Functionally referential alarm calls in noisy miners communicate about predator behaviour

Abstract

Many vertebrates have alarm calls that warn conspecifics about danger, and some species even communicate about the type of predator or its behaviour, allowing for appropriate responses. However, such ‘functionally referential’ communication has been shown experimentally in only a handful of species, and requires demonstrating that individuals give acoustically distinct calls to different threats, and that the calls alone are sufficient to prompt listeners to behave as if a specific threat is present. We carried out model presentations, acoustic analyses and a playback experiment to test whether the alarm calls of noisy miners, Manorina melanocephala , are functionally referential. Miners gave different calls to airborne raptor models compared to terrestrial or perched raptor models, and even switched from ‘aerial’ alarm calls to ‘chur’ alarm calls when a hawk glider landed on the ground. They also behaved differently to these two types of threats, showing avoidance to aerial threats, including fleeing or freezing, but deterrent behaviour to terrestrial threats, including vigilance, approach and mobbing. The two alarm call types were acoustically distinct, and consistent with calls to live predators. Blind scoring of video revealed that birds responded appropriately to playbacks of alarm calls alone, typically fleeing to aerial alarm calls yet becoming vigilant, approaching and calling to chur calls. Noisy miners produce alarm calls that therefore meet both criteria for functional reference, and thus become one of the few bird species in which such calls have been confirmed. Many birds appear to give different calls to airborne predators compared to during mobbing of terrestrial or stationary predators, so functionally referential alarms are likely to be common and may often categorize predators by their behaviour and not simply their taxonomic type.