A songbird mimics different heterospecific alarm calls in response to different types of threat

Birds frequently mimic other species' alarm calls, but the type of information conveyed to receivers, and therefore the function of mimetic alarm calls, is often unknown. Alarm calls can convey predator-specific information that influences how receivers respond: aerial alarms signal the presence of flying predators and provoke receivers to flee, whereas mobbing alarm calls signal the presence of less dangerous predators and provoke receivers to mob. The function of mimetic alarm calls may therefore depend on the type of heterospecific alarm calls mimicked. We examined the use of alarm call mimicry by brown thornbills (Acanthiza pusilla) across different contexts of danger: terrestrial threat, aerial threat, when captured by a predator and during nest attack. Thornbills were most likely to include mimetic alarm calls among their vocalizations when captured and during nest attack, less likely in response to terrestrial and aerial predator threats, and least likely in the absence of any threat. Furthermore, the type of danger affected the type of mimetic alarm calls used: thornbills mimicked mobbing alarm calls in response to terrestrial threat and aerial alarm calls in response to aerial threat but also during capture and nest attack where no aerial danger was present. We suggest that mimicking predator-appropriate heterospecific alarm calls in response to aerial and terrestrial threats may facilitate interspecific alarm communication with other prey species, whereas mimicking heterospecific aerial alarm calls in the absence of aerial predators might deceptively startle predators to release captured callers or offspring when attacked.