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. 2008 Apr 22;275(1637):955-62.
doi: 10.1098/rspb.2007.1619.

The Anna's Hummingbird Chirps With Its Tail: A New Mechanism of Sonation in Birds

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Free PMC article

The Anna's Hummingbird Chirps With Its Tail: A New Mechanism of Sonation in Birds

Christopher James Clark et al. Proc Biol Sci. .
Free PMC article

Abstract

A diverse array of birds apparently make mechanical sounds (called sonations) with their feathers. Few studies have established that these sounds are non-vocal, and the mechanics of how these sounds are produced remains poorly studied. The loud, high-frequency chirp emitted by a male Anna's hummingbird (Calypte anna) during his display dive is a debated example. Production of the sound was originally attributed to the tail, but a more recent study argued that the sound is vocal. Here, we use high-speed video of diving birds, experimental manipulations on wild birds and laboratory experiments on individual feathers to show that the dive sound is made by tail feathers. High-speed video shows that fluttering of the trailing vane of the outermost tail feathers produces the sound. The mechanism is not a whistle, and we propose a flag model to explain the feather's fluttering and accompanying sound. The flag hypothesis predicts that subtle changes in feather shape will tune the frequency of sound produced by feathers. Many kinds of birds are reported to create aerodynamic sounds with their wings or tail, and this model may explain a wide diversity of non-vocal sounds produced by birds.

Figures

Figure 1
Figure 1
A composite image of a male Anna's hummingbird diving to a female, created using high-speed video. Consecutive images are 0.01 s apart. During the dive, males spread their tails for 0.06 s (n=5 videos) and simultaneously produce a loud sound (element Cdive in figure 2a) for 0.05 s (n=53 sound recordings). Videos of two dives are available in the electronic supplementary material.
Figure 2
Figure 2
Spectrograms (showing frequency versus time) and waveforms (showing amplitude versus time) of sounds. Greyscale, relative sound energy, in which black indicates high sound energy, and white indicates low sound energy. All five sounds are from different recording and therefore their waveforms cannot be directly compared due to different recording conditions. (a) Dive sound from an unmanipulated male, (b) song from a male hovering close to the microphone, (c) dive sound of a male with no trailing vane (T.V.), (d) dive sound of a male with no leading vane (L.V.) and (e) is the sound of an R5 feather producing sound in a wind tunnel at an airspeed of 26.2 m s−1. A, B and C are the sound elements of the display dive or song, as defined by Baptista & Matsui (1979). With (a) unmanipulated birds and (d) birds missing the L.V. of R5, (b) Cdive is present with a fundamental frequency of 4 kHz, and is louder than the song. With birds missing the T.V. of R5 (c), Cdive is missing, and instead the bird produces a broad-spectrum ‘whoosh’ sound (indicated by the arrow). When R5 is placed in a wind tunnel (e), it produces a sound with a fundamental frequency of 4 kHz that matches a normal Cdive (initiation of sound production indicated by the arrow). The sound recordings used to create these spectrograms are available in the electronic supplementary material.
Figure 3
Figure 3
A description of the fluttering of the trailing vane (T.V.) of R5. (a) A male Anna's hummingbird with tail feathers (rectrices) labelled as R1 through R5. (b) Frequency of the T.V. flutter and that of sound (f) are highly correlated (linear regression, unmanipulated feathers: slope=0.996, r2=0.94, p<0.0001, n=18); circles, feathers missing L.E.; diamonds, unmanipulated feathers. The correlation between sound production and the T.V. flutter is unaffected by removing the leading vane (L.V.) of the feather (linear regression, feathers lacking L.V.: slope=1.00, r2=0.98, p<0.0001, n=8). (c) Photos of an unmanipulated R5, R5 with the L.V. removed and R5 with the T.V. removed. (d) An outline of the R5 showing the base, tip, shaft, L.V. and T.V.. (e) Four end-on views showing the up-and-down fluttering motion of the T.V. (in grey), relative to the direction of airflow. The tip of the feather is not shown in this perspective. Two videos showing the feather fluttering are available in the electronic supplementary material.
Figure 4
Figure 4
Frequency of sound produced by six male R5 feathers over a range of air velocities in a wind tunnel. Frequency (f, kHz) is positively correlated with velocity (v, m s−1): f=0.056×v+2.6 (linear regression, slope: p<0.001, intercept: p<0.001, n=34 samples). The minimum velocity at which each feather would produce sound is circled. (a) The predicted sound frequency for the aeolian whistle hypothesis, according to equation (1.1), and assuming St=0.2 and d=4 mm. (b) The birds travel an average of 23.1 m s−1 at the bottom of the dive, which is greater than the average critical velocity of 19.6 m s−1.

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