You know how some lorries sing a little song when they reverse? Diddle-peep, diddle-parp! Diddle-peep, diddle-parp! I heard a bird singing that phrase this week. It was a song thrush, a bird that, like most musicians, is equally in love with repetition and innovation.

It’s not exactly mimicry. The song thrush doesn’t think that way: rather, each new chunk of song is an opportunity for reinterpretation. The artist in the song thrush doesn’t allow for slavish echoing: rather, it is a theme on which it can improvise, recompose, establish a variation, put on a personal stamp.

You, too, can hear the song thrush. You, too, can recognise its song, even if the only birdsong you have previously recognised is a cuckoo. All you have to do is to listen out for a bird that repeats itself and you have found your song thrush.

You don’t have to listen hard, because they turn up in most places where there are people and trees. A song thrush takes a phrase and then repeats it, sometimes only once, sometimes two or three times.

This, of course, is Browning’s thrush, the Oh-to-be-in-England thrush, the one that sings each song twice over, lest you think he never could recapture the first fine careless rapture. And if you hear a bird recapturing its first fine careless rapture, then beyond all doubt you are listening to a song thrush.

You can hear them not only in the open countryside or at the edge of a wood, but also in suburban gardens across the land, because song thrushes have always found human company extremely congenial. Song thrushes have enriched human settlements for as long as such things have existed. But here’s an odd thing: on the Continent song thrushes are shy woodland birds. It is only in this country (and New Zealand, where English people imported them) that song thrushes are so comfortable around people.

A song thrush will take all kinds of musical scraps as the basis for its song. You can still hear them incorporating notes from Trimphones, for the songs tend to be passed on from one generation to the next. They will do car alarms, or whistling milkmen. An aristocratic birder reported that the song thrushes on his estate used the call of the African whistling duck he kept on his lake. If you know birdsong you can amuse yourself in trying to track the song thrush’s sources. Nuthatch is a real favourite, also lapwing, redshank, great tit. They even have the temerity to throw in chunks of nightingale.

With some species of bird, the songs are hard-wired. They require practice and experience but not learning, and they have little variation. These singers are content to sing the same song again and again, and it works well enough for them. It establishes a territory, it sees off rivals, it calls in a mate. What more could any singer want from his song?

But some species are repertory singers. They learn. They accumulate different songs throughout their lives: they add different variations with every new season. The song thrush is a classic example, and a top thrush is reckoned to have more than 100 musical phrases in his vocabulary.

The theory is that the more variations a repertory singer possesses the sexier he is to females and the more intimidating he is to rival males. Scientists will tell you that this theory lacks incontrovertible experimental verification; but it makes intuitive good sense and it is backed up by observation. But does this scientific austerity not hint at something else?

Sex and territory are important enough. They are nothing else than life itself to a song thrush. And yet there is one nagging point that remains. Why is the song so beautiful? This is not a good scientific question, I admit, but the fact is that the song is more beautiful than it needs to be.

If you are talking only about biology it would be more economical to give each bird a voice that can sing diddle-parp, diddle-parp and nothing else. But, instead, the birds have created a soundscape of demented excess, of wild and — it seems — quite unnecessary beauty. We have the careless raptures of song thrush, the laid-back fluting of blackbird, the rich fruitiness of blackcap and the almost unbearable passion of nightingale. Why this, and not mere peeping and parping?

A couple of years ago I spent an afternoon with an American musician and philosopher, David Rothenberg, who plays his clarinet to birds (you can listen to him — and them — on www.whybirdssing.com). We can all be a bit territorial about our special subjects, and Rothenberg was given a rough time by scientists who couldn’t comprehend the questions he was asking. But I am not a scientist, and Rothenberg and I sat and listened to birds. I found his view of repertory singers as essentially creative beings perfectly easy to deal with — and if a song thrush makes music to make itself more sexy, then so does every guitar hero and every operatic tenor ever spawned.

This is the season of birdsong. The next six weeks will give us the very best that the birds can offer, the very best that this country can offer. So when you have a drink in the garden on the next fine evening, take a short moment to open your ears and listen. Are you listening to blind automata? Or are you listening to creative artists making music because — for all the biological imperative involved — they are in love with music itself?

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