Choosing the Best Bird Box for Your Garden

Start with the hole, not the roof

Walk into any garden centre in spring and you will find rows of nest boxes that all look much the same: a little wooden cottage with a pitched roof and a round hole. The trouble is that the hole is often 32mm, which is a sort of compromise nobody asked for. A blue tit cannot comfortably use a box designed for a starling, and a house sparrow squeezed into a small blue tit box may abandon it altogether.

If you get one thing right, make it the entrance diameter. Everything else — the timber, the position, the little perch you may be tempted to add — matters far less than whether your intended tenant can actually fit through the door and feel safe once inside.

Match the entrance to your likely tenants

British garden birds fall into fairly predictable size bands, and the hole should be a snug fit rather than generous. A snug hole keeps larger, more dominant birds out and gives smaller occupants a sense of security.

• 25mm — blue tit, coal tit, and most of the small woodland tits. This is the classic small bird box and the one most gardens should own at least one of.
• 28mm — tree sparrow, and a sensible option if you want to exclude great tits while still offering something to slightly larger birds.
• 32mm — great tit, house sparrow, nuthatch and pied flycatcher. The most common size sold, and by far the most over-used in gardens where blue tits are the realistic tenants.
• 45mm — starling. Starlings nest in loose colonies, so two or three boxes on the same wall or tree often works better than one.

House sparrows are sociable nesters and will readily take a terrace of three or four boxes fixed side by side, holes facing slightly different directions. If sparrows have vanished from your street, a terrace is one of the simplest things you can do to tempt them back.

Robins, wrens and flycatchers need a different design

Not every bird wants a hole. Robins, wrens, spotted flycatchers and pied wagtails are all open-fronted nesters, and a box with a round entrance will simply be ignored by them. What they need is a box with an open front — usually with a low lip to stop the nest sliding out — tucked into dense cover.

Robins are notoriously fussy. They favour a box placed low, between 1.5 and 2 metres, hidden among ivy, honeysuckle or a thick shrub, ideally with a partially obscured opening. They will rarely use anything mounted on a bare wall. Wrens are even more secretive and often prefer a natural cavity, but a small open box in a log pile or a dense hedge will sometimes be taken. Hang them at a slight angle if you can, and never in full sun.

Siting the box: wind, aspect and height

In Britain our prevailing winds arrive from the south-west, which means a box facing that way takes the brunt of the driving rain. A wet nest is a cold nest, and chicks chill quickly. Face the box between north and east where you can, or at least shelter it behind a wall, hedge or the trunk of the tree itself.

Height matters too. For tits, mount the box between 2 and 4 metres, high enough to be out of reach of cats but low enough to clean safely from a step ladder. Tilt it very slightly forward so rain runs off the roof rather than into the entrance.

• Avoid full midday sun — overheating kills chicks in a hot June.
• Keep the box away from a bird feeder; constant traffic nearby puts nesting birds off.
• Do not add a perch below the hole. It helps predators far more than it helps birds.
• Space boxes a few metres apart so pairs do not squabble over territory.

Build it well, clean it once a year

Use untreated timber around 15 to 18mm thick, which gives decent insulation. Avoid pressure-treated wood inside, and never treat the inside or the area around the entrance hole. A water-based preservative on the outside only is fine. Make sure the roof overhangs and that the base can be opened for cleaning.

Clean the box out in September or October, once breeding has finished. Remove old nesting material and any parasites, then scald the inside with boiling water and let it dry thoroughly before closing it up. Never disturb a box between March and August. If grey squirrels or woodpeckers are a problem locally, a metal plate around the entrance hole will save the box from being enlarged.

Plant around the box to keep the food coming

A nest box is only half the story. Blue tit chicks need hundreds of caterpillars a day, and those caterpillars come from native trees and shrubs. Oak, birch, willow and hawthorn support the richest insect life, so if you have room for one tree, choose one of those.

For structure and cover, plant hazel, blackthorn, guelder rose and dog rose, and let honeysuckle scramble through them. At ground level, British woodland plants such as native bluebell, wood anemone, primrose, dog violet, wood sorrel and ramsons will thrive in the dappled shade beneath, feeding early pollinators and building the kind of damp, leafy habitat that insects need.

Leave a log pile in a quiet corner, let some leaf litter settle, and resist the urge to tidy every last corner. Do that, and your box will not be a lonely ornament — it will be one small part of a garden that works all year round.