The neuromuscular control of birdsong

Birdsong requires complex learned motor skills involving the coordination o f respiratory, vocal organ and craniomandibular muscle groups. Recent studi es have added to our understanding of how these vocal subsystems function a nd interact during song production. The respiratory rhythm determines the t emporal pattern of song. Sound is produced during expiration and each sylla ble is typically followed by a small inspiration, except at the highest syl lable repetition rates when a pattern of pulsatile expiration is used. Both expiration and inspiration are active processes. The oscine vocal organ, t he syrinx, contains two separate sound sources at the cranial end of each b ronchus, each with independent motor control. Dorsal syringeal muscles regu late the timing of phonation by adducting the sound-generating labia into t he air stream. Ventral syringeal muscles have an important role in determin ing the fundamental frequency of the sound. Different species use the two s ides of their vocal organ in different ways to achieve the particular acous tic properties of their song. Reversible paralysis of the vocal organ durin g song learning in young birds reveals that motor practice is particularly important in late plastic song around the time of song crystallization in o rder for normal adult song to develop. Even in adult crystallized song, exp iratory muscles use sensory feedback to make compensatory adjustments to pe rturbations of respiratory pressure. The stereotyped beak movements that ac company song appear to have a role in suppressing harmonics, particularly a t low frequencies.