Hummingbird feather sounds are produced by aeroelastic flutter, not vortex-induced vibration

Abstract

SUMMARYMales in the 'bee' hummingbird clade produce distinctive, species-specific sounds with fluttering tail feathers during courtship displays. Flutter may be the result of vortex shedding or aeroelastic interactions. We investigated the underlying mechanics of flutter and sound production of a series of different feathers in a wind tunnel. All feathers tested were capable of fluttering at frequencies varying from 0.3 to 10kHz. At low airspeeds (U air ) feather flutter was highly damped, but at a threshold airspeed (U*) the feathers abruptly entered a limit-cycle vibration and produced sound. Loudness increased with airspeed in most but not all feathers. Reduced frequency of flutter varied by an order of magnitude, and declined with increasing U air in all feathers. This, along with the presence of strong harmonics, multiple modes of flutter and several other non-linear effects indicates that flutter is not simply a vortex-induced vibration, and that the accompanying sounds are not vortex whistles. Flutter is instead aeroelastic, in which structural (inertial/elastic) properties of the feather interact variably with aerodynamic forces, producing diverse acoustic results. Supplementary material available online at

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

“…The equipment and setup were the same as, and the protocol similar to, that described in Clark et al (2013b) and is repeated here briefly. The feathers were mounted by inserting an insect pin (small feathers) or dissecting pin (large feathers) into the calamus and anchored with a small amount of cyanoacrylate glue.…”

“…The other end of this pin was then inserted into a pin vise, which projected vertically on a sting down into the freestream of the tunnel, with the feather's long axis perpendicular to flow, as in fig. 2 of Clark et al (2013b). Because of the floor and ceiling boundary layers (Clark et al, 2013b), there was only approximately 20 cm of usable space within the working section.…”

“…2 of Clark et al (2013b). Because of the floor and ceiling boundary layers (Clark et al, 2013b), there was only approximately 20 cm of usable space within the working section. For feathers longer than 20 cm (from ducks and common raven), either the feather shaft was cut and only the distal portion was tested, or the sting was retracted into the roof of the tunnel so that the distal portion of the feather projected out of the boundary layer into the freestream.…”

“…The feather's orientation was then adjusted to find modes of flutter, and airspeed was increased, as needed. If a mode of flutter was found that was similar to the flight sound of the bird from which the feather came, we then collected data at constant orientation, over a range of airspeeds, as in Clark et al (2013b). If after testing 5-10 orientations/airspeeds, no matching mode of flutter was found, we returned to conditions that elicited the mode of flutter that produced the loudest sound and/or was the most stable over varying airspeeds, and obtained measurements over a range of speeds, at a constant orientation.…”

“…Previous results on hummingbird feathers suggested that all individual, isolated feathers can flutter under the right aerodynamic conditions (Clark et al, 2013b). Many of the inducible modes of flutter do not correspond to sounds produced by birds in flight and are thus spurious (Clark et al, 2013a).…”