DUSK by the River Devon and bats are beginning to stir by the Black Bridge, swooping low across the trout rippled water.

At first there is just a couple of them, but another one soon appears, and then another, and within a matter of seconds the river is alive with tumbling bats.

The mayflies are now under sustained attack from trout and bats in an insect feast in the gloaming.

I turn on my electronic bat detector, and the earphones fill with a marvellous mix of rapid-fire staccato clicks from their echolocation calls.

The bat detector is an ingenious hand-held device that picks up the ultrasonic calls of bats as they hawk for insects and converts them into sounds that can be heard readily by the human ear.

The detector also provides information on the frequency range of the calls, which is an important means for identifying species when on the wing.

These calls were coming in at just below 50 kHz, which combined with their behaviour of swooping low over the water, made me deduce they were Daubenton's bats.

This river-loving species is habitually late in leaving their daytime roosts compared to some other types, which further confirmed my identification.

Bats are enigmas, the only mammals that fly and their nocturnal habits further add to their mystique.

Gilbert White, the pioneering 18th century English nature diarist, was fascinated by bats, and wrote of knowing two species – the pipistrelle and the long-eared bat.

In his seminal book, The Natural History of Selborne, he recounted about being much entertained by a tame bat, which would take flies out of a person's hand: "The adroitness it showed in shearing off the wings of flies, which were always rejected were worthy of observation and pleased me much."

After watching the Daubenton's bats on the Devon for a while, I rise from my grass-hummock seat and head for home.

In the fading light, a goosander slides across the water surface on the hunt for fish and a heron lifts into the air on ghostly wings.

I hear a large splash in the water – possibly an otter or beaver, or perhaps a large trout jumping. Night's dark grip had taken hold and the Devon was stirring into life.