Mark Lennihan/AP File Photo

Big Bird tapes an episode of “Sesame Street” in New York in this April 10, 2008, file photo. Big Bird, who has appeared on “Sesame Street” since the beginning, is turning 40 but remains a 6-year-old.

New York — Big Bird is leaving Sesame Street!

That’s what he decides on the “Sesame Street” season opener. A rapping real-estate agent pitches him on migrating to a new habitat (“habitat,” the episode’s “Word on the Street”). After sizing up a beach and a swamp for his new habitat, Big Bird chooses a rain forest.

But then he comes to his senses with a musical number.

“Sesame Street is my habitat!” he sings. “Sesame Street is my home!”

40th anniversary

Indeed, Big Bird — that towering, yellow-feathered 6-year-old — has been calling Sesame Street home for four decades, ever since the show premiered on Nov. 10, 1969.

Now, as it marks its 40th anniversary on Tuesday on PBS (check local listings), he remains an essential member of the flock.

He is still brought to life by Caroll Spinney, who also plays trash-can denizen Oscar the Grouch.

Hand-picked by Muppet-meister Jim Henson, Spinney was 35 when “Sesame Street” began. He turns 76 the day after Christmas. In his dressing room at Kaufman Astoria Studios in Queens, where the show is taped, he was pondering an existential question not long ago.

“If you didn’t know when you were born, how old would you think you are?” he mused. “I can apply that to Sesame Street’s longevity: It seems like years, but I’d NEVER guess 40!”

Ever evolving

Maybe that’s because the self-renewing “Sesame Street” is forever young.

A realm of sunny days where everything’s A-OK, the series starts its new season with episode 4187, which features the letter H and, naturally, the number 40. With it and the 25 new hours that follow, “Sesame Street” will continue to explore its chosen habitat — and experiment with how it does the job.

“We think of every year as experimental,” says Carol-Lynn Parente, the show’s executive producer, “and this new season is just part of that continuing evolution.

“It was always designed to emulate the TV-viewing environment,” she notes. “Back in 1969, it had a magazine format that emulated what was then on television.”

To meet expectations of its audience 40 years later, each new episode has been reformatted as an hourlong block composed of modular programming parts.

Murray Monster, a lively orange Muppet, hosts each episode’s four segments. These include Abby Cadabby in the new “Abby’s Flying Fairy School,” which marks the first time a “Sesame Street” character has been transformed into CGI animation.

The program is also kicking off “My World Is Green & Growing,” a two-year science initiative designed to increase positive attitudes toward nature and the environment.

With that in mind, First Lady Michelle Obama visits Sesame Street to plant vegetable seeds with Elmo and several young flesh-and-blood gardeners.

Then Big Bird steps up.

“Wow, did I just hear right?” he says. “The first lady eats seeds? I love seeds!”

Not exactly, says Mrs. Obama, but “I do eat what grows from these seeds.”

Big Bird absorbs this information with the thoughtfulness of any curious 6-year-old, which is what he is.

But that wasn’t how he was originally hatched.

“For the first few shows, he was just a silly, goofy guy,” recalls Spinney. “Then one day I said, ‘Big Bird should be a kid. Forget the fact that he’s 8 feet tall.’ And real children accepted him.”

Forever young

Indeed, Big Bird fast became a signature figure on “Sesame Street.” Early on, he appeared solo on the cover of Time magazine, which dubbed his show “TV’s Gift to Children.”

But even if he has never grown older, he has never stood still. Spinney continues to refine the performance.

“I study tapes to see how to get new expressions out of his face,” Spinney says. “I see something good that I did, and I take note to make sure I do it again.”

As the silver-haired, nattily bearded Spinney speaks with a reporter in his dressing room, Big Bird’s lower half is hanging in the closet: fuzzy orange fleece pants with platter-size feet, into which Spinney climbs almost like pulling on waders.

Then, on the set, with an assistant’s help, he encases himself in the feathered yellow body and head before each scene is taped. A tiny television monitor harnessed to his chest lets him glimpse the outside world. He recites Big Bird’s lines as his upraised right hand supports the head and animates its mouth and eyes.

“The head weighs about 4 1/2 pounds,” reports Spinney. “One fellow says, ‘That’s no big deal, I can do that.’ And I said, ‘All right. Let’s hold our hand up for five minutes. You don’t even have to put anything in it.’ And in a couple of minutes, he said, ‘My God!’

“There’s a rule with puppetry: If it’s comfortable, you’re probably doing it wrong.” Spinney laughs. “My arm has gotten much stronger than it was when I started. I’m really great at painting ceilings.”

Spinney is one of but a few charter members of the show still on the Street. Among them: Bob McGrath (Bob) and Loretta Long (Susan), as well as camera man Frankie Biondo.