The physiology of vocalization by the echolocating oilbird, Steatornis caripensis

1. Oilbirds (Steatornis caripensis; Steatornithidae) have a bilaterally asymmetrical bronchial syrinx (Fig. 2) with which they produce echolocating clicks and a variety of social vocalizations. The sonar clicks typically have a duration of about 40 to 50 ms and can be classified as continuous, double or single. Agonistic squawks typically have a duration of about 0.5 s and contain multiple harmonic components (Figs. 5, 6). 2. Both sonar clicks and agonistic squawks are initiated by contraction of the sternotrachealis muscles (Figs. 7 and 16) which stretch the trachea, reducing the tension across the syrinx and causing the cartilaginous bronchial semi-rings supporting the cranial and caudal edges of the external tympaniform membranes (ETM) to hinge inward, folding the ETM into the syringeal lumen (Fig. 17). Bernoulli forces created by expiratory air flowing through the restricted syringeal aperture presumably initiate vibration of the internal and/or external tympaniform membranes. 3. Social vocalizations such as the agonistic squawk, continue until the sternotrachealis muscles relax (Fig. 10), allowing the cranial portion of the bronchus to move anteriad and abducting the ETM. 4. Sonar clicks are terminated by rapid contraction of a previously undescribed intrinsic syringeal muscle, the broncholateralis, which inserts on the semi-ring supporting the anterior edge of the ETM and causes it to rotate about its articulation with the next anterior bronchial cartilage in such a way that it abducts the ETM (Figs. 8, 16, 17). Musculus broncholateralis contracts only during sonar clicks, appears to have a high proportion of twitch-type fibers, and is specialized for the rapid abduction of the ETM to produce short duration, click-like vocalizations. 5. Tracheal airflow and sternal air sac pressure reflect the changes in the syringeal aperture. Tracheal airflow at first increases as expiratory effort increases subsyringeal pressure. The initial high rate of airflow drops at the onset of phonation due to the increased syringeal resistance. In the case of a double click, airflow momentarily ceases during the intraclick interval when the ETM temporarily closes the syrinx. Air sac pressure rises to its maximum level at this time. Expiratory airflow rapidly increases as the ETM is abducted from either its closed or phonatory position to its open, resting position. Each sonar click requires about 1 cm3 of air; a typical agonistic squawk may use about 27 cm3 of air (Figs. 11, 12, 13; Tables 3, 4). 6. Each sonar click is often accompanied by a complete respiratory cycle or ‘mini-breath’ (Fig. 11). Pulmonary ventilation can be controlled independently from the clicking rate by varying the tidal volume of the mini-breaths, which may be as small as the tracheal and bronchial dead space. Mini-breaths permit oilbirds to produce click trains having a long train duration uninterrupted by a long inspiration. 7. A dual flow probe was used to simultaneously measure the rate of airflow through each semi-syrinx during vocalization. The rate of airflow through the right semi-syrinx was 40 to 60% greater than that through the left. Both syringes functioned together except during the middle portion of some continuous type sonar clicks when sound was sometimes generated only by one semi-syrinx, the other being closed (Figs. 14, 15; Tables 5, 6). 8. The fluid subsyringeal power reaches approximately 100 and 150 mW in the left and right semi-syrinx, respectively, during the second member of a double sonar click (Table 6). Total syringeal power during agonistic squawks reaches at least 60 mW and syringeal resistance during these vocalizations is as high as 1500 cm H20/LPS (Table 7).