AT the click of a mouse, the peaks and troughs on the screen are translated into patches of blue, red, and yellow. ''That's more than one,'' says one of the 10 people gathered in the room. ''That's a 55,'' says another excitedly. ''Could it be a learner? A young one?'' Then there is another sound. This time it's a feeding frenzy.

None of this is very extraordinary in the world of the bat detective. This small group from the Central Scotland Bat Group have gathered in a house in Dunblane to analyse the calls they have been collecting all evening. They are used to being called eccentric or batty, but they don't care what other people think.

In fact, they are proud to be associated with animals whose

acrobatic lives have come to fascinate them.

''Some of our friends think we're nuts, but, then, they play golf,'' says Fay Pascoe, a stalwart of the group, who, as a biology teacher, has

a background of professional

knowledge to draw on. She and her husband, Ian, began their personal odyssey into the nocturnal world of the furry flyers when they discovered bats hanging in their attic. It turned out that their guests were rare Nathusius' pipistrelles, which are more than a cut above the common or garden pipistrelles.

The couple have also given houseroom to brown long-eared bats, which are particularly endearing because of their disproportionately long, rabbit-like ears attached to a mouse-like body.

The Pascoes, like hundreds of other enthusiasts across the country, are part of a network of volunteers who, every year, take part in surveys of the bat population, such as this week's Sunrise Survey, which is organised by the Bat Conservation Trust. It is these amateur investigations that give Britain a wider perspective than mainland Europe, where the study of bats is strictly the province of academics.

Anne Youngman knows what drives these volunteers to take part. She first saw a brown long-eared bat at close quarters while studying for her degree in science and the environment, and it sowed the seeds of an interest that eventually became a career with the Conservation Trust, where she is their Scottish officer.

It's a job she relishes, despite the fact that the evening work with other enthusiasts goes with the territory. She takes everyone from the Brownies to the Women's Rural Institute on batting expeditions as dusk turns to darkness. Although she is too diplomatic to say so, her favourites seem to be the boys from the BB and cub groups who get really excited when the bat detectors begin beeping. ''You get a raspberry noise

at the end of the call, which causes great amusement,'' she says with a laugh. ''Then they get so absorbed, that, if you're near a river, you have to be ready to stop them falling in.''

The chance to examine a bat close up is offered by John Haddow, another biology teacher with expertise in the species. Someone has brought in an injured bat, whose chocolate-coloured face identifies it as a common pipistrelle.

It is a tiny creature dwarfed by John's fingers. On close examination one wing is so badly damaged the bat will never fly again. It will have to be ''euthanased''.

It is a sombre reminder, before we go out to look for its cousins, that these tiny creatures are fragile and vulnerable. Bats have traditionally had a bad press as creatures of the night, as blood-suckers, or agents of something evil, but Anne Youngman says she's never seen anyone who has failed to be charmed once they make the acquaintance of the miniature mammals.

Nicky Credland would certainly fit that category. She is an enthusiastic amateur who has become interested in birds and particularly birdsong through her husband's interest in wildlife.

After discovering a roost of bats near their home in Powmill, she wanted to know more and is becoming hooked on their echo-location calls.

Anne Youngman believes it is those calls that are the key to finding out more about bats. She is convinced that we are still on the beginning of a journey of discovery.

''As bat detectors become more sophisticated, we are discovering more and more about bats, and I

am convinced that we will find

there is far more to know than we imagined.''

As the light fades, we head out towards Allan Water, passing the cathedral, the well-maintained roof of which has no bats, despite the species' popular association with ecclesiastical buildings.

Thanks to a lightning tutorial from Anne, I know that we are likely to spot Daubenton's bats, the ones commonly seen skimming across the surface of smooth stretches of water as they scoop up insects with their feet and stuff them into

their mouths.

First, though, we pick up signals from pipistrelles at both 45 and 55 kHz, meaning both common and soprano varieties are

hunting.

We stop to listen, comparing bat detectors: heterodyne versus time expansion, both of which turn

the ultrasonic calls, which

are not audible to our ears, into sounds we

can hear. After a little practice, it's possible to tune into the bats' echolation language.

Regular listeners can glean much

more information, distinguishing not just

one species from another, but a social call from a feeding frenzy.

We huddle on the pavement, listening as instructed for the ''wet, slapping sound'' that shows we're tuned into the right frequency. Then we are transported to a parallel world governed by its

own rules.

When a light goes on in a house across the street, Ian Pascoe confides that hanging about in the dark has some interesting consequences. On one occasion he was accompanied by his son and son-in-law. ''They were both wearing leather jackets and we must have looked as if we were loitering with intent, because a passing motorist saw us and phoned the police. The first thing we knew was when the lady of the house came out and told me I'd been reported as a suspicious person.''

Ian's interest in bats is now a passion. He and Fay have charted the bat population of their local area, which includes a colony of whiskered bats, a species thought until recently to be confined to England and Wales, and he's a regular leader of bat walks for members of the public.

As darkness encloses us, Jules Agate, the field project officer of the National Bat Monitoring Project, suddenly breaks off her cool analysis of bar surveys.

Her earrings, necklace, and sleek hair hidden beneath a large anorak, she scrambles down the river bank hoping for a better view of a Daubenton's bat which has just flipped across our line of vision. For the bat lover, this is what it's all about.

The Bat Conservation Trust's sunrise survey continues for the next two weeks. For more information on how to take part, call 020 7627 2629 or log on to www.bats.org.uk