Figure 1.

When an insectivorous bat pursues an insect it changes the calls in a typical sequence of search, approach and terminal buzz phase calls, where duration gradually decreases, while band width and repetition rate increase. A shows a series of cartoon images, depicting a bat pursuing an insect. The sonar beam pattern is aimed in different directions during the search phase. Towards the end of the approach phase and through the terminal buzz, the sonar beam is directed at the prey. B shows spectrograms of typical FM echolocation signals produced by the big brown bat, Eptesicus fuscus, at each of the phases of insect pursuit. The lower panels illustrate that a bat hunting insects out in the open sky, where all other objects are far away, will often encounter fairly “clean” echo returns from isolated objects, whereas a bat hunting close to vegetation will encounter a far more complex auditory scene composed of echoes from many objects arriving at different delays and intervals.