 "Birthday" contains elements of funk-pop, synthpop, and disco house. The song is backed by a "busy but buoyant" instrumentation featuring a "deeply rhythmic impulse". Randall Roberts of the Los Angeles Times felt "Birthday" was a musically updated version of disco, achieved by replacing "cheesy strings" with "jerky breaks and synth washes". Its "joyful" lyrics present the commemoration of her partner's birthday as a metaphor for sexual intercourse. The bridge section of "Birthday", which sees Perry singing "Let me get you in your birthday suit / It's time to bring out the big balloons", was found by Ben Rattliff to resemble French duo Daft Punk's works.
