Motor dynamics of song production by mimic thrushes

Abstract

In brown thrashers (Toxostoma rufum) and grey catbirds (Dumetella carolinensis) neither side of the syrinx has a consistently dominant role in song production. During song, the two sides operate independently, but in close cooperation with each other and with the respiratory muscles which are capable of adjusting expiratory effort to maintain a constant rate of syringeal airflow despite sudden changes in syringeal resistance. Phonation is frequently switched from one side of the syrinx to the other, both between syllables and within a syllable. When both sides of the syrinx produce sound simultaneously, their respective contributions are seldom harmonically related. The resulting "two-voice" syllables sometimes contain difference tones with prominent sinusoidal amplitude modulation (AM). Rarely, both sides simultaneously produce the same sound. In general, however, the frequency range of sound contributed by the right syrinx is higher than that of the left syrinx. The right syrinx is also primarily responsible for producing a rapid cyclical amplitude modulation which is a characteristic feature of some syllables. This kind of AM is generated by either repetitive brief bursts of sound from the right side that modulate the amplitude of a continuous sound arising on the left side or cyclically opening the right syrinx, allowing unmodulated expiratory air to bypass the phonating left side.

The neuromuscular control of birdsong

“…The two sides receive di¡erent motor programmes, however, and often give rise to vocalizations containing harmonically unrelated frequency components originating simultaneously on separate sides of the syrinx. At other times phonation is switched from side to side (Suthers 1990;Suthers et al 1994Suthers et al , 1996b. Left^right switching of sound production is a prominent feature of the introductory note clusters in songs of the brown-headed cowbird (¢gure 6) where successive notes are produced on opposite sides of the syrinx (Allan & Suthers 1994).…”

Vocal tract articulation revisited: the case of the monk parakeet

“…This enables songbirds to sing with two voices simultaneously or to switch between the two sets of labia while singing, depending on the frequencies produced (Suthers, 1990;Suthers et al, 1994;Zollinger and Suthers, 2004). The greater complexity of the vocal organ of songbirds initially led to the hypothesis that acoustic variation predominantly arises at the sound source and that, in contrast to human speech, acoustic filtering by the vocal tract only plays a minor role in birdsong production (Greenewalt, 1968).…”

Bilateral coordination and the motor basis of female preference for sexual signals in canary song

“…By phonating sequentially on each side of the syrinx, instead of simultaneously as during two-voice syllables, domestic canaries may have evaded production constraints that could otherwise limit the difference between simultaneously generated left and right fundamental frequencies. In two-voice species, such as brown thrashers (Toxostoma rufum) and gray catbirds (Dumetella carolinensis), the difference between the left and right fundamental frequencies is usually substantially less than that between the fundamental of each side during unilateral phonation (Suthers et al, 1994). In the case of twovoice syllables the frequency difference between sides of the syrinx might be limited by the bandwidth of the resonance filter in the suprasyringeal vocal tract or there may be biomechanical or physical interactions between the left and right sides of the syrinx (Nowicki and Capranica, 1986;Riede et al, 2006) that restrict their ability to simultaneously oscillate at widely different frequencies.…”

“…The rapid switching of phonation between the left and right sides of the syrinx is a prominent feature of the songs of many birds as they take advantage of lateralized differences in frequency range, frequency modulation, etc. (Suthers et al, 1994).…”