Eavesdropping on the neighbours: Fledglings learn to respond to heterospecific alarm calls

Young birds and mammals suffer from a high risk of predation, and should be under strong selection for early response to cues indicating danger, including the alarm calls of other species. Despite this prediction, there has been little investigation of the development of response by young animals to heterospecific alarm calls, and none on fledgling birds. Previous studies have suggested that learning is important in the recognition of heterospecific alarm calls in adult birds, but when learning occurs is unknown. We examined the responses of fledgling white-browed scrubwrens, Sericornis frontalis, at three ages to playback of the aerial alarm calls of the sympatric superb fairy-wren, Malurus cyaneus, and New Holland honeyeater, Phylidonyris novaehollandiae. Fairy-wren and scrubwren alarm calls are acoustically similar, while honeyeater alarm calls are distinct, but adult scrubwrens respond to both. Recently fledged scrubwrens responded strongly to conspecific alarm calls, but weakly to heterospecific alarm calls. By contrast, only 2. weeks after leaving the nest most fledglings responded to fairy-wren and honeyeater alarm calls similarly to conspecific alarm calls. However, fledglings in territories without honeyeaters ignored honeyeater alarm calls. Three weeks later, fledglings on all territories responded to all heterospecific alarm calls, an identical response to that of their parents. These results demonstrate that fledglings can quickly develop appropriate responses to heterospecific alarm calls, and present strong evidence that response is learned over a microgeographical scale.