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. 2008 Aug 23;4(4):341-4.
doi: 10.1098/rsbl.2008.0252.

Fluttering Wing Feathers Produce the Flight Sounds of Male Streamertail Hummingbirds

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Free PMC article

Fluttering Wing Feathers Produce the Flight Sounds of Male Streamertail Hummingbirds

Christopher James Clark. Biol Lett. .
Free PMC article

Abstract

Sounds produced continuously during flight potentially play important roles in avian communication, but the mechanisms underlying these sounds have received little attention. Adult male Red-billed Streamertail hummingbirds (Trochilus polytmus) bear elongated tail streamers and produce a distinctive 'whirring' flight sound, whereas subadult males and females do not. The production of this sound, which is a pulsed tone with a mean frequency of 858 Hz, has been attributed to these distinctive tail streamers. However, tail-less streamertails can still produce the flight sound. Three lines of evidence implicate the wings instead. First, it is pulsed in synchrony with the 29 Hz wingbeat frequency. Second, a high-speed video showed that primary feather eight (P8) bends during each downstroke, creating a gap between P8 and primary feather nine (P9). Manipulating either P8 or P9 reduced the production of the flight sound. Third, laboratory experiments indicated that both P8 and P9 can produce tones over a range of 700-900 Hz. The wings therefore produce the distinctive flight sound, enabled via subtle morphological changes to the structure of P8 and P9.

Figures

Figure 1
Figure 1
(a) Adult male streamertail's wing during the mid-downstroke, traced from a high-speed video. (b) Subadult male streamertail's wing during the mid-downstroke, traced from a high-speed video. (c) Images from the downstroke (i) and upstroke (ii) of a male streamertail's wingbeat. Consecutive images are 1 ms apart. P8, P9 and P10 bend during wing pronation (frames 10–13), resulting in a gap between P8 and P9 during the downstroke, which is absent in the upstroke.
Figure 2
Figure 2
(a) Spectrogram of the flight sound of a caged adult male Red-billed Streamertail, over seven wingbeats. Most sound below 650 Hz is background noise. (b) Spectrogram of the sound produced by an isolated P9 in a jet of air, at the same frequency as the flight sound. (c) Consecutive images of P9 projecting into a stream of air (0.1 ms apart), exhibiting the fluttering of the tip. (d) Flutter and sound frequencies correspond perfectly (regression, slope=1.04, r2=0.97). (e) Feather frequency as a function of air velocity in a wind tunnel. Black, P9 (n=2); grey, P8 (n=2); the three lines are coincident.

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