A 3 mm change in hole size can alter nest box species

A 3 mm change in an entrance hole can admit a larger, more competitive bird. Match the clear entrance diameter, box dimensions and placement to birds already using your garden.

Pick the hole size by the bird you want

The entrance is a filter rather than a species label, so measure the clear finished opening after fitting any metal plate.

Match each entrance to its likely user

Use a 25 mm entrance hole for coal tits and other small tits where reducing larger-bird access matters. A 28 mm entrance hole is the standard choice for blue tits and great tits, while 32 mm gives house sparrows easier access. A 45 mm entrance hole is for starlings only when the entire box is larger and securely fixed.

Use open fronts for robins and wrens

Robins, wrens and pied wagtails usually prefer open-fronted nest boxes, with a wide sheltered opening rather than a round drilled hole. A large circle in a closed tit box does not recreate this habitat. Swifts are different again: they need a purpose-built high-mounted box with a narrow horizontal slot, often about 28 by 65 mm.

Compare size, box and placement together

Choose the hole alongside the floor area, chamber depth and mounting height. A suitable entrance does not make an undersized box safe, and the table below is a starting point rather than a guarantee that one named species will occupy it.

Open-fronted nest boxes are measured by the height and width of their open section rather than by a drilled circle. A wren nest box usually benefits from a smaller, well-sheltered opening, often around 50 mm high, whereas a robin nest box commonly uses a more open front of about 100 mm high and should be hidden among vegetation rather than placed on an exposed wall. A swift nest box slot is different again: its low horizontal slot, commonly about 28 by 65 mm, leads into a shallow purpose-built chamber.

Put swift boxes high under eaves or on a tall wall, ideally where swifts already fly, rather than treating the slot as an alternative front for garden birds.

A larger hole invites competing species

Increasing an entrance size can favour stronger, larger occupants rather than simply attracting more birds.

A 28 mm hole can be used by both blue tits and great tits, so it cannot reserve a box for blue tits alone. A 32 mm hole is more suitable where house sparrows are the priority, but great tits may also enter. Where both species occur, use separate 28 mm and 32 mm designs instead of one oversized compromise.

A 45 mm opening is for starlings, not a scaled-up tit box. Choose an internal floor of at least 150 by 150 mm, a depth near 250 mm and a strong fixing on a solid wall or mature tree; a loose box can become unstable when young are nearly ready to fledge.

Predator plates must keep access clear

A predator guard can stop squirrels or woodpeckers enlarging a wooden entrance, but it must preserve the intended clear diameter and sit flush. Avoid sharp edges, loose screws and thick lips that turn the opening into a tunnel; birds need a clean route while carrying food.

Build and place a box birds can use safely

A safe box needs enough internal room, a dry base, weather protection and a stable fixing.

Give eggs space and drainage

For most tit boxes, allow around 180 to 200 mm from the floor to the bottom of the entrance. Drill drainage holes in the base and ventilation holes high in the side walls, then fit an overhanging roof and tilt the box slightly forward so rain runs away from the opening.

Keep feeders away from the nest

Keep feeders and food stations away from nest boxes. Heavy traffic, dropped seed and regular visits from squirrels or cats can make the entrance conspicuous and the site less calm. Do not add a perch: tits, sparrows and starlings can cling to the front board, while a perch may assist predators.

Clean only when the box is empty

Clean only when you are certain the box is vacant, usually in autumn or winter. Remove old material, inspect the timber and fittings, and restore any worn entrance to its original size. Do not open, move or clean an active box during nesting season: the Wildlife and Countryside Act 1981 protects wild birds, active nests and eggs in England and Wales.

Among birdhouse entrance hole sizes, the nest box entrance diameter is only one part of a safe site. Face the entrance away from the strongest prevailing rain and, where possible, avoid prolonged midday sun that can overheat a box fixed to an exposed south-facing wall. A clear flight path matters, but so does cover nearby: a shrub or tree a short distance away gives adults a place to pause without allowing cats or squirrels an easy route to the box.

Nest box mounting height should suit the species, while the fixing should keep the box stable and slightly forward-tilted so water drains away from the entrance.

Keep the nest box entrance diameter separate from the other measurements when comparing plans or products. The diameter is the clear width of the finished hole, including any predator plate; nest box internal dimensions describe the usable floor width and length; and nest box chamber depth is the vertical distance from the internal floor to the underside of the roof. For a closed-front tit box, a chamber depth of roughly 180 to 200 mm can work with the lower edge of the entrance about 125 to 150 mm above the floor.

This separation prevents a deep box from being mistaken for one with an unnecessarily high entrance, which can make access harder for adults and fledglings.

FAQs

What are standard UK bird box hole sizes?

Common UK sizes are 25 mm for smaller tits, 28 mm for blue and great tits, 32 mm for house sparrows and 45 mm for starlings. Robins and wrens generally need open-fronted boxes.

What birds can use a 28 mm nest box hole?

Blue tits and great tits can both use a 28 mm hole. It is a useful standard tit-box size but cannot guarantee blue tits alone.

What birds use a 32 mm hole?

House sparrows commonly use 32 mm holes, and great tits may use them too. Choose this size for broader access or a sparrow terrace.

Is a 25 mm hole too small for blue tits?

Blue tits may use 25 mm, but 28 mm is more usual. Use 25 mm when smaller tits are the priority.

What size hole does a starling nest box need?

A starling box needs an entrance of about 45 mm, a floor at least 150 by 150 mm and secure mounting around 3 to 5 metres high.

Can I make a robin box by drilling a larger hole?

No. A robin box needs an open or semi-open front and sheltered placement, usually 1 to 2 metres high.

Should I put a perch under a nest box entrance?

No. British garden birds do not need one, and it can help predators.

Editorial Team

Our team consists of passionate British birdwatchers and garden wildlife enthusiasts. We provide honest, practical advice on feeding garden birds, choosing the right feeders, and making your outdoor space a haven for UK wildlife — guided by RSPB best practice and real garden experience.