Everyone joins the fun as “Sesame Street” turns 40 years old. Big Bird, voiced by Carroll Spinney, is one of the few who’s been there from the beginning: Nov. 10, 1969.

PBS

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!”

Indeed, Big Bird — that towering, yellow-feathered six-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 (Ch. 2, 10 a.m.; Ch. 36, 7 a.m.), 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!”

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 4,187, 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.”

As part of that evolution, the program is 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!”

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!’ ”

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.

They and so many others pioneered a strategy for channeling television to help underprivileged youngsters. Cradled by a nonprofit organization (now called Sesame Workshop), the mission continues, its mandate expanded to reach middle-income kids, too.

Just as in the formulation of the show’s original game plan, research continues to play a major role.

“That is the model that ‘Sesame Street’ has always been based on: The education and research department works hand-in-hand with producers,” says Rosemarie Truglio, who heads up Sesame Workshop’s research effort.

Last season, “Sesame Street” averaged more than 5 million viewers each week, and beyond that, logged 135 million impressions through media sources other than PBS between January and September.

And the show goes on. “Sesame Street” is currently midway through production of its 41st season, and one recent afternoon was shooting a scene on a rare rainy day. In Studio J, the diminutive Muppets Elmo and Rosita are having a problem sharing an umbrella with Big Bird.

“You can’t fit under the umbrella if I’m holding it,” Rosita worries.

“Oh, sure I can,” says Big Bird. “I’ll just make myself short.” And down Spinney sinks into a Big Bird crouch. Good knees!

With no sign of slowing down, Spinney says he aims to keep at it as Big Bird and Oscar.

“I still have the job, and I have contracts for the future in hand,” he says with a smile, “and I’m delighted.”

After 40 years and counting (plus spelling and other explorations), on “Sesame Street” everything’s A-OK.