Why a Nest Box Is Worth the Effort

Britain's garden birds have had a rough few decades. Blue tits, great tits, house sparrows, starlings and tree sparrows have all declined, and the reasons are largely the same: fewer hedgerows, tidier gardens, the loss of old trees with rot holes, and modern buildings that offer no cavities at all. A single well-made box in a suburban garden is a small gesture, but when natural nest sites are scarce, small gestures get taken up quickly. In some streets, boxes are so rare that anything put up in late winter is occupied within days.

The good news is that a bird box is one of the simplest woodworking projects you can take on. You do not need a workshop, a router or any great skill. You need a plank of untreated timber, a saw, a drill and an hour of your time.

Wood, Fixings and the Perch Question

Use untreated timber about 15–18mm thick. Tanalised or stained timber gives off fumes inside a sealed box, where there is no airflow to carry them away, and the inside of the box should always be bare, dry wood. Reclaimed boards, an offcut of exterior-grade ply or a piece of rough sawn timber all work well. Ply is not ideal for the front panel, where a bird may peck, so keep solid timber for that face if you can.

• No perch. They are not needed and they are actively harmful: perches give predators, especially cats and grey squirrels, a foothold from which to reach into the hole. They also help nothing, since birds land directly on the entrance.
• Drainage. Drill four small holes in the corners of the floor so any water that blows in can escape.
• Ventilation. Leave a narrow gap between the top of the side panels and the roof, or drill a couple of holes high on the sides. Warm, humid boxes are unhealthy boxes.
• Fixings. Galvanised nails or stainless steel screws will hold up for years. Fit a hinged side or front panel with a hook and eye so you can clean the box out later.
• Roof. Give it a decent overhang — 25mm or so at the front — and a slight slope so rain runs off rather than into the hole.

Dimensions That Actually Work

An internal floor of about 120mm by 120mm, walls about 150mm high at the front and slightly taller at the back to create the slope, and roughly 150mm from the floor to the bottom of the entrance hole. These proportions suit our common small hole-nesters. The hole size is the bit that matters most, and it is worth getting right:

• 25mm — blue tits and coal tits.
• 28mm — great tits and tree sparrows.
• 32mm — house sparrows and nuthatches.
• 45mm — starlings.

Too large a hole lets in bigger, more aggressive birds and makes it easier for predators to reach the nest. Too small and your intended tenants cannot squeeze in. If you want to attract robins, wrens or pied wagtails, build an open-fronted box instead — roughly 150mm by 150mm with the front left partly open — and site it deep inside a climbing plant or hedge where cats cannot sit and wait.

Where to Put It

Position is where most attempts fail, and it is worth taking ten minutes to think it through.

• Height. Between 2 and 4 metres, fixed to a wall, fence post or tree trunk. Higher is often safer from cats.
• Aspect. Face it between north and east. Our prevailing winds come from the south-west, and a box facing into that will take the worst of the driving rain. A little morning sun is welcome; all-day heat is not.
• Tilt. Angle the box very slightly forward so rain runs off the roof rather than sitting on it.
• Predators. Keep it clear of low shrubs, shed roofs and other launch points. On a smooth post, fit a baffle. A metal plate around the entrance hole stops greater spotted woodpeckers and squirrels enlarging it.
• Space. Leave at least five metres between boxes; tits are territorial and will squabble over anything closer.

Timing and a Light Touch

You can hang a box at any time of year, but autumn and winter are best. Birds use boxes as winter roost sites — a dozen wrens or a queue of blue tits tucking in together on a cold night is a genuinely cheering sight — and they will scout for nest sites from January onwards. Aim to have everything up by the end of February.

Once March arrives, leave it alone. Watch from a distance rather than lifting the lid: a brief glimpse of a bird carrying moss, then feathers, then food tells you the nest is active. Resist the temptation to check inside, as disturbance at the wrong moment can cause a nest to be abandoned.

Cleaning and Autumn Upkeep

Clean the box out between October and December, once breeding is finished and any late broods have fledged. Old nests are riddled with fleas and mites, and removing them reduces the parasite load for next spring.

• Empty out the old material and scrub the inside with boiling water — no chemicals, no detergents.
• Let it dry thoroughly before closing it up.
• Check the fixings, clear the drainage holes and make sure the roof is still watertight.
• Note down what you saw this year: which species, roughly when, and how many young. Patterns emerge over a few seasons.

Put the box back up promptly, and consider adding a second one facing a different direction. A pair of boxes, a metre or two apart but with different aspects, gives prospecting birds a choice — and gives you a far better chance of a nest to watch next spring.