On a recent sunny afternoon, David Rothenberg was trekking through the Foundry Cove Nature Preserve near his home in Cold Spring, N.Y., identifying birds.

''Wow, there's a yellow warbler,'' he said, ''and some red-bellied woodpeckers, and listen, listen up there to the right are some blue indigo buntings, four or five of them. We can't see them right now, but they have this really long, involved song.''

Dr. Rothenberg, a 43-year-old clarinetist, composer, philosopher, teacher and amateur ornithologist, has just published ''Why Birds Sing: A Journey Into the Mystery of Bird Song'' (Basic Books) about his adventures playing music to songbirds and with them.

He has also produced a CD of human and bird music as a companion piece to the book (Terra Nova Records). A cut of Dr. Rothenberg's duet with ''George,'' an Albert's lyrebird that resides in the Lamington National Park in Queensland, Australia, can be heard at nytimes.com/science.

Q. How did your book ''Why Birds Sing'' start for you?

A. In March of 2000, I was invited to the National Aviary in Pittsburgh to play my clarinet with the birds there. As I played, one bird -- a white-crested laughing thrush -- really responded to what I was doing. Could this bird really be interacting with me, I wondered?

Was he actually changing his song in relation to mine? He was!

That's not within the conventional wisdom of what birds can do. I'd always been told they had their set song and they just sang it. But with him, I played some notes and he'd join in. If my note changed, his did, too. Wow! I was jamming with a laughing thrush!

After that, I wanted more. And so I began this five-year journey, where I traveled the globe, trying to find the answer to a question that poets, philosophers and scientists had been asking for centuries: Why do birds sing? I thought that being a musician might give me a special line into it.

What I hoped to do is learn how music emerges from nature and how melodies can move from one species to another.

Q. Has bird song been a longtime interest of yours?

A. Sort of. I'm a professor of philosophy. I teach engineers nonquantifiable things at the New Jersey Institute of Technology. That's my day job. But I'm also a jazz musician.

When I was in high school in the 1970's, I was influenced by saxophonist Paul Winter's ''Common Ground'' album, which had his own compositions with whale and bird sounds mixed in. That got me interested in using music to learn more about the natural world.

I believe there is music in nature. Song and melody may very well have evolved before words and language.

Q. In all your travels, did you ever learn why birds sing?

A. I think they sing for the same reason people do: because they can. It's part of their essence. Birds sing because that's what they evolved to do. They must.

The science on this is inconclusive. Biologists generally say that birds sing to protect territory and attract mates. And this is true of many species, though not always. It's not always clear that the bird that sings better has more mating success.

According to a study by Peter Merritt, a mockingbird that had the best song had neither the most mating success nor the best territory. The Australian lyrebird, which has a really complex song, sings for hours when there are few females in the area to even hear him.

The thing about birds with extremely complex songs -- mockingbirds, nightingales, indigo buntings -- their songs are much more extensive than they need to be. If they didn't sing as elaborately, they'd still do O.K. with mating and defending territory. Most songbirds sing far more than necessary. They go on and on and on when they don't have to.

This is not to say that science is wrong. It's just we know a lot less about why birds sing than we think.

Q. So what do you think is going on in nature that makes the birds sing?

A. Charles Darwin in ''The Descent of Man,'' has two chapters on birds. Darwin says birds have a natural aesthetic sense. He says they appreciate beauty, which is why they have beautiful feathers and why they sing beautiful songs.

Stephen Jay Gould suggested that this was a spandrel, which is a term from architecture for ornaments that aren't really useful but are the result of fashion. He wondered if evolution didn't produce features like that.

Q. What do biologists have to say about the song theories of Darwin and Stephen Jay Gould?

A. Most contemporary biologists wouldn't go as far as Darwin. Aesthetics isn't something you can test. Science has to be more precise.

I was fascinated to read about some researchers, who were studying the clicking sounds made by sperm whales -- Moby-Dick. They couldn't figure out how the sounds fit together. So they called in an African drum master and he said: ''Oh, there's a kind of a guiding rhythm there, and they do seem to be listening to each other. I really hear a pattern.'' He heard something the scientists didn't.

Q. Did you learn anything about bird-song scientists as you did your research?

A. I learned they are as territorial as they birds they study. When you go among them, one of them will claim: ''I discovered this. This theory is mine!'' From what I noticed, they didn't seem to spend a lot of time reading each other's papers or trying to duplicate someone else's results. Of course, the same can be said for philosophers, and musicians.

Q. You have a track on the companion CD to ''Why Birds Sing,'' where you play your clarinet to the songs of the bird in Lamington National Park. How did you discover your co-star?

A. I went to Australia last year to play along with lyrebirds as part of the research for my book. The lyrebird, he's amazing. He's like a pheasant with a huge plumed tail and an extremely complex song structured like human music, with a beginning, middle and end.

This bird will sing all day long, flipping his huge tail right over his head. He dances to his own beat, and his song is made up of sounds picked up from other birds: snippets of calls and squawks and even the sound of wings flapping.

But lyrebirds tend to be shy. George is more tolerant of people than most. So I went into the forest, found him, heard him singing and began interjecting my clarinet sounds into his song. I tried to play along with him in a way that made musical sense. The experts told me he'd run away, but George stayed.

He kept singing because that's the strongest desire a lyrebird has: to complete his song. Interestingly, he responded to this foreign clarinet sound by changing his song ever so slightly.

And that's how we got that cut. When I went on a book tour for ''Why Birds Sing,'' that cut was played quite a bit on AM radio stations. Nobody said, ''Boy this is some weird music here.'' I felt I had broken through the species barrier. And even people seem to like it.

Photo: David Rothenberg playing his clarinet this summer. Birds sometimes change their tunes when he plays. (Photo by Librado Romero/The New York Times)