Smithornis broadbills produce a loud klaxon-like song during an elliptical flight display. Naturalists have hypothesised that the birds produce the sound mechanically with their outer primary wing feathers. While most birds that make non-vocal sounds have distinctly modified feathers for sound production, such modifications appear absent in Smithornis. We investigated the mechanism of sound production in Smithornis broadbills using a combination of synchronised high-speed video and sound recordings of displays, wind tunnel experiments on individual feathers and a spread wing, and field manipulations of flight feathers. Synchronised video and sound recordings demonstrated that sound pulses were produced during the downstroke, with subtle gaps appearing between primaries feathers. Tests of a spread wing in a wind tunnel demonstrated that at a specific orientation, primary feathers P6 and P7 flutter and produce sound.  Wind tunnel tests on individual outer primary feathers from males of each species reveal that while they can all produce sound via aeroelastic flutter, P6 and P7 produce the loudest sounds that are consistent with the frequency of wing song, at airspeeds consistent with flight. In support of their role in tonal sound production, field manipulations of P6, P7, as well as P8 changed the timbre of the wing song, reducing its tonality. Smithornis broadbills have also evolved reduced syringeal complexity, suggesting that the functional wing song replaced vocal song in these birds.