Double identity for Britain's most common bat

The chiffchaff is not the only small flying creature to have had its identity split up by the sound it makes.

Britain's commonest bat, the pipistrelle, has recently been divided into two species on the basis of the high-frequency squeaks used for navigating and hunting.

The new species, Pipistrellus pygmaeus, has been named the soprano, as its call centres around 55kHz, a little higher than the original species, P. pipistrellus, still named the pipistrelle, which centres around 46kHz. Bat calls are usually between 18kHz and 50kHz. As with the chiffchaff, the findings were confirmed by DNA testing.

Julia Hanmer, chief executive of the Bat Conservation Trust, said "the sounds are made over a whole range of frequencies and you have to look for the strongest to identify it".

There is great variety in bats' ultrasound calls, which are complex, textured sounds rather than simple pips. Children can often hear them, although adults soon lose this ability - men quicker than women. The human ear, which listens from around 20kHz to 20kHz, requires electronic "bat boxes" equipped with microphone and speaker to make slightly altered versions of the cries audible to us.

Such kits cost from a basic £25 to thousands of pounds for hi-tech models. Audio compact discs of bat calls are also available for training the novice.

The Bat Conservation Trust says different species have varied and distinct characteristics on the detectors. The noctule makes a "chip-chop" sound, while the pipistrelle sounds like a "wet slap". Other sounds are compared to the "rat-tat-tat- tat" of a machine-gun or a "rhythmic rap-artist sound".

Britain has 16 species of bat but the total population is unknown and difficult to establish. "We're 50 years behind bird monitoring," Ms Hanmer said. However, a five-year monitoring programme which concludes this year hopes to provide much information.

The Bat Conservation Trust's website is www.bats.org.uk