Mechanisms of mobbing call recognition: exploring featural decoding in great tits

Recent research on bird calls has unearthed a striking result: birds sometimes react to the calls of other species that are neither geographically nor phylogenetically close. One mechanism explaining this response may be the recognition of specific acoustic features, also present in their own vocalizations, rather than the recognition of complete notes, with the result that unfamiliar calls may be understood if they contain the critical feature. Parids and other passerines produce mobbing calls with similar properties that are responded to across species (i.e. noisy, large frequency range notes reaching low frequencies and with numerous harmonics) that are therefore good candidates for recognition based on features. In a playback experiment, we explored the featural interpretation hypothesis by testing the response of free-ranging great tits, Parus major, to artificial mobbing calls with varying acoustic properties. We first confirmed that they respond to artificial calls sharing all the targeted spectral properties (large frequency range, low frequency, noise and harmonics). In contrast, great tits did not respond to calls with the same rhythmicity but without the targeted features. We then tested whether great tits respond to calls that possess only one of the four above-mentioned properties. We show that great tits did not respond to any of the four treatments, and therefore no single specific spectral feature seems likely to explain great tits' response to unknown calls. We discuss alternative mechanisms for decoding novel calls, notably through a similarity threshold.