REPETITION:

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”.

CALL VARIATION:

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.

PEAK FREQUENCY

This is the frequency at which a bat’s call is loudest and can be a good indicator of the species of bat (see here and here)