Eavesdropping on other species: mutual interspecific understanding of urgency information in avian alarm calls

Some birds eavesdrop on the alarm calls of other species, but little is known about the specific information obtained. Fleeing in response to nonurgent alarms, such as those given for distant predators, wastes time and energy and so individuals could benefit from decoding information about urgency. White-browed scrubwrens, Sericornis frontalis, and superb fairy-wrens, Malurus cyaneus, flee in response to each other's aerial alarm calls, and scrubwrens communicate urgency by including more elements in their alarms when a threat is closer. We carried out a model-presentation experiment to test whether fairy-wren alarm calls also encode risk-based information, followed by a playback experiment to compare how fairy-wrens and scrubwrens respond to graded information in both their own calls and those of the other species. Fairy-wrens encoded urgency in a way similar to that of scrubwrens, by including more elements when the model predator was closer and by increasing the maximum frequency of elements. Each species was more likely to flee in response to both conspecific and heterospecific alarm calls that included more elements. Fairy-wrens were more likely than scrubwrens to flee regardless of call urgency, particularly when responding to their own species' calls, perhaps because they are more vulnerable to predators or because they use a different scale for decoding risk-based information. Fairy-wrens also spent more time in cover after fleeing multielement conspecific calls with a greater number of elements. Our study reveals that urgency information in the form of graded alarm call variants can be transferred through mutual eavesdropping between bird species.