Birds sing to mark out territory or attract a mate, right? Perhaps not. According to musician and philosopher David Rothenberg, they sing for fun. PETE KELLEY listens closely and finds out more ahead of the Rothenberg's date in Norwich this Tuesday.

It was 2000, in Pittsburgh and a musician visiting an aviary did something odd. He picked up his clarinet and began playing to the birds. Then something odder happened. A white-crested laughing thrush responded - joining in and singing a 'duet' with him.

“I had no idea a bird could interact so spontaneously with a human musician,” said David Rothenberg - who will be speaking at Borders bookshop in Chapelfield this Tuesday. According to his account, the bird not only copied him, but altered its song repeatedly, 'challenging' Rothenberg to keep up with its variations.

The five-year quest that followed this experience resulted in a fascinating new book, Why Birds Sing.

Rothenberg, a professor of philosophy and music at New Jersey Institute of Technology, now believes they sing because - like us - they enjoy it. They are creative.

“What I hope people will get from this book,” he said this week, “is that they will start to listen to familiar sounds in a new way. Instead of - perhaps - just wanting to identify birds and 'ticking them off', I want them to really listen.”

The idea that birds are being creative could sound sentimental.

But the fascination of this book is the detailed work which backs it up - ranging through anthropology, evolutionary theory and biology to myth and poetry.

Science has, of course, long favoured the less romantic view that birds sing for good Darwinian reasons: to mark territory and to attract mates. Rothenberg doesn't discount that, but says what they do goes much further.

So here we read about the tiny marsh warbler which journeys from Belgium to Zambia and back - learning new songs along the way. Why?

Then there is the Albert's lyrebird of the Australian rainforest - with a song so complex it takes it five years to learn. It is then performed along with a special complex dance routine.

Why?

It's a mating call. But, if that's all it is, it seems a bit excessive.

To Rothenberg, who is also a composer and jazz clarinettist with six CDs out under his own name, birdsong also shows organisation.

As a musician, he says: “I notice how many of the qualities that define human music are here in the songs of birds: rhythm, structure, pattern, repetition and variation.”

A website accompanying the book (www.whybirdssing.com) includes slowed-down audio clips to illustrate this. He likens the results to jazz. Usually, he says, “it all goes by too fast to grasp. Only with slowdown technology can we make sense of all these streaming motifs.”

And although Rothenberg doesn't plan to bring any Australian rainforest birds to Norwich, he promises some live music during his talk.

“I will be playing alongside tapes of birdsong,” he said, “to try to give people an idea of what I am talking about.”

The book is light in tone, with a pacey style. But the philosopher in Rothenberg uses his theme to raise some thought-provoking questions. For example:

If birdsong is 'unnecessarily' beautiful, perhaps humans are not the only creatures to have an aesthetic sense. He points out that Darwin himself favoured this view.

If we want to build bridges to other species, perhaps we should play together rather than seeking a common 'language'.

The day we think we fully understand nature, something will be lost. For Rothenberg, birdsong is mysterious, and should remain so. The answer that birds sing just because “God made it so” or because “it's just a survival mechanism” are equally unsatisfying, he thinks - precisely because they seem to settle the matter, cutting off our continuing curiosity.

He quotes ecologist Paul Shephard as saying that: “Anything we say about [birds] is a human abstraction from the truth.”

And what is our relationship to a natural world in which birds sound so much more sure of themselves?

Birdsong, he says, however often repeated always sounds fresh, and always sounds “right” in a way human music never can.

“Humanity will never 'get it right' because we are the species that is always revising how we live and what will work for us...human life is not like that of birds. We are never as sure as they.”

Rothenberg also makes a clear link to environmental concerns. In the book he writes of his hope that by looking more closely at birdsong “we may widen the realm of art, just as we expand ethics to include the environment, and honestly find a way for our species to care about the rest of this fragile world”.

Speaking this week, he said: “What I am trying to show is that to understand the world fully, we may need both artistic and scientific approaches. Neither may be enough in itself.

“I am going to continue to play music with birds. But my next book will be about whalesong ...something we have only been able to appreciate and study in the last few decades because, until then, we didn't have the technology to listen.”

David Rothenberg will be speaking at Borders at 6.30pm on Tuesday, November 29, reservations on 01603 66453. He will be introduced by naturalist and writer Richard Mabey.

A CD of the same name - Why Birds Sing - has been released by Terra Nova Music. It includes 'inter-species jams' plus music inspired by birdsong.

Why Birds Sing, by David Rothenberg, is published by Allen Lane, part of the Penguin Group, at £17.99.