Courting Bird Sings with Stridulating Wing Feathers

Abstract

In birds and other vertebrates, most acoustic signals are produced pneumatically by moving air through a vocal apparatus. Here we describe a unique mechanism used to produce a tonal acoustic signal in vertebrates. Video recordings of the courtship displays of male Club-winged Manakins, Machaeropterus deliciosus, reveal that males produce sustained harmonic tones through interactions among oscillating secondary wing feathers. This mechanism of sound production shows morphological and mechanistic convergence with arthropod stridulation. Intersexual selection for production of a nonvocal courtship song has led to major morphological, functional, and likely physiological modifications in the wing of this flying bird.

Female mate choice based upon male motor performance

“…In the New World manakins (Pipridae), males of many species produce non-vocal courtship sounds by extraordinary movements of their wings (Bostwick and Prum, 2003;Bostwick and Prum, 2005). Several of these sounds are produced as birds slap the dorsal surfaces of their wings together at very high repetition rates, close to the contraction speed limits of vertebrate muscle.…”

Resonating feathers produce courtship song

“…The authors proposed that the repeated oscillations of the modified feathers allowed for stridulationmultiplication of the frequency of motion-generating events-induced when each of the seven ridges on the surface of the modified feathers' shafts are rubbed twice (once as the feathers converge medially, and once as they diverge laterally) for each cycle of wing oscillation. In order for the stridulation mechanism to function as proposed and produce the seamless tone, however, the authors predicted that the modified feathers should also exhibit distinctive resonant properties at or near the fundamental frequency of the sound produced in nature, such that each percussive event excites resonance in the thickened shafts, and that this resonance is sustained by the stridulatory impulses (Bostwick & Prum 2005).…”

“…Immediately following this collision, the wings are shivered laterally and medially, pulling these feathers just millimetres apart, only to be adducted approximately 9 ms later to produce another collision. The pair of feathers knock and rebound as the wing shivers, and the sonation tone is produced continuously throughout this process (see electronic supplementary material, movie S2, Bostwick & Prum 2005).…”

“…These two syllables differ primarily in their duration, and but also in their peak frequencies. Measured from 10 tick-ting calls in one male, the peak frequency of the tick is 1.59 + 0.05 kHz, and the duration is 0.014 + 0.007 ms (n ¼ 17), while the peak frequency during the sustained ting is 1.49 + 0.01 kHz, and the duration is 0.335 + 0.015 ms (n ¼ 10; unpublished details from Bostwick & Prum 2005). Variation within and between individuals in this species is believed to be relatively low, with tings measured in two other individuals measured at 1.44 + 0.08 kHz and 1.48 + 0.04 kHz (n ¼ 20 for each of two individuals ;Bostwick 2000).…”

“…Courting male Club-winged Manakins, Machaeropterus deliciosus, are hypothesized to use a mechanism of sonation, or non-vocal sound production (sensu Bostwick & Prum 2003), unique to vertebrates: resonant stridulation of modified feathers to produce a briefly sustained, unwavering, tonal sound (Bostwick & Prum 2005). The resonant stridulation hypothesis was inferred from: (i) description of the two modified feathers which show enlarged and ridged shafts; (ii) analysis of high-speed videos which show the males actively drive approximately 107 Hz medio-lateral oscillations of the modified feathers, and thereby repeatedly knock the modified feather shafts together across their midline; and (iii) analyses of the acoustic structure of the sound which show it is a sustained (approx.…”