Making a Bird Box for Small Woodland Birds

Why Small Woodland Birds Need a Helping Hand

Walk through a British wood in spring and much of the birdsong comes from tiny cavity nesters: blue tits, coal tits, marsh tits, great tits, nuthatches and, in the right places, pied flycatchers and redstarts. All of them need a hole in a tree with a dry, sheltered chamber behind it. Old, decaying trees once supplied those holes in abundance, but tidier woodland management, the loss of ancient trees and the routine removal of dead limbs have made natural cavities scarce.

A well-made nest box is a genuinely useful substitute, and it gives you a front-row seat for one of the best bits of British wildlife watching. The secret is to build it properly, from the right materials, and to put it in the right spot. Get those three things right and a box can raise brood after brood for a decade or more.

Start with Untreated Timber

Timber choice matters more than most people realise. Use untreated softwood — pine, spruce or larch offcuts are ideal — between 15mm and 20mm thick. Tanalised or pressure-treated timber, along with varnish, wood stain and anything creosote-based, gives off fumes inside a small enclosed chamber and can harm developing chicks. Protect the outside if you like, but leave the inside bare.

• Untreated softwood board, 15–20mm thick
• Galvanised or stainless steel screws — plain steel nails rust and work loose
• A metal hole plate for the entrance
• Roofing felt, or a second board fixed over the roof to shed rain
• A brass hinge or secure catch for the access panel

Rough-sawn timber is better than planed, and cutting a few shallow horizontal grooves into the inside of the front panel below the hole gives fledglings something to grip as they clamber up towards daylight.

Getting the Dimensions Right

Different species need different entrances, and the wrong size means either a squabble at the doorway or an empty box. Too large a hole lets in bigger, more aggressive birds and gives predators easier access.

• 25mm hole — blue tit, coal tit, marsh tit. Floor about 100mm square, hole roughly 150mm above the floor.
• 28mm hole — great tit, tree sparrow. Floor 120mm square, hole about 150mm above the floor.
• 32mm hole — nuthatch, pied flycatcher, redstart. Floor 150mm square, with a deeper drop of around 200mm for nuthatch.

Whatever the species, three details matter. Drill a few drainage holes in the floor corners, leave small gaps or drilled holes under the eaves for ventilation, and slope the roof forward with a generous overhang so rain is thrown clear of the entrance. Skip the perch. Perches help predators and do nothing at all for the birds.

Fitting a Metal Predator Guard

A metal plate around the entrance is the single best upgrade you can make. Grey squirrels, greater spotted woodpeckers and even jackdaws will enlarge a wooden hole in minutes, and once the entrance is wide enough the nest is lost. Fit a galvanised or stainless steel plate with a hole matching the box entrance exactly, and make sure the plate stands proud of the wood by at least 25mm so there is no edge to lever against. Fix it with four countersunk screws so nothing snags on a wing.

Then think about access routes. A box mounted on a smooth metal or plastic pole with a baffle beneath is far safer than one nailed to a trunk. If you must use a tree, choose one without low branches and keep the box well clear of anything a cat or squirrel could leap from. Angling the box slightly forward also makes it harder for a predator to reach inside.

Positioning: Face North or East

Position the box so the entrance faces somewhere between north and east. This keeps it out of the strongest midday sun and away from the prevailing south-westerly rain. South-facing boxes overheat, which can kill chicks in a warm June, and west-facing boxes take a soaking that leaves the entrance damp and the nest cold.

A few other rules of thumb are worth following. Mount the box between 2m and 4m up: high enough to feel safe, low enough to inspect without a ladder. Tilt it very slightly forward for drainage. Keep the flight path to the hole clear of twigs and foliage, and site it in dappled shade if you can. Don't place two boxes for the same species within about 10m of each other, as tits defend a territory, and avoid hanging a box right beside a busy feeding station, where the constant traffic puts birds off. Check the fixing is solid — a box that swings in the wind will be abandoned.

Cleaning, Repairing and Watching

Once the breeding season is over, from September onwards, open the access panel and clear out the old nest material by hand or with a scraper. Don't use insecticides, disinfectants or any chemical cleaner. Pouring boiling water through the box and letting it dry naturally is enough to deal with parasites, and the old nest should be binned or composted well away from the box.

Autumn is also the time to check hinges, screws and the metal plate, and to make any running repairs before winter. From March to August, leave the box alone apart from a distant look with binoculars — a single disturbance at the wrong moment can cause a clutch to be deserted.

If you can, note down the dates eggs appear, how many chicks fledge and when, and pass the records to your local wildlife recording scheme. It takes a minute or two, and it turns a pleasant garden project into something genuinely useful. Then hang the box, step back, and let the birds do the rest.