Repetition rate of calls used in multiple contexts communicates presence of predators to nestlings and adult birds

In multispecies communities, animals may come to recognize the signals of other species both by responding to common signal features and by learning about associations between signals and relevant threats. However, some signals are produced in multiple contexts. To a given receiver, such a signal may only sometimes be relevant. Here, I demonstrate that receivers use contextual variation in signal form as a cue to their relevance. Individuals from 15 species of songbirds repeated their calls rapidly when confronting widely threatening predators, but repeated the same calls more slowly during other types of social interactions. In playback experiments, repetition rate was a cue to nestling Ficedula flycatchers, which reduced their activity in response to quickly but not slowly repeated calls, and also to adult birds from a variety of species, which responded more strongly to the calls of their own and other species produced at faster rates. These results show that repetition rate is an innate or early learned contextual cue and, in combination with learning about heterospecific signals, allows receivers to fine-tune their responses to the calls of their own and other species according to their relevance, suggesting that simple rules facilitate widespread heterospecific communication networks.