Graded Frequency Variations of the Tremolo Call of the Common Loon (Gavia immer)

Abstract

The tremolo is an alarm call generally associated with some behavioral indication of a tendency to flee. It is also given in approach-avoidance conflict situations. Data from field recordings indicate that there are 3 distinct structural variations (call types), which are formed by the addition of progressively higher harmonically unrelated frequency components. The pitch frequency of each of the call types varies depending on its duration: the longer the call, the higher its frequency. The amplitude and the interval between calls are correlated with the type given. Each of the call types and their variations occur in every context in which the call was recorded, and may be correlated with changes in stimulus intensity. They are therefore considered graded forms of the same signal.