Why Hole Size Is the Single Most Important Detail

A nest box is not a generic bird home; it is a very specific piece of habitat, and the measurement that matters most is the diameter of the entrance hole. Choose wrongly and you will either attract nothing or, worse, watch a great tit or a starling evict the blue tit you were hoping to help. For small garden birds in the UK, three options cover almost everything:

• Twenty-eight millimetres for blue tits, coal tits and marsh tits
• Thirty-two millimetres for house sparrows, great tits and nuthatches
• An open front for robins, wrens, pied wagtails and spotted flycatchers

Each one sends a clear signal to a different species, and mixing them up is the commonest reason a nest box stays empty.

Twenty-Eight Millimetres: The Blue Tit Standard

If you want blue tits, a twenty-eight millimetre hole is the gold standard. It is just wide enough for a blue tit or a coal tit to slip through, but too tight for a great tit, a nuthatch or a house sparrow. That small difference matters enormously in spring, when competition for nest sites is fierce.

A good blue tit box has a floor of roughly 100 by 100 millimetres, internal walls around 150 to 200 millimetres deep, and the hole positioned about 120 to 150 millimetres above the floor. The depth gives the chicks a dark, safe chamber, while the hole height stops cats and squirrels reaching in from below. Put the box two to four metres up, facing somewhere between north and east, and avoid a position in full afternoon sun.

Thirty-Two Millimetres for House Sparrows

House sparrows have declined sharply across much of the UK, so a nest box is a genuinely useful thing to offer. They need a thirty-two millimetre hole, which is also the size favoured by great tits and nuthatches. If you put up a thirty-two millimetre box, be prepared for a great tit to move in first.

The key with sparrows is that they are sociable. A single box may be ignored, whereas two or three boxes fixed close together, or a purpose-built terrace of boxes, is far more likely to be used. Mount them under the eaves, on a sheltered wall, or on a mature hedgerow tree, ideally two to four metres up. Sparrows like a clear flight path to the entrance, so keep the immediate approach free of dense twigs.

Open Fronts for Robins, Wagtails and Wrens

Robins, wrens, pied wagtails and spotted flycatchers do not use a hole at all. They want an open-fronted box, essentially a wooden shelf with three sides and a roof. The open front should be at least ninety millimetres wide, and the box itself needs to be deep, around 200 to 300 millimetres, so that the nest sits well back in the shadows.

Placement is different here. Robins and wrens prefer a box tucked into ivy, a climbing plant or a shed wall, usually lower down at one and a half to two metres. Pied wagtails are particularly associated with water, so a box on a wall or post near a garden pond or a damp, boggy corner is an excellent bet. The open front does leave eggs and chicks more exposed, so keep the box hidden from the prevailing wind and out of easy reach of cats.

Siting, Aspect and the Wetland Edge

Whichever box you choose, a few siting rules apply across the board. Face it between north and east so that it does not overheat in summer or take the brunt of driving rain. Mount it at least two metres above ground, and use a metal predator guard around the hole or a baffle on the pole or tree if squirrels and cats are a problem. Never fit a perch below the hole; perches help predators and are no use to the birds.

A wetland or damp garden is a real asset here. Ponds, boggy margins and damp borders teem with insects, and that protein is exactly what growing chicks need. Mud from a pond edge is also valuable nesting material for many species. If you have space, let a corner go a little wild with reeds, rushes or soft planting to give fledglings cover as they leave the box.

Materials, Cleaning and a Few Final Touches

Use untreated timber at least fifteen millimetres thick, with drainage holes in the floor and a small ventilation gap near the top. Avoid paint inside the box, and if you treat the outside, use a water-based preservative rather than creosote or old-fashioned wood treatments. A side-opening or front-opening panel makes cleaning far easier; a box with a hinged roof is awkward.

Clean the box out each autumn, once breeding is finished, usually between September and November. Remove old nesting material and scrub the inside with boiling water, not chemicals. Avoid disturbing the box between March and August, when birds may be nesting or roosting. Finally, a box that is left up through winter does double duty, offering small birds a sheltered roost on the coldest nights. One well-chosen box, matched to the right species, is worth more than half a dozen that are simply the wrong size.