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Respective Parts of the Carrier and of the Frequency Modulation in the Semantics of Distress Calls

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Distress calls are complex signals, i.e. they are made up of numerous harmonics. Their spectral composition suggests that there are two components: a carrier frequency and a modulating frequency. We have distinguished a primary modulation (slow increase or decrease of frequencies from the beginning to the end of the call) and an additional secondary modulation (fast frequency sweeps within a narrow bandwidth). We have attempted to examine the role of these different signal components in the semantics of distress. To resolve this problem, we have used digital synthesis methods. We have performed playback experiments in the field with starlings (Sturnus vulgaris). These experiments give the following results: a carrier frequency presented alone, without frequency modulation, does not elicit distress reactions. A signal with the carrier frequency only, modulated by a secondary modulation, gives the same results. But obvious distress reactions are observed if the carrier frequency is modulated by a signal which presents a slight increasing slope (primary modulation), be it with or without a secondary modulation. The primary modulation appears to be a very important parameter in the decoding of starlings distress calls. On the other hand, secondary modulation appears to be unnecessary for this decoding. We have demonstrated that distress calls, in spite of their complex structure, follow a very simple law of decoding.

Affiliations: 1: (Laboratoire d'Ethologie Expérimentale, Station de Terrain de St. Lucien, 28210 Nogent-le-Roi, France


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