Why dormice need a helping hand at the woodland edge

The hazel dormouse is one of Britain’s most charming and elusive mammals. Golden-brown, with large black eyes and a furry tail, it spends almost its entire life in the branches, rarely descending to the ground. Dormice need a rich, connected canopy of hazel, bramble, honeysuckle and oak — exactly what you find along a well-managed woodland edge or a thick hedgerow. But suitable nesting sites are in short supply. Natural cavities in old trees, dense tangles of honeysuckle bark, and abandoned bird nests are all used, yet these can be scarce in fragmented landscapes.

A wooden box fixed to a tree offers a warm, dry alternative. It mimics a tree hole: sheltered from rain, raised above the reach of ground predators, and warmed by the sun on a south-facing edge. For conservationists, these boxes are also a practical monitoring tool. Dormice are difficult to survey otherwise, and a network of boxes can reveal whether a local population is holding on or even expanding.

Choosing the right spot on the edge

Woodland edges are ideal because they receive more sunlight than the interior, so boxes stay warmer — and dormice favour warm nests. Pick a tree or large shrub that is part of a connected canopy. Hazel, hawthorn, blackthorn and oak are all good hosts. Avoid isolated trees, as dormice dislike crossing open ground.

• Height: fix the box 1.5–2 metres above ground, clear of low branches that could give predators a step up.
• Aspect: face the entrance south-east or south to catch morning sun. In very hot summers, a little dappled shade at midday prevents overheating.
• Wind: shelter the box from the prevailing south-westerly winds. A dense hedge or a slope behind the tree helps.
• Spacing: if you are putting up several boxes, place them 15–20 metres apart along the edge. This mimics natural territory spacing and makes monitoring easier.

Look for signs of dormouse activity before you commit: gnawed hazelnuts with a smooth, round hole and tooth marks around the rim are a good clue. So are nests of woven honeysuckle bark in dense shrubs.

Building the box: materials and dimensions

Use untreated timber, 15–20mm thick. Exterior-grade plywood or solid pine works well. Avoid tanalised or chemically treated wood, which can be harmful to wildlife. You will need about a metre of 150mm-wide board, plus a offcut for the roof.

• Base: 150mm x 150mm, with three or four 3mm drainage holes drilled through it.
• Front and back panels: 150mm wide x 200mm high.
• Side panels: 150mm deep x 200mm high.
• Roof: 170mm x 170mm, with a 20mm overhang on all sides.
• Entrance hole: 35mm diameter, drilled in the front panel about 50mm up from the base.

Assemble the box with galvanised screws, pre-drilling to avoid splitting. Make the lid removable but secure — a single screw hook or a small brass catch is ideal. For monitoring without disturbance, you can fit a 3mm perspex panel in one side, but keep it out of direct midday sun to prevent a greenhouse effect. A small strip of rough bark or wire mesh under the entrance helps young dormice get a grip, though they are accomplished climbers.

Fixing the box safely to a tree

The best fixing is one that does not damage the tree. Use a wide strap — an old belt or a length of webbing — passed around the trunk and stapled to the back of the box. Slip a piece of rubber hose or a folded piece of inner tube between the strap and the bark to prevent rubbing. Check the strap each year and loosen it as the tree grows.

If you must use nails, choose two galvanised round wire nails and drive them into the tree’s outer bark only, never deep into the heartwood. Aluminium nails are softer and less likely to split the trunk. Either way, inspect annually and remove the box if the tree is suffering. Angle the roof slightly forward so rain runs off away from the entrance, and make sure the hole is not blocked by leaves or twigs.

Monitoring without disturbing

Dormice are a European protected species. It is an offence to disturb them, so you must not open a box unless you hold a current licence from the relevant statutory conservation body. Licensed surveyors check boxes in dry weather, usually in spring or autumn, and handle any occupants with extreme care. If you are not licensed, you can still monitor by looking for signs.

• Hazelnut shells: a dormouse leaves a smooth, circular hole with distinctive tooth marks around the inner rim. Squirrels and wood mice leave jagged, irregular holes.
• Nests: look for a woven ball of honeysuckle bark, often with a few green leaves, visible through the entrance hole.
• Trail cameras: a small camera aimed at the box entrance can record nocturnal visitors without any handling.

Never clean out a dormouse nest. Dormice often reuse nests, and disturbing them — especially in winter when they are torpid — can be fatal. If you find a nest, simply record it and leave it alone.

A quiet, long-term habit

Putting up a dormouse box is a small act, but it can pay dividends over years. A single box on a good woodland edge might be used for a decade or more, and a series of boxes can help build a picture of how dormice are faring in your area. Keep a simple notebook: date, box number, and what you saw or found. That record, shared with your local biological records centre, becomes part of the bigger effort to conserve this remarkable little mammal. Warm, dry, and safe — your box offers exactly what a dormouse needs.