Why the Right Box Matters

Putting up a nest box is one of the simplest and most rewarding things you can do for the birds in your garden. But a box is not simply a box. The difference between a nesting box that sits empty year after year and one that is packed with chicks each spring often comes down to a handful of small details: the size of the entrance hole, how high the box is mounted, and which way it faces.

Get those right, and you give blue tits, great tits, house sparrows and other familiar residents a genuine chance of raising a brood just a few metres from your kitchen window.

Match the Hole to the Bird

The entrance hole is the single most important feature of any nest box. Too large and you invite larger, more aggressive birds such as starlings to take over. Too small and your target species simply cannot get in.

As a general guide for a UK garden:

• 25 mm — blue tits, coal tits and marsh tits. Blue tits are small enough to squeeze through this, and the size also keeps out most larger competitors.
• 28 mm — great tits and tree sparrows. A common all-rounder, but be aware that house sparrows will sometimes try their luck.
• 32 mm — house sparrows and nuthatches. Good for sparrows, though it will also admit great tits.
• 45 mm — starlings. Only use this if you specifically want starlings, as they will dominate smaller boxes.

If you are undecided, a 32 mm hole is a sensible compromise for a mixed garden. One more tip: choose a box with a metal plate around the hole. Squirrels and woodpeckers will happily enlarge a plain wooden entrance, ruining the box for the birds you had in mind.

Open-Fronted Boxes for Robins and Wrens

Not every bird wants to squeeze through a hole. Robins, wrens, pied wagtails and spotted flycatchers prefer an open-fronted box — essentially a wooden shelf with sides, a roof and a low lip at the front.

These boxes are much more exposed, so placement is critical. Site them low down, ideally between one and two metres, and tuck them well into ivy, a climbing rose or a dense shrub. A robin will rarely use an open box that is hanging in the open air, no matter how well built it is.

Height, Aspect and Position

Once you have the right box, think about where it goes.

• Height: between 2 and 4 metres for most hole-nesting birds. High enough to be safe from cats, low enough that you can still check and clean it.
• Aspect: face the box between north and east. This avoids the full heat of the afternoon sun and, crucially, keeps driving rain out of the entrance. A box facing south or south-west will often overheat, and in a wet British summer it may fill with water.
• Tilt: a slight forward tilt helps rain run off the roof rather than down the front of the box.
• Clear flight path: make sure birds can approach without branches, fences or foliage in the way.

Try to avoid mounting boxes right beside a bird table or feeder. The constant traffic can put off nesting birds, and a crowded feeder may attract predators.

Materials, Drainage and Ventilation

Wood remains the best material: 15–20 mm thick, untreated except for a coat of water-based preservative on the outside only. Avoid treated timber on the inside, and never use creosote or similar products anywhere near a box.

A well-designed box has three features that are easy to overlook:

• Drainage holes in the base, so any water that does get in can escape.
• Ventilation near the top, allowing air to circulate on warm days.
• A removable roof or side panel, so you can clean the box out at the end of the season.

Clean your boxes between September and January, once the last brood has fledged. Remove old nesting material, which can harbour parasites, and rinse with boiling water. Never disturb a box between March and August, as you may abandon eggs or chicks.

Getting the Details Right

A nest box is a small investment that pays back many times over. Get the hole size right, mount it two to four metres up with a northerly or easterly aspect, keep it weatherproof and clean it out each winter. Do those things and you stand a very good chance of watching a pair of blue tits ferrying caterpillars to a box full of hungry chicks next spring — one of the finest sights a British garden has to offer.