Adaptation au transfert de l’information sonore dans l’environnement. Rôle des paramètres de durée dans le cri de détresse de l’Étourneau (Sturnus vulgaris)

Abstract

According to information theory, the extension of the duration parameters of acoustic signals increases their efficacity. The aim of our study was to test this hypothesis on distress call of a bird, the starling Sturnus vulgaris. Distress calls are high intensity alarm signals produced by birds when they are seized by a predator. Earlier studies revealed that frequency modulation constitutes an important feature of distress calls for triggering responses. The coding-decoding process is thus precisely defined. On the contrary, the coding of duration parameters seems imprecise since individuals produce distress calls of variable length. As predicted by the information theory, our experiments show that longer signals are more efficient than shorter ones. An explanation is that an increase in duration improves the signal resistance against degradation occuring during transmission through the environment. Our experiments indicate that signal recognition is affected by a noisy environment, particularly when noise overlaps a part of the signal. The greater the overlapping, the lower the signal recognition, independently of the position of the noise (at the beginning or the end of the signal). Consequently, to improve message efficiency in the case of transmission through a noisy environment, birds must increase the signal duration. The flexibility of duration parameters in the coding process gives birds the possibility of adapting their distress signals to different conditions of propagation.