Build a bird box: proper instructions

A bird box is most useful when it is sized for a particular type of bird, kept dry and mounted where predators and people cannot easily interfere with it. This guide makes a plain small-hole nest box suitable for species such as blue tits, coal tits and great tits. A 25 mm entrance suits blue tits; 28 mm admits great tits and tree sparrows. Do not enlarge a finished hole casually: a larger hole also admits more competitors and predators. Build in autumn or winter if possible, so the box is weathered and ready before nesting begins.

Materials and tools

• Untreated exterior-grade softwood, ideally 15–18 mm thick; one board roughly 150 mm wide and 1.2 m long is sufficient.
• Exterior wood screws, about 35–40 mm long; use screws rather than glue so repairs are possible.
• Galvanised or stainless screws for fixing the box to its support, plus a suitable spacer if fitting to a tree.
• Hand saw or circular saw, drill, 25 mm or 28 mm wood bit or hole saw, 3 mm pilot bit, screwdriver, tape measure, square, pencil and sandpaper.
• Eye protection, hearing protection for powered tools, dust mask for cutting, and a clamp or stable workbench.

Cut list

Cut a 120 mm-wide front and back, two 120 mm-wide sides, a 120 mm-wide floor and an overhanging roof. A practical set of dimensions is: back 300 mm high; front 250 mm high; sides 250 mm high at the front and 300 mm at the back, cut with a sloping top; floor 120 × 120 mm; roof 180 × 180 mm. The roof overhang protects the entrance from rain. The internal floor area should be near 120 × 120 mm and the distance from the floor to the bottom of the hole about 125–150 mm.

Build the box

1. Measure and mark every cut with a square. Mark the two side pieces as mirror images: their top edges must slope up towards the back. Clamp the timber before sawing and keep hands clear of the blade path.
2. Cut the pieces. Smooth only splinters and sharp outer edges. Leave the inside face below the entrance slightly rough; fledglings can use it to climb towards the hole.
3. On the front, mark the centre line. Mark the entrance-hole centre at least 150 mm above the bottom edge. Drill the chosen 25 mm or 28 mm hole cleanly, backing the timber with scrap wood to reduce breakout. Do not add a perch: it is unnecessary for small birds and can help a predator gain purchase.
4. Drill four or five 5–6 mm drainage holes through the floor. Drill two tiny ventilation holes near the top of each side, below the roof line, but do not make large gaps that create draughts.
5. Pre-drill pilot holes in all screw positions. Screw the two sides to the back, then screw in the floor. Keep the floor set slightly above the bottom edges if that suits your construction; drainage still needs to escape freely.
6. Screw the front to the sides and floor. Fix the roof last with two screws at the rear and one or two at the front. For cleaning, make one roof screw removable or hinge the roof with a strip of rubber; the opening must nevertheless be secure against wind and squirrels.
7. Check the box has no internal screw points, wide gaps or loose joints. Do not varnish, pressure-treat or paint the inside. If an exterior finish is wanted, use a water-based, low-odour exterior paint only on the outside and let it cure fully before installation.

Mounting and checks

Mount the box in late winter before birds establish territories. Face the entrance between north and east where practical, avoiding strong midday sun and the prevailing driving rain. A height of roughly 2–4 m gives access for maintenance without making disturbance likely. Ensure there is a clear flight path to the hole, but some nearby vegetation is useful cover. Keep it away from feeders and busy paths.

Fixing to a wall, post or building is usually simpler than fixing to a living tree. If using a tree, never wrap wire tightly around the trunk: use a broad, adjustable tree-friendly strap or a fixing method that allows future growth, and inspect it annually. Avoid protected sites or work that could damage trees. Do not place a box where cats can jump directly onto it, and do not use grease, spikes or traps to deter predators.

Testing, cleaning and common mistakes

• Before mounting, pour a little water onto the roof and check it runs away from the entrance. Look through the hole for daylight around joints and repair major gaps.
• Shake the box gently. It should not rattle, flex or have a loose roof. Confirm drainage holes are open.
• After the breeding season, normally from October onwards, open the box, remove old nesting material and brush it out. Wear gloves, wash hands afterwards and do not use insecticide. Do not inspect an occupied box routinely.
• Never disturb active nests, eggs or young. In the UK, wild birds, their nests and eggs have legal protection; deliberate damage, taking or disturbance may be an offence, with stronger protections for some species. If in doubt, leave the box alone and seek advice from a wildlife organisation.
• Common failures are thin timber that overheats or leaks, an entrance hole too large, no drainage, a box facing hot afternoon sun, and frequent checking during nesting. A box may remain unused for a season; do not move it repeatedly in spring.

Record the installation date and check the mounting each autumn. Re-tighten a loose wall fixing, clear blocked drainage and replace cracked timber. The useful outcome is a safe, dry cavity; occupancy is the birds’ choice.