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Aeroelastic Flutter Produces Hummingbird Feather Songs

Science  09 Sep 2011:
Vol. 333, Issue 6048, pp. 1430-1433
DOI: 10.1126/science.1205222

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  1. Fig. 1

    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.

  2. Fig. 2

    Evolution of tail feather sound production in bee hummingbirds [phylogeny from (5)]. Outlines traced from tail feathers tested in this study are shown; colored numbers indicate rectrix identity and the mode we elicited in the wind tunnel (colors are as in Fig. 1H); the asterisk indicates feathers that may not make sound in wild birds (table S1). Lineage color indicates inferred mode of vibration (black indicates ambiguous), and the black numbers indicate which tail feather(s) produce sound.

  3. Fig. 3

    Sympathetic vibrations increase the loudness of the tail-feather display sounds of a male (A) Anna’s hummingbird (C. anna). (B) Courtship dive sound of intact (left) and missing (right) R4. Bdive and Cdive, two acoustic elements of the dive sound. Cdive of wild birds is present but not as loud after manipulation (arrow) (6). (C and D) Lab experiments with R5 and R4. (C) R4 placed ~1 mm behind (not touching) R5 increases loudness by 12 dB relative to R5 alone (n = 8 feather pairs; P < 0.001, Tukey test), whereas a flat plate in the same position does not increase loudness (P > 0.05). (D) SLDV shows that R5 vibration amplitude is similar for all arrangements. When R4 is placed behind R5, R4 vibrates at a comparable velocity to R5, whereas a flat plate does not. See movie S2.

  4. Fig. 4

    Heterodyne interactions in tail feathers R4 and R3 of (A) a male Allen’s hummingbird (S. sasin). (B) Courtship dive sounds of an intact S. sasin (left), a bird missing R4 (center), and a bird missing R3 (right). Blue arrows indicate a 2-kHz sound produced by R3 and second to fifth harmonics (f1). Red arrows indicate a 7- to 9-kHz tone produced by R4 (f2). Purple arrows indicate heterodyne interaction frequencies of f2 ± f1 (left). Heterodyne frequencies disappear with removal of either rectrix (center and right). (C) Vibration frequency spectra from SLDV of isolated R3 (top), R4 (bottom), and R3 + R4 (middle) in the wind tunnel; R3 + R4 treatment replicates heterodyne interaction (U = 22.8 m s−1). Heterodyne frequencies (purple arrows) produced in the wind tunnel include the integer harmonics of f1, (f2 – 2 f1, f2 + 2 f1, f2 + 3 f1) in addition to f2 ± f1 recorded in the wild (B).