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 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!” Maybe that's because the self-renewing “Sesame Street” is forever young. 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 eight 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.” 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. 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. With no sign of slowing down, Spinney says he aims to keep at it as Big Bird and Oscar.