Fig. 1.

The perception of ISOL may provide birds with valuable information during flight, such as the speed, location in 3-dimensional space, and the wing-beat frequency of neighbors. Such information will travel in all directions; hence, it might be used in mutual adaptation among neighbors. The distance may be assessed from a stereotypic sound, such as ISOL, breathing, or vocal call, since the amplitude will be influenced by distance. Moreover, when a sound travels through air, its timbre changes, since higher frequencies are damped more (Photo: Torbjörn Arvidson)