When do bats hibernate in the UK?

When do bats hibernate in the UK?

The Woodland Trust puts the start of UK bat hibernation at November, and says bats "may not be fully active again until mid-May". The Bat Conservation Trust (BCT) gives the deepest months as December to February, with limited activity picking up in March and bats mainly out and hungry by April.

They aren't really disagreeing. Bats enter brief periods of torpor (a short, shallow version of hibernation) from October onwards whenever the weather turns cold, then settle into proper hibernation when there aren't enough insects to make flying worth it. The BCT describes the settled state as "a state of inactivity characterised by lower body temperature, slower breathing, and lower metabolic rate". It is cold that decides, not the calendar.

So a mild October keeps bats out later. A sharp cold snap pushes them in sooner. In most years, late November to late March is the honest answer.

Where bats spend the winter

The Woodland Trust estimates about three-quarters of UK bats roost in trees, especially mature or veteran ones with natural cavities or holes made by woodpeckers. Where there aren't suitable trees, bats move into human buildings: brick walls, roof voids, barns, cellars and abandoned outbuildings. Caves and old mines work for the species that use them, so long as the site stays at a constant, cool temperature and is free from damp and frost.

Garden bats in southern England and the Midlands are often common or soprano pipistrelles, Britain's smallest bat species. In the game, the common pipistrelle is the one hunting over the shed roof in summer. In winter you won't see it, because it has crawled into a crevice somewhere: a loft, a cladding gap, a hollow tree.

Why bats sometimes wake in winter

Hibernation isn't continuous sleep. The Woodland Trust says a bat's heart rate falls to about four beats a minute in hibernation, from more than 1,000 beats a minute in flight. That drop is what makes months without food survivable. But it isn't permanent.

Bats wake up for two reasons. The weather turns mild, and there are a few midges or moths on the wing, so it is worth burning some precious fat to go out and feed; or something disturbs the roost. The Woodland Trust puts it plainly: "If you spot a bat before the weather gets warmer, it's probably been rudely disturbed and is looking for a snack."

Each wake-up costs a lot of fat, so bats that are disturbed repeatedly over a winter may not survive it. That's the reason behind most of the advice below.

What to do if you find a bat in your house in winter

The Bat Conservation Trust runs the National Bat Helpline on 0345 1300 228, Monday to Friday, 9.30 to 4.30. That is the number to ring for anything bat-related: a bat flying inside, a grounded bat, a cat catch, or a bat you have found tucked behind a curtain.

A few practical points while you wait for advice:

• Don't handle a bat with bare hands. Use gloves or a cloth. UK bats can carry a rabies-like virus, European bat lyssavirus, in a small number of cases: handling bare-handed is the risk.
• If you need to contain it, put it in a ventilated box with a shallow lid of water and leave it somewhere cool and quiet. Don't try to feed it.
• If a cat has caught it, the BCT treats that as an emergency. Ring the helpline the same day, because bats that have been caught by cats need specialist care even if they look unharmed.
• If it is flying inside, open a window and close the door to the rest of the house. A healthy bat will usually find its own way out at dusk.

A single bat in a loft or a cavity isn't a problem and isn't a sign of an infestation. The helpline will usually suggest leaving it alone until spring.

Bats and the law

All UK bats are protected. The Bat Conservation Trust says it is a criminal offence to "deliberately capture, injure, or kill a bat", to "deliberately disturb a bat", or to "damage or destroy a breeding site or resting place", even when the roost is unoccupied. The protection sits under the Wildlife and Countryside Act 1981 (in England, Wales and Northern Ireland) and under the Conservation of Habitats and Species Regulations 2017 across Britain.

The practical version is this. Before any loft insulation, re-roofing, tree surgery or demolition, call the helpline. The BCT can advise on whether a bat ecologist survey is needed, and the time of year it should happen. Most building work on a known or suspected roost needs a licence from the statutory nature body for your country, and starting without one is a problem the courts take seriously.

Helping bats through winter

Most of the help is doing less.

• Leave old buildings, cladding gaps and hollow trees alone in winter. If you are planning work, do the survey first.
• Keep cats in at dusk in late autumn and on mild winter nights, which is when bats are most likely to be stirring.
• Put up a bat box before next spring if you have a south- or south-east-facing tree or wall at four metres or more. The BCT publishes designs.
• Don't shine outdoor lights at trees, roofs or walls where bats might emerge. Bright lights push many species away from their roost entrances.
• Record what you see. The BCT's National Bat Monitoring Programme uses garden sightings and droppings to track populations.

Bat populations have "declined considerably over the last century", the BCT says, from habitat loss, roof and barn conversions, cat attacks, and light pollution. Gardens are one of the places most of us can help, and most of the help is giving a tired, cold-blooded mammal somewhere quiet to sleep until April.