Meeting Abstract
Wing trills are pulsed sounds produced by modified wing feathers during flight. We investigated the kinematic basis for the ~9 kHz trill of male Allen’s Hummingbird (Selasphorus sasin). The wingtip velocity hypothesis posits that trill production is modulated by the airspeed of the wingtip, whereas the wing rotation hypothesis posits that trill production is a function of wing rotation kinematics. To test these hypotheses, we flew six Allen’s Hummingbirds in an acoustic (open jet) wind tunnel at flight speeds of 0, 3, 6, 9, 12 and 14 m s-1, and recorded their flight with ‘acoustic camera’ below, behind, and lateral to the flying bird. The acoustic cameras were a phased array of 40 microphones that used beamforming to paint sound sources onto a camera image. Loudness exhibited a U-shaped relationship with flight speed in all three camera positions. The sound field was loudest perpendicular to the stroke plane. The wing rotation hypothesis was supported by two lines of evidence. The trill is produced during supination, and the trill was up to 20 dB louder during maneuvers than it was during steady state flight in the wind tunnel, across all airspeeds. Support of the wing rotation hypothesis implies that Allen’s Hummingbird may have some voluntary control over trill production, meaning they can partially modulate it to be louder or quieter depending on social context. Moreover, the trill may not be correlated with aerodynamic force production, and thus may not signal flight performance during courtship displays produced for females.
