The story goes that Bob Dylan was in the studio a few years ago with a top record producer, and while the singer warmed up on a tune, the producer heard a turn of phrase he especially liked. The producer persuaded a reluctant member of Dylan's band to ask the singer to do it the same way for the record.

Dylan blew up at the messenger: "Don't you know by now I never sing anything the same way twice?"

Musicians have been covering the singer's tunes for almost 40 years — even Duke Ellington, Marlene Dietrich and Spike Jones cut "Blowin' in the Wind" — and are still at it. The new A Nod to Bob: An Artists' Tribute to Bob Dylan on His 60th Birthday features a brace of folkies, and jazz clarinetist Michael Moore is preparing his all-Dylan CD, Jewels and Binoculars. But Bob remains his own best and most ruthless interpreter.

For Dylan, who turns 60 on Thursday, those iconic old songs are mutable objects, to be remade every time he performs them. Compare multiple (early vs. late, studio vs. live, acoustic vs. electric) versions of, say, "Like a Rolling Stone" (RealAudio excerpt), "Tangled up in Blue" or "Most Likely You Go Your Way and I'll Go Mine." From one to another, he may vary the melody, phrasing, feel and even lyrics so much that a song's identity can blur before your ears.

It's a daredevil routine, singing like Dylan. Rather like Jimmie Rodgers, a country-music progenitor as steeped in African-American music as Dylan is, he may leisurely stretch out the words in the first part of a line, then dare himself to squeeze a clown-car full of syllables into the back half. He might even throw in a few unnecessary words to give himself that much more to inject, as in "Visions of Johnanna": "And the country music station plays soft/ But there's nothing, really nothing to turn off."

Meanwhile, from year to year he might alter the quality of his voice itself; remember how the Woody Guthrie mumble of the early protest songs gave way to the gleeful heckling of Blonde on Blonde to his Nashville Skyline soothing croon. And those transformations are but first steps toward his Gabby Hayes sandpaper voice on '90s discs World Gone Wrong (where he became one of the weathered old bluesmen the album pays tribute to) and Time out of Mind.

The premium he places on improvised melody and a changing timbre recalls a great jazz soloist — for example saxophonist Coleman Hawkins, who recast a melody every time he played it and altered his signature sound several times over his long career.

As deeply informed by African-American blues and gospel music as Dylan is, in many ways he seems remote from jazz. The closest he ever came to the idiom was the jazz-and-poetry send-up "If Dogs Run Free," on 1970's New Morning. But the backing band on his first electric single, 1962's near-rockabilly "Mixed up Confusion," was mostly jazz musicians; Dylan once claimed to have done sessions with Don Cherry and Billy Higgins from Ornette Coleman's free-jazz quartet; and the cover of Billie Holiday's Lady in Satin is in plain view on the cover of Bringing It All Back Home. That's a clue: You might hear Holiday, in her final, cracked-voiced "Lady in Satin" phase, as a model for artful phrasing that transcends the limitations of the singer's pipes and touches you all the more for that reason.

At certain times, potent ideas are in the air, and minds sensitive to cultural weather may follow parallel courses unawares. Had you access to Columbia's New York studios in 1966, you might have witnessed one of the label's stars improvising arrangements on the spot, coaxing a take out of the musicians when they've barely had a look at the material. A performance would jell as the tape rolled, something so electrifying it hardly matters the ending fell apart: Dylan, cutting "She's Your Lover Now" — or Miles Davis, recording Miles Smiles.

Bob Dylan cooks his lyrics the same way sometimes, throwing them together just before the tape rolls, as on much of Blonde on Blonde. He'd written "Tangled up in Blue" months before recording the Blood on the Tracks version, but by one account, he came back from a vending-machine break during the session with one newly minted verse. But even where a text is set, his halting/fluid delivery can make the words sound like they're tumbling from his brain for the first time.

One of many reasons to celebrate Dylan at 60 is that he's still out on the road defying pop's conventional wisdom of getting out there and reproducing your hits as closely as possible. Any recent Dylan set list confirms he likes his '60s classics as much as the rest of us do, but any Dylan concert affirms that for him, performing oldies has nothing to do with nostalgia and everything to do with endless reinvention. That takes guts, smarts, and mental and physical energy — qualities he's never lacked, not even on the cusp of 60. Way to go. Happy birthday.

For more on Dylan, be sure to check out "Bob Dylan At 60: Weiland, Stipe, Wyclef Tip Their Hats" and "Bob Dylan, Ten Essential Albums."