Learning to listen? Nestling response to heterospecific alarm calls

Many nestling birds go silent in response to parental alarm calls, potentially lowering their risk of being overheard by predators. Parents are not always nearby, however, and so offspring could also benefit if they respond to the alarm calls of other species. Response could be innate, particularly if heterospecific alarm calls are acoustically similar to conspecific alarms, or learned through experience or association with parental behaviour. We investigated the responses of both young (5-6. days old) and older (10-11. days old, close to fledging) nestling white-browed scrubwrens, Sericornis frontalis, to the mobbing alarm calls of three heterospecifics whose nests are vulnerable to similar predators. Brown thornbills, Acanthiza pusilla, produce the most similar alarm calls to scrubwrens, while superb fairy-wren, Malurus cyaneus, and New Holland honeyeater, Phylidonyris novaehollandiae, produce alarm calls that are distinct. Heterospecific mobbing assays demonstrated that nestlings were likely to overhear the alarm calls of all three species. In support of innate response, even young nestlings suppressed calling after hearing both thornbill and parental alarms. However, young nestlings ignored or increased calling to fairy-wren and honeyeater alarms. Older nestlings continued to suppress calling to thornbill and parental alarm calls, but also suppressed calling to honeyeater and fairy-wren alarms, suggesting that they could have learnt to recognize those calls. This study thus demonstrates that nestlings can respond to the alarm calls of other species, and that these responses are likely to be enabled through both innate mechanisms and learning.