RESUMO
For centuries, naturalists have suggested that the tonal elements of pigeon wing sounds may be sonations (non-vocal acoustic signals) of alarm. However, spurious tonal sounds may be produced passively as a result of aeroelastic flutter in the flight feathers of almost all birds. Using mechanistic criteria emerging from recent work on sonations, we sought to: (1) identify characteristics of rock pigeon flight feathers that might be adapted for sound production rather than flight, and (2) provide evidence that this morphology is necessary for in vivo sound production and is sufficient to replicate in vivo sounds. Pigeons produce tonal sounds (700±50â Hz) during the latter two-thirds of each downstroke during take-off. These tones are produced when a small region of long, curved barbs on the inner vane of the outermost primary feather (P10) aeroelastically flutters. Tones were silenced in live birds when we experimentally increased the stiffness of this region to prevent flutter. Isolated P10 feathers were sufficient to reproduce in vivo sounds when spun at the peak angular velocity of downstroke (53.9-60.3â radâ s(-1)), but did not produce tones at average downstroke velocity (31.8â radâ s(-1)), whereas P9 and P1 feathers never produced tones. P10 feathers had significantly lower coefficients of resultant aerodynamic force (CR) when spun at peak angular velocity than at average angular velocity, revealing that production of tonal sounds incurs an aerodynamic cost. P9 and P1 feathers did not show this difference in CR These mechanistic results suggest that the tonal sounds produced by P10 feathers are not incidental and may function in communication.
Assuntos
Columbidae/fisiologia , Plumas/fisiologia , Voo Animal/fisiologia , Som , Animais , Fenômenos Biomecânicos , Plumas/anatomia & histologia , Feminino , Masculino , Gravação em VídeoRESUMO
The shape of remiges (primary and secondary feathers) is constrained and stereotyped by the demands of flight, but members of the subfamily of New World ground doves (Peristerinae) possess many atypical remex shapes with which they produce sonations of alarm. Within the genus Columbina specifically, the seventh primary feathers (P7) have elongated barbs that create a protrusion on the trailing vane which varies in size and shape between species. These feathers are hypothesized to have been coopted to produce communicative sounds (i.e., sonations) during flight, but the mechanism of this sound production is unknown. We tested the sound-producing capabilities of spread wing specimens from three species of ground doves (C. inca, C. passerina, and C. talpacoti) in a wind tunnel. High speed video and audio analyses indicated that all wings of adult birds produced buzzing sounds in the orientation and flow velocity of mid-upstroke. These buzzing sounds were produced as the protrusion of elongated barbs fluttered and collided with adjacent P6 feathers at a fundamental frequency of 200 and 400 Hz, respectively. Wings from juvenile C. inca produced significantly quieter buzzes and most (three of four individuals) lacked the elongated barbs that are present in adults. Buzzing sounds produced in the wind tunnel were similar to those produced by wild birds indicating that these P7 feathers have been coopted to produce acoustic signals (sonations) during flight. The shape and mechanism of sound production described here in Columbina appear to be unique among birds.