Why the Hole Matters More Than You Might Think

When you hang a nest box, the entrance hole is doing three jobs at once. It lets the right bird in, it keeps the wrong birds out, and it decides how safe the chicks are from predators and from the weather. A blue tit measuring roughly 10cm from beak to tail needs to slip through 25mm without difficulty, but a great tit — noticeably chunkier across the shoulders — simply cannot manage it. Choose too small and the box stays empty. Choose too large and you may find starlings or even squirrels squabbling over it, or a grey squirrel chewing the hole wider until nothing small can nest safely inside.

So the question to ask before buying is simple: which bird am I actually hoping to attract? Match the hole to that species and everything else follows.

The Standard Hole Sizes at a Glance

Nest box manufacturers in the UK have settled on a handful of entrance diameters. These aren't arbitrary — they've been refined over decades of ringing and nest recording.

• 25mm — blue tit, coal tit, marsh tit. The smallest size worth buying, and the best all-rounder for a small urban garden.
• 28mm — great tit, tree sparrow, and blue tits too. The classic "small bird" box.
• 32mm — house sparrow, nuthatch, pied flycatcher. Also lets great tits and blue tits in.
• 45mm — starling, and occasionally jackdaw. Needs a deep, heavy box to be any use.
• Open-fronted — robin, wren, pied wagtail, spotted flycatcher. No hole at all.

If you only ever buy one box, a 32mm entrance gives you the widest range of potential occupants. If you specifically want tits and nothing else, stay at 25mm or 28mm.

Blue Tits and Great Tits: The 25 versus 28 Question

This is where most people go slightly wrong. A 25mm hole is a blue tit box, full stop. Great tits will inspect it, cling to the front, and give up. If your garden has both species — and most do — hang two boxes rather than compromising on one. Put them at least a few metres apart, ideally facing different directions, so the pairs don't spend spring arguing over the same patch.

One lovely detail: blue tits often use nest boxes as winter roost sites, squeezing in together on cold nights. A 25mm box left up all year round earns its keep well beyond the breeding season.

Sparrows, Starlings and the Bigger Visitors

House sparrows are sociable nesters and will happily take a 32mm box, but they prefer company. Three or four boxes clustered under the eaves, or arranged in a loose group on a shed wall, will be far more attractive than a single box in splendid isolation. Keep the cluster away from the busiest part of the garden.

Starlings need a 45mm entrance and a box with real depth — around 30cm from hole to floor — because the chicks grow fast and crowd the base. Starlings are also notorious for enlarging holes with their beaks, so fit a metal plate if you want the box to last. For nuthatches, a 32mm hole with a protective plate is ideal; they will plaster mud around the rim to reduce the opening to their preferred size, which is a wonderful thing to watch.

Robins and Wrens Need Something Different Entirely

Robins are not cavity nesters in the tit sense. They want a shelf or an open-fronted box with a wide, low opening roughly 10cm high, tucked into ivy, a hedge, or against a shed wall behind foliage. A hole box will be ignored, however carefully you position it. The same design suits wrens, pied wagtails, and spotted flycatchers. Site these lower than a tit box — around 1.5 to 2 metres is plenty — and angle them slightly forward so rain runs off.

Getting the Details Right

The hole is the headline act, but a few practical points decide whether the box actually gets used.

• Keep the hole at least 12cm above the floor inside, so chicks can't be reached by a predator's paw through the entrance.
• Never fit a perch below the hole. It looks charming and it hands squirrels and cats a ladder.
• Site boxes two to four metres up, facing between north and east. Avoid full midday sun and prevailing wind.
• Use untreated, rough-sawn timber around 15–19mm thick, with drainage gaps in the floor and ventilation near the top.
• Clean the box out in September or October once breeding is finished, then leave it up for winter roosting.

Measure twice, buy once. A well-chosen box, sited sensibly, will be inspected within days and occupied by spring.