Turning the tables: a tiny bird uses alarm calls and mimicry to deceive its nest predator.

Animals often eavesdrop on other species' alarm calls to gain information about danger, but this can allow for deception by callers. Such deception often uses 'aerial' alarm calls, which normally warn of airborne predators and prompt immediate fleeing. The calls are deceptive if they are given when no flying predator is present and the caller benefits from the victim's response, typically by gaining food dropped when the listener flees. We studied deceptive alarm calling by brown thornbills, Acanthiza pusilla, defending offspring against predatory pied currawongs, Strepera graculina. Thornbills give their own and mimetic aerial alarm calls when defending nestlings against currawongs, who are fooled into scanning for danger or flying away. We tested whether deception works by exploiting the predator's response to aerial alarm calls, and what role mimicry plays. Currawongs were more likely to flee, and delayed feeding longer, after playback of purely aerial compared with purely mobbing alarm choruses. They responded the same regardless of what type of mimetic alarm followed the thornbill's aerial alarm. We conclude that vocal deception is effective because it exploits currawong response according to call meaning, while mimicry likely creates an illusion of a multi-species alarm chorus.