Bats repeat their calls at various intervals (to ensure they hear the echo from the previous call), this interval will get shorter as they home in on their prey and catch it. This is called the “feeding buzz”.
The calls of different bat species vary in their amplitude, duration and frequency. This variation is related not only to their insect prey but also their foraging strategy and the habitat in which they forage. Generally, bats that hunt in a cluttered environment (e.g. amongst trees) tend to put more emphasis on frequency modulation (i.e. changes of frequency) whilst bats that hunt in the open tend to use a more constant frequency.