Like a cordless phone looking for a clear channel, it can hop on to a less crowded frequency to send the next burst.

FORBES: Copper Hopper

Then the bat detectors - which can be set to the frequency of pipistrelle, Daubenton's water bats and the impressive long eared bat - burst into life.

BBC: NEWS | UK | Scotland | Highlands and Islands | Seeking 'a wet slap' in the dark