Sound propagation in forest and habitat desertion in the Western Capercaillie (Tetrao urogallus L.)

Abstract The Western Capercaillie Tetrao urogallus L. uses its calls for breeding, territorial and hierarchical purposes. We investigated whether changes in the vegetation structure of the forest might affect call transmission and contribute to Western Capercaillie’s lek desertion in the Pyrenees, as predicted by the acoustic adaptation hypothesis. We measured forest structure and the sound propagation properties of the forest at nine occupied lek sites and eleven deserted lek sites in the Alt Pirineu Natural Park in Catalonia (NE Spain). We found similar values of forest Maturity Index (MI), Human Footprint Index (HFI) or Spatial Integrity Index (SII) in both groups of stands. Abandoned stands exhibited higher Dominant Tree Height (Ho), higher Basal Area (BA), higher Volume of Living Trees (FV) and higher Volume of Dead Trees (FM). Deserted sites exhibited higher values of Background Noise (N) and lower values of scattering of the sound on the vertical and horizontal plane (JLFH and JLFV), resulting in longer reverberation time (T20), higher sound pressure level (SIGC) and a slower decay curve (K) due to lower attenuation. We suggest that the background noise and the sound propagation characteristics of the deserted forest stands might be affecting the transmission of Capercaillies’ calls, creating distortion and masking of the acoustic signal by excessive reverberation and noise. Our results suggest that changes in the acoustic properties of forests, partially driven by changes in the forest structure, may contribute to lek desertion. If confirmed, conservation-oriented forest management for Capercaillies should aim not only to maintain food and shelter resources, but also to preserve acoustic conditions that facilitate non-distorted and correct signal discrimination during display.