ROLE OF SYRINGEAL MUSCLES IN CONTROLLING THE PHONOLOGY OF BIRD SONG

1. The contribution of syringeal muscles to controlling the phonology of song was studied by recording bilateral airflow, subsyringeal air s ac pressure, electromyograms (EMGs) of six syringeal muscles, and voca l output in spontaneously singing brown thrashers (Toxostoma rufum). 2 . EMG activity in musculus syringealis ventralis (vS), the largest syr ingeal muscle, increases exponentially with the fundamental frequency of the ipsilaterally generated sound and closely parallels frequency m odulation. 3. The EMG activity of other syringeal muscles is also posi tively correlated with sound frequency, but the amplitude of their EMG s changes only a small amount compared with variation in the amplitude of their EMGs correlated with changing syringeal resistance. The elev ated activity in all syringeal muscles during high-frequency sounds ma y reflect an increased need for structural stability during the strong contractions of the largest syringeal muscle (vS). 4. Several syringe al mechanisms are used to generate amplitude modulation (AM). The most common of these involves modulating the rate of syringeal airflow, th rough activity by adductor (m. syringealis dorsalis and m. tracheobron chialis dorsalis) and abductor (m. tracheobronchialis ventralis) muscl es, which change syringeal resistance, switch sound production from on e side of the syrinx to the other, or produce rapid oscillatory flow c hanges. Variation in the phase relationship between AM and EMG bursts during oscillatory airflow suggests complex biomechanical interaction between antagonistic muscles. 5. AM can also arise from acoustic inter actions of two independently generated sounds (beat notes) including c ross talk signals between the two syringeal halves. In this latter mec hanism, sound generated on one side radiates slightly out of phase wit h the source from the contralateral side, resulting in lateralized AM generation.