A nest box is one of the kindest and least expensive things you can add to a British garden. It takes twenty minutes to put up, costs about as much as a decent bag of bird seed, and for a blue tit or a house sparrow it can be the difference between a brood raised and a season lost. But boxes are not one-size-fits-all, and the single most important measurement is the diameter of the entrance hole, because that decides who can get in — and who cannot.

Hole Size Decides Which Birds Move In

Get the hole right and most other things fall into place. Too large, and starlings or jackdaws will claim a box meant for tits. Too small, and your intended tenants simply cannot squeeze through. Most boxes sold in Britain come with one of four standard entrance sizes:

• 25 mm — blue tit, coal tit, marsh tit, lesser spotted woodpecker in wooded areas
• 28 mm — great tit, tree sparrow, pied flycatcher
• 32 mm — house sparrow, nuthatch, tree sparrow where it is competing with its larger cousin
• 45 mm — starling, and occasionally great spotted woodpeckers inspecting in spring

That 25 mm hole is the classic garden choice, and it is worth buying a box with a metal plate around the rim. Tits will peck at a wooden edge until the hole is wide enough to suit a starling, and a cheap reinforcing plate prevents years of slow, accidental renovation.

Open-Fronted Boxes for Robins, Wrens and Wagtails

Not every garden bird wants a cavity. Robins, wrens, spotted flycatchers and pied wagtails nest in sheltered ledges and hollows, so they need a box with three sides, a roof and a floor — no front panel at all. Site these very differently from a tit box. Tuck them deep into cover: inside a climbing rose, behind a thick ivy curtain, in the crook of a mature hedge. Keep them low, roughly one to two metres up, ideally angled slightly forward so rain runs off rather than in.

Wrens are incorrigible. The male will build several nests in several boxes and let the female choose, so if you find a bundle of moss in an open-fronted box that never seems to be used, that is not failure — it is courtship.

Where to Put It: Face It North to East

This matters more than most people realise. An entrance facing between north and east stays out of the strongest sun and away from the prevailing south-westerly rain. A box that faces south can become an oven in a hot June, and overheating is genuinely lethal to nestlings. It also invites driving rain straight through the hole.

As for height:

• Small tits and sparrows: two to four metres, on a wall, fence post or tree trunk
• House sparrows: three metres or more, and they will happily nest in loose colonies
• Starlings: four metres plus, high on a gable or mature tree
• Robins and wrens: one to two metres, hidden in vegetation rather than out in the open

Give the box a clear flight path to the entrance, tilt it very slightly forward for drainage, and keep it a few metres away from busy feeders, where constant traffic can disturb sitting birds.

Materials, Perches and Predators

The best boxes are made from untreated timber at least 15 mm thick, with a sloped roof, drainage holes in the base and a couple of ventilation gaps near the top. Wooden-concrete boxes are heavier but last for decades. Avoid anything painted or preserved on the inside, and avoid boxes with a perch below the hole — the perch does nothing for the birds and gives predators a foothold. A cat can reach almost anything a ladder can, so position boxes away from shed roofs, wheelie bins and overhanging branches, and fit a baffle if the box is on a pole.

Clean It Out Every Autumn

Once the last brood has fledged, usually from late September through to January, take the box down and empty it. Old nesting material harbours fleas, mites and damp, and a clean box is far more likely to be used again next spring. Scrape out the contents, wash the inside with boiling water — never insecticides, disinfectants or household cleaners — and let it dry thoroughly before refitting. If you find unhatched eggs or dead chicks, remove them and clean as normal; it is sad but entirely usual, and it is not a sign that you did anything wrong.

Then Be Patient

A new box may sit empty for a season. Birds need time to find it, and natural holes are often still available nearby. Leave it up year-round, as many birds roost in boxes on cold winter nights, and resist the urge to peek inside during the breeding season. Once a pair moves in, the reward is a front-row seat to one of the best bits of British spring.