John Lennon (in 1972) - "Both of us (wrote this song.)"
John Lennon (in 1980) - "'Birthday' was written in the studio. Just made up on the spot. I think Paul wanted to write a song like 'Happy Birthday Baby,' the old fifties hit. But it was sort of made up in the studio. It was a piece of garbage."
Paul McCartney (in 1994) - "We thought, 'Why not make something up?' So we got a riff going and arranged it around this riff. So that is 50-50 John and me, made up on the spot and recorded all in the same evening."
