Propagation of bird acoustic signals: comparative study of starling and blackbird distress calls.

Previous works have demonstrated that the information supported by a bird distress call is encoded both by the energy distribution among harmonics and the temporal evolution of the frequency modulation. In the present study, using these parameters, we compared long-range information transfer in a dense vegetation environment between the starling Sturnus vulgaris and the blackbird. Turdus merula distress calls. It appears that excess attenuation of high frequencies (higher than 4 kHz) after a long-range propagation is responsible for modifications in distress call spectra. The energy of propagated signals tends to be concentrated in a 1.5-4-kHz bandwidth whatever the initial spectrum. Owing to its broad spectrum (0.8-7 kHz), the starling distress call is greatly modified. On the contrary, owing to a narrower spectrum (2-5.5 kHz), the blackbird distress call is relatively preserved. The blackbird distress call appears to be well-adapted to long-range transmission in an environment with dense vegetation. In contrast, the starling distress call is far more easily degraded by propagation. Nevertheless, frequency modulation of the starling call is preserved and message decoding remains possible even if message reliability may diminish. This result is examined from an etho-ecological point of view, taking into account both habitat and social structure of both birds.