Sound Production and Modification in Birds – Mechanisms, Methodology and Open Questions

Elaborate and diverse sounds are an important aspect of many bird behaviors, and these sounds are generated by sophisticated multiple motor systems regulating respiration, vocal organ and upper vocal tract structures. The avian vocal organ, the syrinx, is a unique sound generator among vertebrates, and its morphology varies substantially between different taxa. In this review an introduction to our current knowledge of the peripheral mechanisms of sound production and modification is presented in light of the methodologies that have been used to study various aspects of phonatory mechanisms. Limitations of these methods are also identified and areas for future study and needed information are discussed for each participating motor system. Respiratory control determines the coarse temporal aspects of vocalizations. Rapid switching between expiration and inspiration enables birds to take mini-breaths during inter-syllable intervals up to syllable repetition rates of approx. 30 Hz. At higher rates, pressure modulation of a sustained expiratory pulse still indicates detailed respiratory involvement in the fine control of sound production. Even during rapid sequences of expiration and inspiration, gas exchange is maintained, allowing birds to sing very long songs. Although syringeal morphology has been studied for centuries, the functional aspects of this morphological variation have only recently become subject of investigation to complement efforts focused on neural control of acoustic features. The interplay of morphology, biomechanics and neural control remains a fertile ground for future investigation of song production mechanisms and differences between avian taxa. The neuromuscular control of sound production is best understood in doves and oscine songbirds. Syringeal muscles contribute to the regulation of airflow and tension of the vibrating tissues (membranes or labia), but complex biomechanical interactions make complete understanding of the control of acoustic parameters difficult. For example, the control of sound frequency in oscines arises from a complex interplay of muscle action, physical parameters (flow and pressure gradients) and morphological specializations (extracellular matrix design of the labia). The presence of two independently controlled sound generators in some bird taxa also creates the potential for an enhanced vocal range and for complex acoustic interactions. Once sound is generated in the vocal organ, it is modified as it exits the bird through trachea and oropharyngeal spaces. This modification can be highly sophisticated, as birds can dynamically adjust resonances to track the fundamental frequency of rapidly modulated song syllables to generate tonal sounds or give rise to complex harmonic content with formant-like quality. At each motor level, many details remain to be discovered, and a thorough understanding of the peripheral mechanisms will be required for decoding the central motor program of song generation. In addition, the morphological variation in syrinx structure across different bird taxa provides a rich source for studying functional aspects of sound generation, but also for investigating evolutionary aspects of this unique and elaborate sound producing organ among vertebrates.

Keywords: Birds, Functional morphology, Muscular control, Peripheral mechanisms of sound production, Respiration, Sound modification, Syrinx, Upper vocal tract.