Why old stone bridges are the best bat-watching spots in Britain

There is something quietly magical about standing on an old stone bridge as the light drains out of the sky. The water below turns from silver to slate, the first midges rise, and then it begins: a flicker, a dark shape, a sudden turn. Bats.

Old stone bridges earn their place as some of the finest dusk-watching spots in the country for two simple reasons. First, many of them are roosts. The gaps and joints in weathered masonry, the sheltered crevices under arches, and the loose stonework on older packhorse and canal bridges make ideal summer quarters for species such as Daubenton's bat and the common pipistrelle. Second, they sit over water. Rivers, canals and mill leats are rich hunting grounds, producing clouds of caddisflies, mayflies and midges that bats exploit with ruthless efficiency.

Put a roost and a larder within metres of one another and you have a site worth returning to, season after season.

Finding your window in the evening

Timing matters more than anything else. Check your local sunset time before you set out, then aim to arrive at your bridge a good 30 minutes before sundown. This gives you time to find a safe spot, settle, and let your eyes adjust. If you arrive in the dark and switch on a torch, you will spend the next twenty minutes seeing almost nothing.

The busiest period is usually the 20 to 45 minutes after sunset. Pipistrelles often appear first, sometimes while the sky is still visibly blue. Daubenton's bats tend to follow slightly later, skimming low and straight over the water surface. On warm, still, humid evenings after a hot day, activity can be exceptional. Wind and heavy rain shut things down quickly.

Late May through September is the core season, with July and August the peak, once young bats are on the wing. A mild October evening can still deliver a few late pipistrelles.

Choosing a spot and settling in

Look for a bridge with a decent stretch of open water on at least one side and a tree line or hedgerow nearby. Bats use these edges as flight corridors and as shelter from wind. A slow, deeper stretch of river is often better than shallow riffles, though both will produce insects.

• Stand back from the parapet by a metre or two. It keeps you safe, reduces your silhouette against the sky, and gives bats room to pass.
• Avoid the arch itself as a viewpoint if you can see obvious crevices. Bats leaving a roost should never be blocked or lit.
• Wear dark, soft clothing and stand still. Bats are not troubled by a quiet human, but they notice movement and bright colours.
• Bring a torch with a red filter or a dim red lamp. Red light disturbs bats far less than white, and it preserves your own night vision.
• Pack midge repellent, long sleeves and a hat. You are standing next to a river at dusk; the insects that feed the bats will feed on you too.

Once you have chosen your place, stay put. Wandering with a torch beam is the fastest way to see nothing at all.

Listening for echolocation clicks

Your ears will pick up some of it if you stand quietly and close to the water — a faint, dry ticking, like a fingernail on a comb, often described as a series of rapid clicks. This is echolocation, and it tells you what is flying even when you cannot see it.

Most of these calls sit above the range of human hearing, which is where a bat detector earns its keep. A heterodyne detector, the traditional choice, lets you tune a dial through the frequencies until the clicks drop into an audible range. The pitch you settle on is a good clue to the species:

• Around 45 kHz — common pipistrelle, the most familiar bat in Britain.
• Around 55 kHz — soprano pipistrelle, often found near water and woodland edges.
• Around 20 to 25 kHz — noctule, a high, fast flyer that often emerges before full darkness.
• Regular wet slaps just above the surface — Daubenton's bat trawling insects off the water with its feet and tail membrane.

A smartphone app can give you a rough starting point, but a dedicated detector will always be more reliable. Local bat groups often run evening walks where you can try one before buying.

The law, and good manners

All British bats and their roosts are protected by law. That means no torches pointed into crevices, no climbing on bridges to look for roosts, and absolutely no handling. If you find a grounded or injured bat, do not touch it — contact a local bat group or wildlife rescue for advice. Disturbance at a roost, even accidental, is an offence.

Keep dogs on a short lead, leave gates as you find them, and take every piece of litter home. Riverside spots are fragile, and the goodwill of landowners and angling clubs is what keeps them accessible.

Making a habit of it

Bat watching rewards repetition. Return to the same bridge three or four evenings across a summer and you will start to notice patterns — the first pipistrelle arriving at the same minute, the Daubenton's working the same stretch of glide, the sudden silence when a tawny owl calls.

Keep a small notebook: date, sunset time, weather, first bat seen, temperature, and which detector frequency produced the loudest clicks. Within a season you will have a genuine local record, and one that bat groups are often glad to receive. Best of all, you will have learned the rhythm of a place — and there is no better way to spend a warm evening in the British countryside.