Characterization of sound and vibration of single feathers in airflow. (A) Feather flutter velocity (Vfeather) against airspeed of S. flammula R2. Range I: feather is immobile; range II: minute vibrations without detectable sound; range III: vibration amplitude increases sharply above a critical velocity (U*) and sound is detected. (B) Sound pressure level (SPL)–airspeed relationship of C. anna R5 above U* (n = 3 feathers). In range IIIa, sound pressure level rises with airspeed (slope: 2.9 dB m s−1), whereas in IIIb it declines. Udive, speed that live birds reach in a dive (21) [see (11)]. (C) SPL of 31 feathers from 14 species of hummingbird as a function of Vfeather. Faint lines are regressions for individual feathers. Color indicates the fundamental frequency of sound (in kilohertz): red, <0.95, orange, 0.96 to 1.55, yellow, 1.56 to 2.41, green, 2.42 to 4.8, blue, 4.9 to 7.2, purple, >7.3. (D and E) Sound frequency power spectra for (D) R4 of Chaetocercus mulsant and (E) R2 of S. flammula in the wind tunnel (black) against background sound of the tunnel (gray; Uair = 22 m s−1). (D) Second harmonic is dominant (arrow). (E) Over 30 integer harmonics are present (select harmonics are numbered). (F) Power spectra of Calliphlox mitchellii R5 in the wind tunnel (11.6 m s−1); sound recording is above, and SLDV of feather flutter is below. All sounds were present as vibrations in the feather (as in arrows). (G) Fundamental frequency and mode of vibration of 31 different hummingbird feathers. Frequency tends to increase with airspeed, but a negative slope is possible (e.g., arrow = C. mitchellii R4). (H) Four modes of vibration: a transverse mode of the trailing vane (blue), whole-feather bending mode (green), and torsional-transverse mode of the tip (red), which in Stellula calliope was sometimes a purely torsional mode (yellow). See movie S1.