Why a well-made box makes a real difference

Blue tits, great tits, house sparrows and tree sparrows all raise their young in holes. What they need is a dark, dry cavity with a small entrance and a floor wide enough for a nest of moss, feathers and wool. The trouble is that old trees with rot holes, and the gaps in timber barns and outbuildings they once used, are far scarcer than they were. Modern gardens are tidier, hedges are cut harder, and many sheds are now metal.

A nest box puts a safe cavity back into the landscape, and it does it in a place you can watch from the kitchen window. The single most important decision you will make is the size of the entrance hole. Get that wrong and you will either exclude the birds you wanted or open the door to bigger, more aggressive species. Get it right, use untreated timber, and a box can last a decade and raise brood after brood.

Choose untreated wood, and not much else

Use untreated timber between 15mm and 20mm thick. Preservative-treated timber, and any wood stained or painted on the inside, can give off fumes in the enclosed space of a box, and some birds will simply avoid it. MDF, chipboard and thin plywood are poor choices too — they swell, delaminate and fall apart after a couple of wet winters.

• A plank of untreated softwood roughly 1.5m long and 150mm wide is enough for one box
• Galvanised nails or stainless steel screws, which will not rust through
• A 32mm drill bit or hole saw
• A small offcut of metal or tough plastic to reinforce the entrance

If you want to extend its life, treat only the outside with a water-based, bird-safe timber treatment, and never the inside face or the entrance hole itself.

Cutting and assembling a simple box

These dimensions suit blue tits and sparrows, and they are forgiving if you are a millimetre or two out:

• Floor: 150mm x 150mm
• Front: 150mm wide x 220mm tall, with the entrance hole cut in it
• Sides: 150mm deep, 220mm tall at the front sloping up to about 270mm at the back
• Back: 150mm wide x 320mm tall, with the extra length left above the roof for fixing
• Roof: 200mm x 200mm, giving a generous overhang at the front and sides

Fix the sides to the floor and back first, then add the front. Drill four 5mm holes in the floor for drainage, and leave a small ventilation gap under the roof or drill two 10mm holes just below it. The roof should be removable — a single screw at each corner is plenty — so you can clean the box out at the end of the season. Do not add a perch below the hole. It looks charming, but it gives predators and competing birds something to stand on, and it is of no use to the birds inside.

Why 32mm is the right size

A 32mm entrance is the standard for blue tits, great tits, house sparrows and tree sparrows. It lets them in comfortably while keeping out most of the larger, more boisterous birds that would otherwise take over the box or predate the chicks. If you are hoping for coal tits or marsh tits, use 25mm instead. Starlings need something much bigger, at around 45mm.

Position the hole so its centre sits roughly 150mm above the floor — deep enough that the nest is sheltered and dark, and high enough that the chicks cannot tumble out. Never go below 125mm. A plain round hole is best; sawing it into an oval or adding a tunnel gives predators more to grip.

There is one weakness. Great spotted woodpeckers can peck a hole wider, and grey squirrels will chew at the edges. A metal plate, or even a square of tough plastic, screwed over the entrance with the same 32mm hole drilled through it, solves both problems and adds years to the box.

Siting the box

Position matters as much as the build. Work through this list before you drill any fixings:

• Face the box between north and east, so it avoids the hottest midday sun and the prevailing wet wind
• Mount it two to five metres up, well out of reach of cats, on a wall, fence, shed or tree trunk
• Tilt it very slightly forward, so rain runs off the roof rather than into the entrance
• Keep it away from bird feeders, where traffic and predators gather
• Leave at least ten metres between boxes of a similar size, as blue tits defend a territory
• Fix it firmly with brackets or a strap, not a single nail that lets it swing in the wind

Put it up by February if you can. Birds prospecting for nest sites in early spring will find it, and some will use it as a winter roost before that. Once a pair has settled, leave it alone.

Cleaning, repairs and watching well

Once the young have fledged, usually by late July or August, take the box down or open the roof and clear out the old nest material. It harbours fleas, mites and parasites that will make next year's brood harder to raise. Scrape it out, rinse with boiling water and let the box dry thoroughly in the sun before replacing it. Avoid insecticides, disinfectants and soap. Check the fixings and the roof at the same time, and replace any softened or split timber.

While the box is occupied, watch from a distance and resist the urge to open it — a disturbed female may desert the clutch. A pair of binoculars and a chair in the shade will show you far more than a torch ever will. If you find wasps or a bumblebee nest inside, leave them be until autumn; they will be gone by then, and the box will be ready for the birds again in spring.