male wild turkey, that grandest and most magnificent bird of the North American woodlands, has in addition to his spectacular feathers and the strange hair-like tassel dangling down his chest, a naked neck hung around with swags of wattles. These vary in size. A bird that is not very fit, plagued perhaps by an infection of internal parasites, will not have such big wattles as a bird who is in the peak of condition. And as a bird gets older, so his wattles grow bigger. But they can also quickly change colour. Usually they are a purplish red, but if a male disputes with another, perhaps over access to females, then they will rapidly flush bright scarlet. So do those of his rival. As the two contestants square up to one another, the difference in the size of their wattles becomes very apparent. The male with the lesser ones, daunted by the sight in front of him, rapidly backs down – and signals his submission by a rush of blood away from his wattles that, within seconds, fade from bright red to pale pink.

Visual signals, however, have a great limitation. They can only be transmitted and received over a relatively short range. To broadcast messages to a wider audience, sound is much more effective. Some birds create it mechanically. Woodpeckers do so by adapting the hammering action of their bills, which they normally use to excavate their food, to beat out characteristic and meaningful tattoos. As drums they select trees or branches that are particularly resonant – a hollow trunk or a dead branch. Some exploit exciting new instruments that have only recently come their way – a corrugated iron roof or a metal stove-pipe. Each species, indeed each sex, drums in its own characteristic way. The most diagnostic feature, it seems, is the frequency with which each tattoo is repeated. The speed with which the blows are delivered is also important. They are mostly so rapid that it is impossible for the human ear to count them. Only analyses of recordings can do that.

They show that the greater spotted woodpecker hammers out twenty blows a second. By contrast, the Magellanic woodpecker, which lives in the southern beech forests in Tierra del Fuego, the most southerly tip of South America, has a very simple call. It consists of only two blows, but the interval between them is important. Imitated correctly – and they are so close to one another that you will need two sticks to do so rather than one – and the spectacular red-headed bird is likely to appear very quickly and beat an angry response on a tree close by, if not on the top of the one you are using yourself.

Several other birds create mechanical sounds, even though none has such a highly evolved and specialised instrument as the woodpecker’s beak. The palm cockatoo in Australia improvises. It breaks off a stick and holding it in its beak, beats it against a hollow tree. Others use their feathers – the African flappet lark by clapping its wings together as it falls from high in the sky down to within 6 metres of the ground; the lyre-tailed honeyguide by diving so that air rushes through slots in its wings to produce a whistle; the European snipe by ‘drumming’, pitching headlong downwards with its tail fanned and the outermost feather on each side sticking out independently so that the wind rushes past and produces this distinctive noise.

Of course the instrument that most birds use to proclaim their identity is their voice. The distance their calls travel and the degree to which they become distorted is greatly affected, like any other sounds, by the character of the environment. Trees impede them and reduce the distance that they will travel. The leaves, particularly if they are hard and shiny like so many of those in a tropical rainforest, reflect the sound back and forth, so that a complex call becomes slurred and confused. Accordingly, birds that live in the canopy of a dense rainforest use calls that are usually very simple in structure, repeated over and over again in case distortions have prevented accurate reception, and are very often piercingly loud. The bare-throated bellbird lives in the South American forest. It is little bigger than a jay but it has a voice of the most extraordinary penetration. It perches in the very top of the trees so that its calls are subject to the minimum of interference and will travel farther, and produces a simple two-note call that it repeats over and over again throughout the day. The consequences can be maddening to a traveller as he trudges wearily through the forest, dripping with sweat, and moves from the territory of one calling bellbird to another. Those out of sympathy with birds sometimes call the bellbird the fever bird.

Low-pitched sounds travel better than higher ones through such congested environments. The small size of the bellbird prevents it from producing deep sounds, but the great bittern has the stature to do so. It lives in European reed beds and produces a deep, resonant booming noise which probably gives it the birds’ long-distance record for sound transmission. It can be heard up to 5 kilometres away across the fenlands.

In one or two celebrated cases, the call is the only way by which, at a distance, two species can be distinguished from one another. Two small warblers visit English woodlands during the summer. They have a faint yellowish stripe across the top of the eye and an equally pale flush of yellow on their white breast. If you look very hard, you may be able to detect that one of them has paler legs than the other. But as soon as they sing, the difference between them becomes plain. One produces a stream of liquid musical notes falling in a melodious cascade. That is the willow warbler. The other, the one with darker almost black legs, has a repetitious two-note song, one that is simple enough to convert into words and use as its name – chiffchaff. There is another pair of look-alikes in the New World that are less famous but equally difficult to separate except vocally – the alder flycatcher and the willow flycatcher. The first lives in the more northerly part of the continent, the second in the south and west. Their ranges overlap where the northern forests give way to the grasslands of the prairies and there the birdwatcher’s only hope of distinguishing between the two is by their calls which the field guides transcribe as ‘way-bee-o’ in the first case and ‘fitz-bew’ in the second.

Like visual signals, songs proclaim not only a bird’s species but its identity as an individual. Robins living in an English woodland stay in the same territory all the year round and both male and female vocalise. A male – and to a lesser extent a female – makes a regular circuit of his territory and sings from specific perches. When he stops, he will cock his head and listen. Usually his song stimulates an answering call from across his frontier in a neighbouring territory. He knows all his neighbours individually and recognises them by their song. If the response is one he recognises, then he will continue on his round unperturbed. But if you play to him a recording of the song of another male from another wood, he is likely to react in a very different manner. He repeats his song with greater vehemence and fluffs out his red breast aggressively, preparing himself for a territorial dispute with the stranger. If there really is an intruder, perhaps a young male who is trying to establish a territory between two existing ones, or one attempting to take over a territory made vacant by death, then there may be a vocal duel during which the two contestants size up one another. Other things being equal, a sitting tenant will manage to hold his ground. If the newcomer is merely taking over a recently vacated territory, then the duel will serve to get the two neighbours acquainted before they settle down to accepting the new situation. Only if the established bird is weakly or elderly will the newcomer be likely to establish a completely new territory alongside for himself.

A bird’s vocal repertory is partly inherited and partly learned from its parents. That is easily shown experimentally. Chaffinch chicks reared in silence make sounds that are just recognisable as simple impoverished versions of a typical chaffinch call. It is only if they can hear the sounds their parents make that they will vocalise in the way that other adult chaffinches do. Individual birds, however, each have their own individual songs. Since these influence the songs of their young, it follows that over many generations a local population of chaffinches, or any other songbird, is likely to develop its own characteristic accent. And so they do. Chaffinches in the north of England sing songs that are recognisably different to the expert ear from those in the south, and blackbirds descended from those that were taken to Australia during the nineteenth century to gladden the ears of European settlers with sounds of home, now have very distinct Australian accents.

Saddlebacks are among the most loquacious and melodious of New Zealand’s native singing birds. They are the size of starlings, black with a chestnut patch across the back and the upper part of the wings so that when they sit with their wings folded, the patch forms a continuous and conspicuous saddle. The males have small orange-coloured wattles dangling from the base of the beak. Local groups do not merely develop their own accent, they introduce so many new phrases and motifs that even a small group develops its own dialect. Each member of a group occupies a territory in the forest and holds it throughout the year, defending it from intruders by singing from regular perches strung out along its frontier. As he does so, his nearest neighbour will respond to the challenge by singing in response, as a robin does.

Young male saddlebacks, when they fledge, wander unobtrusively through the forest, keeping low in the trees and often feeding on the ground so as not to challenge any of the established territory holders. As they wander, so they come to recognise all the different dialects being sung for miles around. Usually they spend most of their time some distance from where they hatched in areas where their parent’s dialect is not used. They are on the lookout for widows.

If a male vanishes, the young bachelors detect his absence extraordinarily swiftly and start to sing. One was observed to do so within ten minutes of the owner’s disappearance. But the song a bachelor uses is not that sung by his father. Instead, he uses the dialect of the area he happens to be in, for this is the version that the widowed female has become accustomed to and which she prefers. If she accepts him, he will take over the territory. As a result of this regular behaviour, inbreeding among saddlebacks is kept to a minimum.

In those cases where a pair of birds hold a territory jointly and the year round, the two may share the task of defending it with song. The ‘to-whit to-whoo’ call of a tawny owl is one of the characteristic sounds of the English countryside at night, the more noticeable in winter since that is a time when many of the summer visitors have left and many of the residents have fallen silent. The call, with those of the cuckoo and the chiffchaff, is one of the few that is simple enough to be accurately conveyed by written words. But it is more complex than it may seem. It is produced not by a single bird but by a pair. The female is responsible for the first section – ‘to-whit’. Then the male, often sitting quite far away, will call ‘to-whoo’, his timing being so perfect that the two calls together form a single statement.

Many birds which live together for several years use this method of keeping their relationship firm. African barbets sing their duets sitting close beside one another on a branch. Their performance is timed to within a fraction of a second so that when one bird stops, the other takes up the song with the greatest precision. You might think that such mutual understanding would take a great deal of practice, but should a male from such a partnership be absent, even temporarily, a young male from the family will take his place and perform a similar version of the song, which the pair will very quickly synchronise.

There is a distinction to be made between a call and a song, even though in some circumstances one may grade into another. Calls, which include the seet and other alarm signals, are usually short, simple and uttered by both males and females. They are not learned but genetically ingrained. Songs, on the other hand, are usually much longer, more complex and produced only by males. Although male turkeys and cockerels sing their own celebrated arias, song is largely restricted to passerine birds, that great section of the bird kingdom that contains all the small perching birds.

In the more temperate parts of the world, in Europe and North America, male passerines give their most glorious performances in spring, joining together in a great chorus at dawn. The woodlands and the hedgerows echo and throb to the songs of their avian inhabitants. So many different species contribute that it can be difficult to disentangle one from the other. This leads one to wonder why so many should be singing at the same time. Would it not be more effective if the choristers spaced their songs by singing at different times during the day? Perhaps it is that dawn suits them all particularly well as a time for singing for then, at first light, it is still too dark to go hunting for insects, and still too cold for the insects themselves to be moving around and making themselves easily visible and catchable. Maybe it is because there is seldom much wind at dawn in summer to distort the calls and diminish their range.

It is also the case that sound travels particularly well in the early morning. The rising sun may warm the air before it reaches the land, so that a layer of cold air many feet deep forms between the ground and the warming skies above. One of the characteristics of sound waves is that, rather than cross the boundary between cold and warm air, they tend to be deflected by it. Sound waves, instead of escaping upwards into the sky, are bent downwards, and funnelled along this invisible aerial conduit. The phenomenon was investigated on the savannahs of Africa, where it was found that the low-frequency calls of elephants travel five times farther at dawn than later in the day. It also occurs in northern woodlands, and even though the effect there is not so marked, it still causes the songs of birds to travel particularly far at this time of day.

But the primary reason that males sing so lustily and splendidly at this season is certain. These songs contain the most important message of their lives. They are directed to females and they say: come be my mate.