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!"

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 tomorrow on Channel 12, 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.

Handpicked 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 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," said 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 noted. "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 hour-long 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 said. "The first lady eats seeds? I love seeds!"

Not exactly, said Mrs. Obama, but "I do eat what grows from these seeds."