A Quantitative Analysis of the Dawn Chorus: Temporal Selection for Communicatory Optimization

Abstract

This study describes a mathematical model designed to explore the relationships between sound transmission and microclimatic conditions present at different times of the day in different habitats. The model includes all attenuation mechanisms and is used to illuminate possible reasons for the dawn chorus observed in birds and primates under a diverse array of environmental conditions. Also explored are the possible effects of calling height and calling frequency on broadcast coverage. Solely from the point of view of atmospheric attenuation mechanism, early morning microclimatic conditions are as optimal for sound propagation as will be present at any other time during the day or night. Depending on the environment, calls voiced at such times are up to 20 times more effective in area of broadcast coverage than the same amplitude signal broadcast at midday. Such an advantage goes far in explaining why many animal species have selected these times for territorial and/or mate attracting communicatory announcements. Increasing height above the substrate also increases the area of broadcast coverage under many environmental conditions, since greater elevation reduces the effects of steep near-surface temperatures and wind velocity gradients in refraction of the sound waves. This mechanism leads to the formation of shadow zones whereby signals can become inaudible at short distances from the source. Decreasing signal frequency also increases area of coverage and thereby leads to a seeming paradox, since animals tend to have higher pitched calls in open environments where wind and turbulence effects predominate. A hypothesis is advanced based on communication theory and sound transmission through a turbulent medium whereby information transfer may be enhanced by the use of higher frequencies.