The Half-Hour That Matters Most

There's a moment on a warm evening in late spring when the garden seems to change hands. The blackbirds go quiet, midges rise over the lawn, and then a dark shape flicks past the apple tree — too quick and too angular for a bird. That's a pipistrelle, and it has probably roosted within a few hundred metres of you all day.

Bats don't appear at random. On a still, mild evening, the first pipistrelles typically leave their roosts 15 to 30 minutes after sunset. Cool, breezy or wet weather pushes that later, or keeps them in altogether. Insects are the whole point: one pipistrelle can eat around 3,000 of them in a night, mostly midges, gnats and small moths.

So check your local sunset time, add twenty minutes, and stand where you have a clear view over a hedge, tree line or pond.

Getting to Know Your Pipistrelles

Eighteen bat species live in the UK, but the ones over most gardens are the common pipistrelle and the soprano pipistrelle. Both are tiny — a body barely longer than your thumb, weighing about as much as a two-pence coin — and almost impossible to separate by eye. Flight style is your best clue:

• Common pipistrelle: fast, jerky, erratic flight along hedges, tree lines and garden edges, turning sharply after single insects. Around 10 to 12 wingbeats a second.
• Soprano pipistrelle: similar in size, but more likely over water — ponds, streams, wet woodland — emerging slightly later and flying a little higher.
• Noctule: much bigger, with long narrow wings, out before sunset and flying high and straight over open ground.
• Daubenton's bat: skims low, just centimetres above the surface of a pond or river.

If you only ever see one or two bats, they are almost certainly pipistrelles.

Hearing the Unheard: Using a Bat Detector

Pipistrelle calls sit far above human hearing, which is why bats look silent. A bat detector makes them audible and turns a vague silhouette into a genuinely identifiable animal.

Most simple detectors are heterodyne: you tune to a frequency and listen. Set yours to about 45 kHz for common pipistrelles and 55 kHz for soprano pipistrelles. What you want is a rapid run of dry clicks — somewhere between a wet kiss and a snip of scissors — at 10 to 15 a second as the bat closes on prey. It sounds like a Geiger counter having a busy night.

Detectors that record and display a sonogram are better for confirming a species, but beginners don't need one. A basic heterodyne unit and a pencil will teach you more in one June evening than a month of reading.

• Point the microphone forward at chest height and sweep slowly across the sky.
• Keep quiet: footsteps on gravel and rustling jackets compete with the signal.
• Note the time, temperature and wind for each bat. Patterns appear within a week.

Making Your Garden a Bat Magnet

Bats need insects and roosts, and you can improve both.

Plant for moths. Night-scented flowers — honeysuckle, evening primrose, jasmine, lavender, nicotiana — draw in the moths that pipistrelles hunt. Leave a patch of grass unmown through summer for leatherjackets, craneflies and moth caterpillars.

A small pond, even a sunken bowl with a few stones and oxygenating plants, brings in midges and makes the garden far more attractive to soprano pipistrelles and Daubenton's bats.

Cut out pesticides, and resist tidying every dead branch. Mature trees with crevices and lifted bark, old sheds and gaps under the eaves are all potential roosts. Bats in a roof are protected by law, and their droppings crumble to dry dust rather than staining, because they are mostly insect fragments and not blood.

Lighting matters enormously. Bright white LEDs near a roost delay emergence and make hunting harder. Where light is needed, use a warm, low-level fitting that points down and away from trees, hedges and water, on a motion sensor so the garden stays dark when nobody is out there.

A Year in the Life of a Garden Bat

British bats hibernate through the coldest months in caves, cellars, hollow trees and bridges, becoming active again from late March or April. Peak activity runs from June to August, when females gather in maternity colonies and feed themselves and a single growing pup.

May and June are often the most rewarding evenings: warm, insect-rich, and busy long before true dark. September and early autumn bring mating swarms around roosts, full of chasing and calling, and by November most bats have gone to ground again.

Watching Without Disturbing

The pleasure of garden bats is that you need do almost nothing but stand still and listen. Keep a respectful distance from any roost, avoid shining torches at an emergence point, and never handle a grounded bat. If you find one, contact your local bat group — and if it must be moved, wear gloves.

After a fortnight of watching you'll have more than a list: a particular corner of hedge that always gets a bat at twenty past sunset, and a rough idea of how many are out there. That is the whole appeal. You needn't go looking for British wildlife — on a warm June evening it usually turns up over the lawn.