Mechanics of the Syrinx in Gallus gallus. I. A Comparison of Pressure Events in Chickens to Those in Oscines

Abstract

The pattern of air sac pressure events during vocalization was simple. Pressure rose shortly before the onset of calling and dropped as vocalization ceased. The peak value was related to the loudness of the call. Internal pressure events in chickens and ducks [Anas platyrhynchos] were similar to those observed in oscines. Tracheal pressures during prolonged, loud calls of starlings, remained near atmospheric, but those of chickens slowed an interaction between the resistance to flow effected by activity of the vibrating membranes and internal pressures. Tracheal pressure in chickens usually was substantially above atmospheric, indicating a substantial flow of air into the trachea. The external labium, a structure found in oscines but not chickens, may function as a valve that serves to increase the loudness of a call while reducing the volume of gas exhausted. A single exception, a loud, wailing call, uttered by some roosters, showed a pattern of high air sac pressures with tracheal pressures near atmospheric. This situation may be explained by the structure of the chicken''s syrinx. If the drum is drawn caudad as far as possible, the vibrating membranes fold inward and meet between the drum and the pessulus, thereby occluding the lumen of the syrinx. Electrical stimulation of tracheal muscles in anesthetized chickens showed this explanation may be mechanically possible. Rupture of the interclavicular air sac rendered both chickens and starlings voiceless. The mechanism appeared to differ between the 2 spp. In the oscine, air flow was rerouted along a pathway of low resistance, bypassing the syrinx. Chickens remained silent even if flow through the syrinx was restored. The loss of vocal ability could be attributed to the reversal of the pressure differential between the interclavicular air sac and syringeal lumen with the resulting movement of the vocal membranes out of rather than into the airstream. Activity of the muscles that constrict the abdomen, in combination with pressure events, showed that staccato calls by chickens are produced by an airflow that is constant in direction but variable in rate. This is in contrast to the mini-breath hypothesis of Calder, which supposes a reversal of flow. Mini-breaths may be available as an alternative technique.