By Steven McKenzie Highlands and Islands reporter, BBC Scotland news website

A pipistrelle captured in flight by Hugh Clark

A "wet slap" seemed a good description for the noise pipistrelle make as they hunt insects using echolocation.

Armed with this information and a handful of bat detectors, about 20 people headed for the banks of the River Nairn in search of the animals.

Bat walks led by Highland Council rangers have proved hugely popular.

The one in Nairn, near Inverness, was the fourth to be held this summer.

Before leaving the warmth of the Seamen's Victoria Hall on Harbour Road, rangers John Orr and Mairi Nicolson warned that the miserable summer appears to have impacted on bat numbers.

And it was not looking too hopeful for the group as it trudged off down a riverbank path.

Peering upwards looking for a flitter of action, the only movement was the pin prick of light from a satellite as it tracked across the starry sky.

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.

There was the distinct wet slap of a pipistrelle's echolocation as it flew unseen among the nearby trees.

Ball bearings

The tiny creatures come in two varieties - the masked, or bandit, and the soprano - and are differentiated by the frequencies they emit their squeaks at to create an image of their prey and surroundings.

Youngsters in the group of bat watchers were delighted.

Closer to the river, the detectors were tuned to the frequency of Daubenton's.

The sound they make was described as similar to ball bearings being dropped on a frying pan.

And it was a noise heard almost immediately when the group stood on a footbridge over the Nairn.

The impressive bats flashed into sight in the beams of torch lights as they skimmed over the water and fluttered up to the bridge parapets.