Referential calls signal predator behavior in a group-living bird species.

Predation is a powerful agent of natural selection, driving the evolution of antipredator calls [1]. These calls have been shown to communicate predator category [2-4] and/or predator distance to conspecifics [5-7]. However, the risk posed by predators depends also on predator behavior [8], and the ability of prey to communicate predator behavior to conspecifics would be a selective advantage reducing their predation risk. I tested this idea in Siberian jays (Perisoreus infaustus), a group-living bird species. Predation by hawks, and to a lesser extent by owls, is substantial and the sole cause of mortality in adult jays [9]. By using field data and predator-exposure experiments, I show here that jays used antipredator calls for hawks depending on predator behavior. A playback experiment demonstrated that these prey-to-prey calls were specific to hawk behavior (perch, prey search, attack) and elicited distinct, situation-specific escape responses. This is the first study to demonstrate that prey signals convey information about predator behavior to conspecifics. Given that antipredator calls in jays aim at protecting kin group members [10, 11], consequently lowering their mortality [9], kin-selected benefits could be an important factor for the evolution of predator-behavior-specific antipredator calls in such systems.