"Birthday" begins with an intro drum fill, then moves directly into a blues progression in A (in the form of a guitar riff doubled by the bass) with McCartney singing at the top of his chest voice with Lennon on a lower harmony. After this section, a drum break lasting eight measures brings the song into the middle section, which rests entirely on the dominant. A repeat of the blues progression/guitar riff instrumental section, augmented by piano brings the song into a bridge before returning to a repeat of the first vocal section, this time with the piano accompaniment.