Document: HAN-3-50-4

Sound transmission in tropical forests: Bird song convergence through habitat exploitation.

SLABBEKOORN, H.* ¹, J.ELLERS ² and T.SMITH ^1,3

San Francisco State University, San Francisco, CA 94132 ¹
Stanford University, Stanford, CA 94305 ²
University of California, Davis, CA 95616 ³

Abstract:
Animal vocalizations that are used for long distance communication are shaped by acoustic properties of the environment. Bird communities living in dense tropical forests show convergence in their species-specific songs, which is reflected in general acoustic design features. Thirty years of research on the relationship between sound transmission and signal design yielded a huge body of literature, focusing on habitat induced limitations due to signal attenuation and degradation. However, part of the signal design of forest birds may not be explained by limitations, but by hitherto overlooked beneficial effects of reverberations. Typical narrow frequency bandwidth notes ('NFB notes') are the principle elements of the acoustic design for bird song in dense tropical forest. These notes contrast with frequency modulated notes, typical for many other closely related species, which do not live in dense undergrowth. We examined the impact of transmission on these NFB notes in dense tropical forest in Cameroon, Africa. We used acoustic analyses of bird song recordings, transmission experiments with artificial sound stimuli, and playback experiments. We show that the change in acoustic characteristics after transmission is qualitatively and quantitatively different for NFB notes, compared with frequency modulated notes. The difference is caused by the constant frequency of the notes, and leads to a relative increase in efficiency after transmission through dense foliage. Our data provide evidence for that the evolutionary convergence into song with NFB notes for many unrelated bird species, may be explained not so much by habitat induced limitations, but rather by exploitation of the transmission properties.

Keywords: Signal evolution, bird song ecology, sound transmission

This abstract is being presented at: 8:15 AM in session:
Oral Session #3: Avian Ecology.