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.
