Sounds audible to migrating birds

Abstract

We have measured sound levels from frog choruses in eastern New York State at altitudes of up to several hundred metres. Rana pipiens choruses from small ponds often could be recorded by a radio microphone up to 500 m, and on an especially favourable night with light winds they were clearly audible even at 965 m at about 20 dB SPL in the 1·5 to 2·5 2·5 kHz frequency band. Sound travels upward much farther and more predictably than along the surface. Many natural sounds, including those from frogs, insects, whitecaps, and perhaps wind-blown vegetation, arise from large areas and therefore act as extended sources. The intensities of such sounds decrease with altitude more slowly than expected from the inverse square law. Natural sound fields provide migrating birds with a potential source of information about the kind of land or water below them, and their progress over acoustic landmarks could inform them about wind velocity. Because atmospheric absorption increases with frequency, several hundred metres of air act as a low-pass filter, so that altitude could be estimated from the relative reduction of higher frequencies in a familiar sound.