It is 9pm and the evening rush hour has begun in a small town on the edge of Dartmoor. In the fading light, bats are starting out on their daily commute from a hidden roost site to feeding grounds several kilometres away.
Standing in a tree-lined street beside their regular route, I wait with local bat expert Pam Barrett and her husband, Phil Wilson, for the first to arrive. Her handheld detector soon begins to emit strange warbling noises as it picks up ultrasonic calls nearby. “Here they come,” she smiles. “Right on time.”
The ragged shape of a large bat appears through a gap in the hedgerow and flies past at speed, banking into a river gully set deep within the shadows of woodland behind us. Then a second, its pale furry body briefly visible as it crosses overhead, followed by a third and a fourth.
The trickle becomes a flood, and Phil rapidly clicks a small counting device as the bats race by, all following the same route across the street and into the river channel, twisting and turning downstream above the dark surface of the water, beneath the arches of a bridge and out of sight.
These are greater horseshoe bats (Rhinolophus ferrumequinum), named after the crescent-shaped noses that they use in echolocation to navigate and find flying insect prey. Communal living and habitual flight paths make this scarce species particularly vulnerable to disturbance and habitat changes. Over the past century they have disappeared from much of Britain and are now largely restricted to southern Wales and the south-west of England, numbering just a few thousand individuals.
Within 20 minutes the air traffic begins to taper off and Phil reveals the final count: 1,092 greater horseshoes, the highest figure recorded here at this time of year. That is roughly half the total number in the roost, says Pam. “The rest take a different route after emerging, and they will all return just before dawn. It is probably the largest regular movement of mammals in Britain.”
As we head off I spot a couple of stragglers weaving through the blackness – late leavers in a hurry to catch up with the rest of the night shift.