Sean Lennon can still see the moment vividly. It was a day in 2006, in the early morning hours, and he and his friend Carrie Fisher were up late at Fisher’s home in Los Angeles. Lennon was working out some chords for a new song on Fisher’s piano, and out of nowhere, the writer sneaked up behind him, light-footed, poised like a cat. She began to sing, making up lyrics on the spot to Lennon’s melody as his hands moved across the keys: “Love comes on cat’s feet; it circles the corner. . .” Lennon recalled this moment in a phone call with Vanity Fair, after he had posted “Bird Song” to his Soundcloud account.

The lyrics continue : “It's all so wrong / To greet the dawn / The birds sing that awful song saying / ‘You don't belong here!’ ”

“It was so late that the birds started singing,” he said of that morning of songwriting with Fisher. “And you know that kind of guilty feeling when you’ve been up all night, and you wish it was earlier, but the sun’s coming up? You start to hear the birds . . . Usually you associate bird songs with something positive. But it sounded like the birds were reprimanding us.”

When Fisher died unexpectedly in late December of last year, Lennon said he wanted to do something to commemorate his friend and process his grief. Following a January memorial service that Fisher’s daughter, Billie Lourd, held for her mother and grandmother Debbie Reynolds, Lennon said he decided to record the old song. Willow Smith, who is featured on the track, agreed to help Lennon with the single.

“I didn’t want to mix it and master it and put it out as a record because I didn’t want to be exploitative in any way,” he said. “Really, it was like a personal way of mourning the loss, because she was one of the closest friends I’ve ever had . . . I was in a really sad place missing Carrie, and I just wanted to play the song.”

Fisher’s friendship had a profound effect on the 41-year-old Lennon. He met her through her stepson, Harper Simon (son of Paul Simon, to whom Fisher was married for just under a year in the 80s). Lennon and Fisher crossed paths at parties and events when Lennon was very young, but didn’t become close until he was in his 20s. Fisher, who famously opened her home to her friends, invited Lennon to stay in her guest house, where he spent many months at a time when he was in town. There, he said, they shared their most special moments, including that memorable night when “Bird Song” came to be.

Lennon, who is the only son of John Lennon and Yoko Ono, comes from strong musical genes. But, he says, Fisher was one of the most talented lyricists he’s ever known.

“She was like the comedic Nabokov of women,” he said of Fisher’s wit and wisdom. He noted that she also helped write one of Harper Simon’s songs, called “The Shine.” “The lyrics are just unbelievably gorgeous,” he said. “It makes me cry just thinking of them.”

When Lennon spoke of Fisher in a recent conversation, he recalled a mentor who held within her the capacity for deep and personal connection. She was someone who understood the careful balance of life as a child of Hollywood, always ready with a quip like this one, Lennon’s favorite: “Resentment is like swallowing poison and expecting the other person to die.” To Lennon, her influence is indelible.

“She grew up as Hollywood royalty, and I kind of grew up as rock ’n’ roll royalty,” he said. “And we definitely connected on that level, having parents who are just larger than life and trying to find your way and navigate through the minefield of innate celebrity—or being born into celebrity—and living under the sort of microscope of that publicly. She sort of helped me navigate that more than anyone in the world, I would say.”

“She was many things to me," he continued. “She was like a sister and like a best friend, but also like a mom and a teacher.”

Above all, he remembered someone who, no matter the time of night, whether in spoken word—or in song—always knew the right thing to say.