Orientation sounds of the echolocating swiftlet,Collocalia spodiopygia (family Apodidae) consist of two clicks, each having a duration of a few milliseconds, separated from each other by a silent interval lasting about 20 ms (Fig. 1). Most of the acoustic energy is between 2 and 8 kHz. These clicks are generated in the syrinx and require syringeal airflow. 3.

The first member of the double click is initiated by contraction of the sternotrachealis muscles (Fig. 3) which reduces tension on the syrinx by pulling the trachea caudad (Fig. 2). This causes the first bronchial ring to rotate inward (Fig. 10), reducing the syringeal aperture. As the external tympaniform membrane approaches the internal tympaniform membrane, the subsyringeal (= sternal air sac) pressure rises with increasing syringeal resistance and expiratory tracheal airflow peaks at about 6.1 ml/s (Figs. 6, 7, Table 1). Bernoulli forces cause the internal tympaniform membranes to vibrate, producing the first member of the double click (Fig. 9b). The syrinx is closed during the silent intraclick interval by apposition of the syringeal membranes (Figs. 9c, 10). Subsyringeal pressure peaks at about 17cm H2O during this interval (Fig. 6), which is terminated by contraction of the tracheolateralis muscles (Fig. 3) that moves the syrinx craniad and causes the first bronchial ring to rotate outward, thus abducting the external tympaniform membranes and opening the syringeal lumen (Fig. 9d). The second member of the double click occurs as the internal and external tympaniform membranes begin to separate, allowing expiratory airflow to resume (Fig. 9d). Tracheal airflow peaks at about 10 ml/s during this click as syringeal resistance and subsyringeal pressure drop (Figs. 6, 7, Table 1). The second click is terminated by further abduction of the external tympaniform membranes to their resting position (Fig. 9a). 6.

The ability of swiftlets to transform a longer duration squeak-like vocalization into two brief clicks by momentarily closing the syrinx probably represents an important adaptation for acoustic orientation. By generating brief clicks they have created a sonar signal with a greater bandwidth and having abrupt rise-decay times that should improve target range determination based on measurement of the pulse-echo interval.