Producing song: the vocal apparatus.

In order to achieve the goal of understanding the neurobiology of birdsong, it is necessary to understand the peripheral mechanisms by which song is produced. This paper reviews recent advances in the understanding of syringeal and respiratory motor control and how birds utilize these systems to create their species-typical sounds. Songbirds have a relatively homogeneous duplex vocal organ in which sound is generated by oscillation of a pair of thickened labia on either side of the syrinx. Multiple pairs of syringeal muscles provide flexible, independent control of sound frequency and amplitude, and each side of the syrinx exhibits a degree of acoustic specialization. This is in contrast to many non-songbirds, including vocal learners such as parrots, which have fewer syringeal muscles and use syringeal membranes to generate sound. In doves, at least, these membranes generate a harmonic signal in which the fundamental frequency is regulated by respiratory pressure in the air sac surrounding the syrinx and the overtones are filtered out by the vocal tract. The songs of adult songbirds are generally accompanied by precisely coordinated respiratory and syringeal motor patterns that, despite their relative stereotypy, are modulated in real time by somatosensory feedback. Comparative studies indicate songbirds have evolved species-specific motor patterns that utilize the two sides of the syrinx in specific ways and enhance the particular acoustic effects characterizing the species song. A vocal mimic tutored with heterospecific song uses the same motor pattern as the tutor species when he accurately copies the song, suggesting that physical or physiological constraints on sound production have had a prominent role in the evolution of species-specific motor patterns. An understanding of the relationship between the central processing and peripheral performance of song motor programs is essential for an understanding of the development, function, and evolution of these complex vocal signals.