Resonating feathers produce courtship song

Abstract

Male Club-winged Manakins, Machaeropterus deliciosus (Aves: Pipridae), produce a sustained tonal sound with specialized wing feathers. The fundamental frequency of the sound produced in nature is approximately 1500 Hz and is hypothesized to result from excitation of resonance in the feathers' hypertrophied shafts. We used laser Doppler vibrometry to determine the resonant properties of male Club-winged Manakin's wing feathers, as well as those of two unspecialized manakin species. The modified wing feathers exhibit a response peak near 1500 Hz, and unusually high Q-values (a measure of resonant tuning) for biological objects (Q up to 27). The unmodified wing feathers of the Club-winged Manakin do not exhibit strong resonant properties when measured in isolation. However, when measured still attached to the modified feathers (nine feathers held adjacent by an intact ligament), they resonate together as a unit near 1500 Hz, and the wing produces a second harmonic of similar or greater amplitude than the fundamental. The feathers of the control species also exhibit resonant peaks around 1500 Hz, but these are significantly weaker, the wing does not resonate as a unit and no harmonics are produced. These results lend critical support to the resonant stridulation hypothesis of sound production in M. deliciosus.

Figure 1.

(a) Ventral view of secondary feathers of M. deliciosus (UMMZ 255 055): secondaries 1–9 ‘attached’ and labelled. Note enlarged rachi of modified sixth and seventh secondaries. (b) Dorsal surface of the fifth secondary, (c) anatomically ‘medial’ surface of the sixth secondary twisted to orient ventrally. (a) Scale bar, 1 cm.

Figure 2.

(a) Waveform, (b) spectrogram and (c) power spectrum of tick and ting of M. deliciosus showing sustained fundamental tone and harmonics. Power spectrum was calculated from segment of sound highlighted in (a,b) using Raven Pro 1.4 (www.birds.cornell.edu/raven).

Figure 3.

Transfer functions and phase spectra for multiple individual isolated secondary feathers 1 (green line) (unmodified, n = 1), 6 (red lines) and 7 (blue lines) (modified, n = 3) from M. deliciosus.

Figure 4.

Transfer functions for multiple points on rachi of isolated secondary feathers 1 (unmodified), 5, 6 and 7 (modified) from M. deliciosus UMMZ 255 054. (Set of graphs show one representative of three sixth and seventh secondaries, as averaging among feathers obscures structure of resonant tuning in individual feathers). Vibration responses of each individual feather (rows) was measured at four different locations along the shaft (columns) where the colour of the plot corresponds to the point on the feather (red being the most proximal, green being one-third length, blue being two-third length and black being the most distal). Dashed lines indicate the fundamental frequency of the ting sound produced during M. deliciosus sonation.

Figure 5.

Transfer functions (i) and phase spectra (ii) for attached secondaries 1–8 of (a) M. deliciosus, (b) P. fascicauda and (c), L. coronata. All species show multiple frequency peaks between 100 and 500 Hz, and their first higher, or fundamental, frequency peaks between 1500 and 1700 Hz. Machaeropterus deliciosus exhibits stronger peaks, and equally strong or stronger harmonics. Phase spectra in lower panels show M. deliciosus feathers oscillate more coherently (a(ii)) than controls (b(ii),c(ii)). Green lines, feather 5; red lines, feather 6; blue lines, feather 7.