Transmission of Urgency Levels in the Alarm Calls of the Oriental Reed Warbler

Many animals communicate predator-related information to conspecifics through alarm calls that exhibit acoustic structural variations encoding key parameters such as predator type, body size, and urgency of danger. In previous studies, the oriental reed warbler (Acrocephalus orientalis) produced a variety of syllable types in its alarm calls when encountering three species of invaders-Eurasian sparrowhawk (Accipiter nisus), common cuckoo (Cuculus canorus), and oriental turtle dove (Streptopelia orientalis). However, the information conveyed by its alarm calls does not include the type or size of the predator. To explore whether these different syllable types convey varying levels of urgency, we conducted playback experiments during the oriental reed warbler's incubation period. Three syllable types with gradient differences in the acoustic parameters were selected: B alarm calls, D alarm calls, and F alarm calls. The results indicate that the oriental reed warbler responds differently to different types of alarm calls. Specifically, the oriental reed warbler exhibited a stronger response to F alarm calls than to B alarm calls, manifested by higher response intensity, increased attack frequency, and greater individual attraction. In addition, the urgency scores triggered by F alarm calls were significantly higher than those triggered by B and D alarm calls. Furthermore, the frequency and duration of different syllable types significantly predicted conspecific responses. Our results suggest that oriental reed warblers utilize higher-frequency calls with longer durations in their alarm signals to convey a greater sense of urgency regarding danger. Further synthetic sound experiments are needed to investigate the relative significance of duration and frequency, and to determine the role of syllable sequence in conveying predator-specific information.