Eavesdropping and contagious alarming in bird communities

Eavesdropping on heterospecific alarm calls can provide valuable information about predator presence and therefore yield survival benefits. However, if, how, and why individuals react to heterospecific alarms is poorly known. If heterospecific alarms trigger a response, individuals might either stop their own vocal activity (acoustic suppression), to avoid being detected, or start alarming (acoustic stimulation), to warn conspecifics or deter predators. Here, we performed playback experiments with the alarms of 14 common forest passerine species and tested whether heterospecifics changed their acoustic behavior after playbacks and whether this involved suppression or stimulation. Moreover, we tested whether bird behavior was influenced by species-specific attributes such as escape capability, the propensity of being predated by raptors and carnivorous mammals, and the acoustic properties of alarms affecting detectability. Birds uttered alarms more frequently after playbacks than after silent controls, gave the same alarms as they normally use in response to conspecifics, and did not suppress their acoustic activity after playbacks. These results indicate that birds perceive threat from heterospecific alarms and are contagiously stimulated to alarm, rather than inhibited. Species with morphological attributes that promote agility and suffering low predation rates were more acoustically stimulated by heterospecific alarms, irrespective of the acoustic properties of the playback or their own alarms, indicating that the propensity to join into an alarm calling bout is mediated by predation risk. This study provides evidence for contagious alarming across species, for potential costs of responding to these stimuli, and shows a facilitative role of signalers within communities.