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

Abstract

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.

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

“…Nearly all animals possess the ability to detect this sound (Budelmann, 1989(Budelmann, , 1992Fay, 2009;Horch, 1971), and many may go to great lengths to mask their own locomotion-induced sounds (Conner, 2014;Graham, 1934;Roche et al, 1999). Conversely, however, many species have also developed remarkable ways to amplify and otherwise modulate these motion-induced sounds for communication (Bostwick, 2006;Clark and Prum, 2015;Darwin, 1871).…”

“…This evolutionary link between incidental sounds and communicative signals is perhaps most easily explored in birds, in which motion-induced sounds associated with courtship behaviors can become the subject of novel female preferences, exaggerating them into complex signals (e.g. strut displays in greater sage grouse, wing-snapping displays in manakins; Clark and Prum, 2015;Prum, 1998). These non-vocal acoustic signals, or sonations, are common among birds, perhaps because of the inherently noisy nature of feathers, wings and flight (Clark and Prum, 2015;Fournier et al, 2013;Wei et al, 2013).…”

“…strut displays in greater sage grouse, wing-snapping displays in manakins; Clark and Prum, 2015;Prum, 1998). These non-vocal acoustic signals, or sonations, are common among birds, perhaps because of the inherently noisy nature of feathers, wings and flight (Clark and Prum, 2015;Fournier et al, 2013;Wei et al, 2013).…”

“…Incidental sounds produced during flight are ubiquitous among volant birds (even owls; Clark and Prum, 2015;Sarradj et al, 2011) and are, in part, due to the natural propensity of all stiff, light airfoils (e.g. feathers) to aeroelastically flutter under certain flow conditions (Clark and Prum, 2015;Clark et al, 2011Clark et al, , 2013a.…”

“…feathers) to aeroelastically flutter under certain flow conditions (Clark and Prum, 2015;Clark et al, 2011Clark et al, , 2013a. Aerodynamic energy input from flow over an airfoil excites one or more resonance frequencies within the airfoil.…”