Right under my feet is air made of bricks That pulls me down turns me weak for you. I find myself repeating like a broken tune, And I’m forever excusing your intentions, And I give in to my pretendings, Which forgive you each time, Without me knowing they melt my heart to stone.
While Lennon never revealed the inspiration behind the song, it is believed to refer to the rivalry between The Beatles and The Rolling Stones. Although the two groups were friends, Lennon saw the Stones as Beatles copyists, and the titular ‘bird’ may have been Marianne Faithfull. (source)