Pipistrelles can be heard in autumn calling for mates
Bat watchers are preparing for a seasonal phenomenon which sees large numbers of the mammals swarm at the entrances to caves and tunnels.
The behaviour comes at a time when males "sing" to attract a mate.
Anne Youngman, Scottish officer with the Bat Conservation Trust, said autumn swarms continued to intrigue experts.
Biologists at the University of Leeds are studying how females pick mates from among masses of males during what was described as "bat discos".
Dunblane-based Ms Youngman said: "Autumn swarms see bats home in on a cave or a tunnel.
"They fly miles and miles to the same sites year after year."
In Scotland, Daubenton's and to a lesser extent Natter's bats are found swarming between September and October.
These species along with Brandt's and whiskered can be seen flying in their hundreds in England.
Prof John Altringham, of the Faculty of Biological Sciences at the University of Leeds, is leading research by PhD students at sites this autumn.
His colleague Dr Dean Waters said mystery surrounds the behaviour.
He said: "One of the interesting things is it is almost exclusively males that fly around and round the entrances to mines and caves.
"It was thought they were seeking out hibernation sites, but when you go into these mines and caves you will find some bats, but nowhere near the number that were seen flying around.
"Essentially swarming is a bat disco."
Dr Waters said females have been found visiting different sites and picking the "biggest and sexiest" males as mates.
How they make their selections remains unclear, but theories include calls and scent.
The scientist said: "Males are thought to be faithful to a particular site, but the females will go for one 'nightclub' to another."
Dr Waters said the swarm sites were like "nightclubs"
Autumn is also the time when males of other species seek out mates.
They can be heard "serenading" by the human ear, but the squeaks are better picked up on devices such as bat detectors.
Ms Youngman said the calls varied among the species.
She said: "Noctule bats - our largest bat - seem to have a much laid back approach.
"They peep out suggestively from tree holes and call to females. A male will attract several females - if he sings sweetly enough."
Leisler's bats have a slightly more active approach and make the occasional "song flight" - flying and calling at the same time.
But they mostly hang from favourite perches on trees singing for females.
Ms Youngman said: "Their special 'here I am girls' call is between 18-10kHz so we humans can just hear it as a high pitched squeak.
"Pipistrelles - our smallest bats - are much more active.
"The males spend their time song flighting to attract females and then lure them into bat boxes or the underside of bridges.
"They also spend time chasing off sneaky males trying to sidle up to their harem. A male pipistrelle may attract up to 10 females to a bat box."