SUMMARYBasic kinematic and detailed physical mechanisms of avian, non-vocal sound production are both unknown. Here, for the first time, field-generated high-speed video recordings and acoustic analyses are used to test numerous competing hypotheses of the kinematics underlying sonations, or non-vocal communicative sounds, produced by two genera of Pipridae, Manacus and Pipra (Aves). Eleven behaviorally and acoustically distinct sonations are characterized, five of which fall into a specific acoustic class of relatively loud, brief, broad-frequency sound pulses, or snaps. The hypothesis that one kinematic mechanism of snap production is used within and between birds in general, and manakins specifically, is rejected. Instead, it is verified that three of four competing hypotheses of the kinematic mechanisms used for producing snaps, namely: (1) above-the-back wing-against-wing claps, (2)wing-against-body claps and (3) wing-into-air flicks, are employed between these two clades, and a fourth mechanism, (4) wing-against-tail feather interactions, is discovered. The kinematic mechanisms used to produce snaps are invariable within each identified sonation, despite the fact that a diversity of kinematic mechanisms are used among sonations. The other six sonations described are produced by kinematic mechanisms distinct from those used to create snaps, but are difficult to distinguish from each other and from the kinematics of flight. These results provide the first detailed kinematic information on mechanisms of sonation in birds in general, and the Pipridae specifically. Further, these results provide the first evidence that acoustically similar avian sonations, such as brief, broad frequency snaps, can be produced by diverse kinematic means, both among and within species. The use of high-speed video recordings in the field in a comparative manner documents the diversity of kinematic mechanisms used to sonate, and uncovers a hidden, sexually selected radiation of behavioral and communicative diversity in the Pipridae.
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.
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.
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