Birds orient their heads appropriately in response to functionally referential alarm calls of heterospecifics

Abstract

Vertebrate alarm calls signal danger and often encode graded or categorical information about predatorproximity or type. In addition to allowing communication with conspecifics, alarm calls are a valuablesource of information for eavesdropping heterospecifics. However, although eavesdropping has beenexperimentally demonstrated in over 70 species, we know little about exactly what informationeavesdroppers gain from heterospecific alarm calls. Here, we investigated whether Australian magpies,Cracticus tibicen, extract relevant information about the type of threat from functionally referential alarmcalls given by noisy miners, Manorina melanocephala. Miner aerial alarm calls signal a predator in flight,whereas mobbing calls signal a terrestrial or perched predator. We therefore tested whether magpiesgain information on the elevation of expected danger. We first confirmed, by measuring bill angles onvideo, that magpie head orientation changes appropriately with differences in the elevation of a con-spicuous moving object. We then conducted a field experiment that measured magpie bill angle inresponse to playback of miner aerial and mobbing alarm calls. The maximum and mean bill angles werehigher in response to aerial than to mobbing calls, sug gesting that magpies use information from mineralarms to search visually at appropriate elevations for the specific type of danger. Magpies were alsovigilant for longer af ter aerial alarm calls that followed mobbing calls, implying perception of an esca-lating threat level. Our work shows that individuals can gain information on the type or location of danger from heterospecific alarm calls, which is likely to increase the effectiveness of antipredator responses. Show more