Vulnerable but not helpless: nestlings are fine-tuned to cues of approaching danger

Vocalizing nestlings are vulnerable to eavesdropping by predators, but may reduce risk through behavioural tactics such as responding with silence to adult alarm calls. Nestlings may also assess danger independently, although there has been little investigation of this possibility. Additionally, nestlings might use parental signals to modify their response to possible cues of danger to reduce the likelihood of going silent to harmless stimuli. Nestling white-browed scrubwrens, Sericornis frontalis, cease calling in response to both parental alarms and the acoustic cues of a predator. However, it is unknown whether their response to the predator cue (a pied currawong, Strepera graculina, walking on leaf litter) is specific to the predator's sound, or whether it is a response to broadband, atonal sounds in general, to a ‘walking tempo’, or simply to any novel sound. Using field playback experiments of synthetic and natural sounds we show that nestlings are finely tuned to cues of danger. Nestlings suppressed calling most strongly to the sound of a real predator, and less strongly to broadband sounds. They did not respond to either novelty or a ‘walking’ tempo alone. Nestlings responded just as strongly to the predator's sound if they first heard the sound of a parent nearby, suggesting that they could discriminate predator cues from the sound of parental arrival, or that their interpretation of sounds was ‘adaptively pessimistic’. Overall, scrubwren nestlings showed specific and independent assessment of predator sounds, which appeared unaffected by cues of parental presence.