Big Bird is leaving Sesame Street!

That’s what he decides on Tuesday’s “Sesame Street” season opener. A rapping real-estate agent pitches him on migrating to a new habitat (“habitat” being 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 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 Tuesday on PBS, 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, N.Y., 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.

The series starts its new season with episode 4187, which features the letter H and, naturally, the number 40.

During 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, and this new season is just part of that continuing evolution,” says Carol-Lynn Parente, the show’s executive producer.

“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. 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.”

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.”

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 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.

In-house testing helps identify key curriculum goals, shape the show’s content and track its success.

Independent academic researchers have conducted more than 1,000 studies, making “Sesame Street” the most researched TV show in history.

One notable study reconnected with adolescents who had participated in “Sesame Street” research as preschoolers. It found that teens who watched the show at that early age had higher grades and spent more time reading for pleasure than other teens who had missed the show as children.

“We feel so passionately about getting ‘Sesame Street’ in the hands of as many kids as possible because we know it works,” says producer Parente.

These days, it’s not only available on PBS, but also on cable’s Sprout network, online and as video podcasts.

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.

One recent afternoon, a scene was being shot for an upcoming episode on a rare rainy day. 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 going 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.”