Adaptive differences in response to two types of parental alarm call in altricial nestlings

Vertebrate alarm calls can contain information about the type of predator and the degree of danger, but young animals often respond to alarm calls differently from adults. The distinct behaviour of young may reflect an imperfect stage in the gradual development of the adult response, or a response adapted to specific risks faced by young. In this study, we tested whether nestling white-browed scrubwrens, Sericornis frontalis, responded to different alarm calls according to their specific risks of predation. As predators on the ground pose a danger to scrubwren nestlings, whereas flying predators do not, we predicted that they would respond to ground alarm calls but not to aerial alarm calls. In a field playback experiment, we tested the response of young to aerial and ground alarm calls, each presented in a shorter (less urgent) and longer (more urgent) form. We found that both 5- and 11-day-old nestlings responded to ground alarm calls, and did so more strongly to the more urgent playback. By contrast, the response to aerial alarm calls started to develop only towards the end of the nestling stage. Thus, scrubwren nestlings can distinguish between different types of alarm calls and react more strongly to calls warning of a predator posing greater danger, appropriate to the nestling stage of development. Furthermore, they use the length of ground alarm calls as an indicator of the degree of danger.