In 1957, a former marine biologist called Rachel Carson set about alerting the world to the perils of chemical pollution. She did it by writing a powerful book whose title, Silent Spring, evoked a barren future without bird song.
The founder of the modern environmental movement (Rachel Carson's book spurred President John F Kennedy to restrict the use of insecticides) knew what she was doing when she named her work. For it seems that we humans rely on nature's musical soundtrack to reassure us that all is right with the world. Why else would hearsay have it that birds never sing in the vicinity of Auschwitz? (Actually, they do.) And why else would an American television network have begun playing recorded bird song during its golf coverage some years back? (They were caught out when sharp-eared ornithologists identified several warblers that had clearly strayed way off course). Whether we actively listen to it or register it only when it stops, bird song means a great deal to us. But what does it mean to the creatures that produce it?
Those who study animal communication divide the vocalisations of birds into two distinct categories: calls and song. Calls, which are usually a pattern of simple sounds, allow birds to communicate with each other and with the rest of the world. They can be divided into a number of different categories, most of which are self-explanatory. They include alarm calls of various sorts, aggressive calls, territorial defence calls, flight calls, nest calls, flocking calls, feeding calls and pleasure calls.
Not all birds use every type of call, and many species have more than one call for each category. Most seem to have five to 15 distinct calls that are recognisable to ornithologists.
While nearly all birds use calls, true singing is a speciality of the order Passeriformes, or perching birds, which make up around half the world's bird population. Chiefly, too, it is the males that do the singing, either as part of their mating routine to attract and court a female, and perhaps even to stimulate her, or as a way of warning off rival males.
Some females do sing, and in the tropics pairs may be heard duetting, probably to reinforce the bond between them. But otherwise bird song, for all its appeal to poets down the ages, is simply a matter of boasting males shouting their beaks off.
Except, of course, that there's really nothing simple about it. Instead of a voice box capable of producing one note at a time, songbirds have a two-part song-box called a syrinx. This arrangement of vibrating membranes inside a bony framework sits deep within a bird's chest at the point where the trachea divides into two bronchi.
With a syrinx, a bird can sing two different notes at the same time, and even sing a duet with itself. Thrushes can produce a rising note with one side and a falling note with the other. In some birds, the windpipe itself is elongated and coiled within the breast bone - an arrangement which gives added resonance.
While many songbirds seem to sing for minutes on end without catching their breath, they actually take many shallow breaths - up to 300 per minute in the case of the canaries. The song of the wren contains 740 different notes per minute and can be heard more than 500 metres away.
Some birds put as much energy into singing as flying, and the yellowhammer repeats its "little bit of bread and no cheese" phrase more than 3,000 times a day.
Some bird song uses scales that match those of human music, while their songs often display familiar structures, such as the sonata form, which incorporates just the right amount of repetition and variation to be pleasing to our ears.
Unlike simple calls, which are genetically hardwired and can be uttered even before hatching, true bird song involves learning from adult tutors - usually fathers - in much the same way that human children learn to speak.
This learning often occurs in two phases. During the first phase young birds must listen to adult song if they are to develop normally, although at this stage they do not sing themselves.
The following spring they begin practising, listening to their own efforts and attempting to match the memory of what they heard from their tutor. It is sometimes possible to hear the lower-pitched, quieter notes of a young male practising his song in early spring, and evidence from brain scans suggest that some young birds practise their singing silently while sleeping.
Experiments have shown that in most songbird species males are born with a relatively crude melodic template, and chaffinches will develop a recognisable chaffinch song even without tutoring. But where much learning is involved, individual birds develop different songs of varying complexity. What's more, different populations of birds develop recognisable local dialects in the same way as people do.
Only when bird song is seen in the context of mating does this learning process, and the variations it produces, begin to make sense. Many male songbirds are much more vocal when they are looking for a mate than when they have found one. Reed and marsh warblers sing less complicated songs, and sing less often, once they are half of a couple, and sedge warblers stop singing altogether as soon as they settle down.
Significantly, female warblers have been shown to favour males with more complicated songs, and this holds true for most songbirds.
The repertoire of the male sedge warbler is extraordinary in terms of both quantity and quality. An individual male may have 50 or so musical elements at his disposal, and by constantly varying the order in which he arranges them, he may never repeat the same sequence during his lifetime.
By mimicking the calls of other species, some birds increase their repertoire further. Many cagebirds are renowned for this, but starlings and some warblers also excel at imitation. Starlings in the Shetland Islands frequently mimic sheep, while others in urban areas reproduce the sound of passing buses, mobile phones, car alarms and, in one case, an entire cricket match.
Scientists have distinguished the calls of more than 200 other bird species being copied by marsh warblers, while some migratory species imitate bird song from other continents, thereby telling females where they spend the winter.
Scientists at Duke University in North Carolina, have suggested that female birds carefully analyse a male's song quality and choose a mate accordingly, since the song effectively tells the story of the singer's life, how well he was reared and his likely physical condition.
Typically, birds go through a period of rapid growth early in their lives, and many suffer poor nutrition or parasite attack when the regions of their brain that deal with song learning are developing. That's why a discerning female will know instinctively that a good song signifies an effective mate, since the singer came through that critical fledgling phase relatively unscathed.
While some birds stop singing once they have mated, others continue all year round. In summer, male robins sing elaborate songs to attract a partner, but in winter males and females sing simple songs, which suggests that, for these non-migratory birds at least, maintaining a territory is as important as pairing.
In most birds, the mere presence of a singing male is enough to maintain a territory, and in this way singing avoids the need for physical confrontation. By singing, it seems likely that birds such as skylarks are also signalling to birds of prey that they are in tip-top condition and will therefore be hard to catch, since only the fittest birds can afford to sing while being chased.
By singing as it soars, the skylark is also maximising the distance that its song will travel. Sound carries when delivered from a height, which is why male thrushes and nightingales (whatever poets may imply to the contrary, those lyrical nightingales are invariably males) can generally be seen broadcasting their elaborate melodies from the treetops.
But the skylark inhabits open grassland where such vantage points are few and far between, so it soars high into the air to deliver its glorious warbling song, before hovering and plummeting almost vertically.
Undoubtedly, the best time to hear bird song is at dawn. Birds the world over, be they in English woodland or tropical rainforest, show the greatest amount of singing activity at this time, and as day breaks around the globe, so the great wall of sound, which is the dawn chorus, happens with it.
Sound travels well at dawn, because the air is still and there is little other noise. Songs broadcast at dawn can be 20 times as effective as those broadcast later in the day (although night-time is even better, as the nightingale knows).
Hunting and foraging are difficult at dawn, when the light is still dim and flying insects have not yet taken to the air. And, as always, there is the mating consideration. By bursting into song at dawn, when energy reserves are low, males are demonstrating their fitness for breeding. And what better time for a spot of courting, since the most effective mating takes place in the hour before a female lays her eggs, and many females lay in the morning.
There is much still to be learned about the dawn chorus. Yet even as scientists get to grips with this extraordinary daily phenomenon, it is already fading out in many parts of the British Isles. Birds that once made up the backbone of the morning choir are rapidly dwindling in numbers as they struggle to find enough food in areas dominated by intensive agriculture.
The Royal Society for the Protection of Birds (RSPB) has warned that loss of habitat has had a devastating effect on many songbirds in the past 30 years, with the skylark population down by 52 per cent, the corn bunting by 84 per cent and the tree sparrow by an alarming 87 per cent.
Meanwhile, 100 million small birds are thought to be killed each year by predators such as sparrowhawks.
Half a century on, and despite countless warnings by the environmentalists who followed in her footsteps, it seems that Rachel Carson's spectre of a silent spring still haunts us.