Meeting Abstract
Broadbills in the genus Smithornis produce a loud breeet, a klaxon-like song that has been hypothesized to be non-vocal. The sound is only produced during a distinctive flight display, in which the male flies in a tight circle, returning to his point of origin. Although most birds that produce non-vocal communication sounds also have feathers distinctively modified for sound production, Smithornis broadbills do not. We investigated the mechanism of sound production of Rufous-sided Broadbill (S. rufolateralis) and African Broadbill (S. capensis). Synchronized high-speed video and sound recordings of displays demonstrated that pulses of sound were produced during the downstroke, that the wingtip speed reached approximately 15.7 m s-1, and that during downstroke, subtle gaps sometimes appear between the outer primaries feathers P6-P10 (P10 is outermost). Tests of a whole spread wing in a wind tunnel at speeds above 15.8 m s-1 demonstrated that at specific orientations, P6 and P7 both may flutter and produce sound. Tests on individual feathers P5 – P10 from males each species reveal that all of these feathers may produce sound via aeroelastic flutter, but that P6 and P7 produce the loudest sounds most similar to the wing song, and at the lowest airspeeds. Field manipulations of P6 and P7 provided consistent results with changes in the timbre of the sound, and specifically a reduction in the tonal quality. Altogether these results demonstrate that P6 and P7 are the sound source. Smithornis have evolved reduced syringeal complexity as the primitive vocal song was replaced by a functionally equivalent wing song.
