Flights of fear: a mechanical wing whistle sounds the alarm in a flocking bird

SUMMARYFeathers can produce sound by fluttering in airflow. This flutter is hypothesized to be aeroelastic, arising from the coupling of aerodynamic forces to one or more of the feather's intrinsic structural resonance frequencies. We investigated how mode of flutter varied among a sample of hummingbird tail feathers tested in a wind tunnel. Feather vibration was measured directly at 100 points across the surface of the feather with a scanning laser Doppler vibrometer (SLDV), as a function of airspeed, U air . Most feathers exhibited multiple discrete modes of flutter, which we classified into types including tip, trailing vane and torsional modes. Vibratory behavior within a given mode was usually stable, but changes in independent variables such as airspeed or orientation sometimes caused feathers to abruptly 'jump' from one mode to another. We measured structural resonance frequencies and mode shapes directly by measuring the free response of 64 feathers stimulated with a shaker and recorded with the SLDV. As predicted by the aeroelastic flutter hypothesis, the mode shape (spatial distribution) of flutter corresponded to a bending or torsional structural resonance frequency of the feather. However, the match between structural resonance mode and flutter mode was better for tip or torsional mode shapes, and poorer for trailing vane modes. Often, the 3rd bending structural harmonic matched the expressed mode of flutter, rather than the fundamental. We conclude that flutter occurs when airflow excites one or more structural resonance frequencies of a feather, most akin to a vibrating violin string. (SLDV). This device shines a coherent laser on the surface of the feather and uses the Doppler shift of reflected light to calculate the instantaneous velocity in the direction parallel to the laser, across a series of points. SLDV does not require a reflectent to be applied to the surface, unlike regular LDV (Bostwick et al., 2010), allowing its use on objects as small as hummingbird feathers. We measured the feathers in the wind tunnel (see Clark et al., 2013), and also measured the resonance characteristics of a series of feathers stimulated across a range of frequencies by a shaker, an experimental paradigm in which the operating deflection shape approximates the normal modes. We also describe a related aspect of flutter with evolutionary significance: feathers may exhibit a dozen or more modes of flutter, and can abruptly switch or 'jump' from one mode of flutter to another. MATERIALS AND METHODSThe wind tunnel, general methods and definitions for this experiment are as described in the companion paper (Clark et al., 2013). The feathers tested were from wild adult males of 14 species of 'bee' hummingbird (McGuire et al., 2009), obtained in the course of our fieldwork on each of these species under the relevant collecting and import permits. The exact feathers used and the species from which they were obtained are tabulated in the supplementary online material of our previous publications (Clark et al., 2011a;Cla...

Section: Implications For Behaviormentioning

confidence: 97%

Section: Implications For Behaviormentioning

confidence: 99%

Structural resonance and mode of flutter of hummingbird tail feathers

Clark

Elias

Girard

et al. 2013

“…For the four species from the other three clades, we posit that the flight sound production requires an interaction with neighboring feathers that our experimental setup failed to reproduce (Table S2). Moreover, crested pigeon actually produces two tones, one on the downstroke, the other on the upstroke (Hingee and Magrath, 2009). Our wind tunnel experiments on P8 replicated only the higher sounds (arrow in Fig.…”

Section: Flutter and The Flight Sounds Of Birdsmentioning

confidence: 99%

“…Typically, they are produced during the breeding season only, such as during courtship displays that are directed to a female, or in replacement of vocal song that is broadcast into the environment, indicating sexual function. In the other two cases, crested pigeon and golden-bellied starfrontlet (Coeligena bonapartei), both sexes produce the sound and the inferred function is non-sexual communication (Hingee and Magrath, 2009). Hummingbirds, nightjars and tyrant flycatchers all forage in flight, which may make them more likely to produce incidental sounds that are subject to subsequent sexual selection, in much the same way that foraging on insects in wood must have contributed to the evolution of drumming signals in woodpeckers (Picidae).…”

Section: Evolution Of Non-vocal Communication In Birdsmentioning

confidence: 99%

Aeroelastic flutter of feathers, flight, and the evolution of nonvocal communication in birds

Clark

Prum

2015

Tonal, non-vocal sounds are widespread in both ordinary bird flight and communication displays. We hypothesized these sounds are attributable to an aerodynamic mechanism intrinsic to flight feathers: aeroelastic flutter. Individual wing and tail feathers from 35 taxa (from 13 families) that produce tonal flight sounds were tested in a wind tunnel. In the wind tunnel, all of these feathers could flutter and generate tonal sound, suggesting that the capacity to flutter is intrinsic to flight feathers. This result implies that the aerodynamic mechanism of aeroelastic flutter is potentially widespread in flight of birds. However, the sounds these feathers produced in the wind tunnel replicated the actual flight sounds of only 15 of the 35 taxa. Of the 20 negative results, we hypothesize that 10 are false negatives, as the acoustic form of the flight sound suggests flutter is a likely acoustic mechanism. For the 10 other taxa, we propose our negative wind tunnel results are correct, and these species do not make sounds via flutter. These sounds appear to constitute one or more mechanism(s) we call 'wing whirring', the physical acoustics of which remain unknown. Our results document that the production of non-vocal communication sounds by aeroelastic flutter of flight feathers is widespread in birds. Across all birds, most evolutionary origins of wing-and tail-generated communication sounds are attributable to three mechanisms: flutter, percussion and wing whirring. Other mechanisms of sound production, such as turbulence-induced whooshes, have evolved into communication sounds only rarely, despite their intrinsic ubiquity in ordinary flight.

“…Although the link between particular morphologies and their sound-producing abilities is tenuous (Clark and Prum, 2015), ornithologists have nevertheless hypothesized a direct connection between unique shape and sound in many species (Bahr, 1907;Craig, 1984;Hingee and Magrath, 2009;Johnston, 1960;Wetmore, 1926), whereas in many others, sounds are produced in the complete absence of obvious feather morphologies (Clark, 2008;Clark and Prum, 2015;Coleman, 2008;Lebret, 1958).…”

Section: Introductionmentioning

confidence: 99%

Specialized primary feathers produce tonal sounds during flight in rock pigeons (Columba livia)

Niese

Tobalske

2016

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±50 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 (53.9-60.3 rad s ), but did not produce tones at average downstroke velocity (31.8 rad s −1 ), whereas P9 and P1 feathers never produced tones. P10 feathers had significantly lower coefficients of resultant aerodynamic force (C R ) 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 C R . These mechanistic results suggest that the tonal sounds produced by P10 feathers are not incidental and may function in communication.