Nest Box Entrance Hole: 28 or 32 mm for Tits?

A nest box can look attractive, be clearly visible and be securely fixed to a tree, yet remain empty year after year. The cause is often neither its position, design nor colour, but an inconspicuous detail: the diameter of the entrance hole. Just a few millimetres can determine whether tits move in, sparrows take over, or the box is simply left unused.

The entrance hole is a lock for tits

For tits, a nest box is essentially a temporary tree cavity. Its opening functions like a precisely fitted lock: if the “key” - the body of a particular species - does not fit, the birds will pass the box by.

The correct entrance-hole diameter determines whether tits will accept the box as a nesting site at all.

An oversized opening invites competitors and predators. One that is too small prevents access altogether. Many nest boxes sold in shops may look decorative, but are ornithologically flawed: their holes either allow sparrows to push in or stop tits from entering in the first place.

Ornithologists have stressed for years that the opening diameter matters more than the roof shape, decoration, colour or overall design. Anyone wanting to support tits specifically needs to know these measurements and select them deliberately.

28 or 32 mm: which measurement attracts which tit species?

Not every tit is the same size, so the ideal hole diameter varies accordingly:

• 28 mm – for smaller species such as blue tits, coal tits and willow tits
• 32 mm – for larger species such as great tits

For typical home gardens, the following has proved effective.

28 mm: the comfortable size for small tits

An entrance hole measuring 28 millimetres is the classic choice for smaller tit species. Birds can pass through comfortably, while many potential rivals are kept out. For a child-friendly garden with regular tit visits, 28 mm is almost never a poor choice.

This size often prevents sparrows from settling in. Although they appear similarly sized, they are generally a little bulkier and are more likely to be stopped by the narrower opening.

32 mm: what great tits need

Great tits have a sturdier, more compact build. A 32-millimetre hole is recommended for them. It lets them enter easily without squeezing through and enables them to use the box as a safe nesting place.

If the aim is to attract great tits, choose 32 mm and keep to that measurement as precisely as possible. If the hole becomes noticeably larger, sparrows are more likely to take over the accommodation.

Below the ideal size, the bird cannot get through; above it, the door opens to other species and predators.

How to correct an incorrectly drilled entrance hole

The good news is that a nest box with the wrong diameter does not need to be thrown away. It can be adapted with only a few simple steps.

• Is the hole too large? – Screw a metal or wooden plate with a pre-drilled 28- or 32-mm hole over the opening.
• Is the hole too small? – Use a suitable Forstner bit to enlarge it neatly to the required diameter.
• Smooth the edges – Remove wood splinters so birds cannot injure themselves or damage their feathers.

These metal plates are often available from garden centres. They also make the edge more durable, so woodpeckers and squirrels cannot enlarge the hole as easily.

Protection from sparrows and predators: why millimetres matter

The diameter affects more than just which tit species moves in. It also acts as a safety measure for the brood that follows.

From around 35 millimetres, house sparrows have a good chance of occupying the box. They can be aggressive towards tits and may drive them away from their nesting site. The risk rises further as an opening approaches 40 millimetres: woodpeckers can then insert their beaks, while martens or cats may be able to reach in with their paws for nestlings.

A few extra millimetres at the entrance hole can ultimately decide the survival of an entire brood.

For the best protection, consider not only the measurement but also where the opening sits. It should be positioned in the upper third of the front panel. The higher it is, the harder it becomes for predators to reach inside.

Material, appearance and smell: what really puts tits off

Natural tree cavities are rough, dark and smell of wood and soil. In contrast, many nest boxes from garden centres resemble small decorative houses - pleasing to people, but unsettling to birds.

What matters is:

• Untreated timber, at least 15 mm thick, to provide good insulation from heat and cold
• No varnish or brightly coloured synthetic-resin paint, because solvent smells can deter birds
• No decorative “landing perch” beneath the hole, as it gives predators a useful foothold

A practical small trick is to rub the interior with a little damp soil and allow it to dry. This creates a “used” appearance, which many birds find much more reassuring than brand-new, pale timber.

Hanging a tit nest box: timing, height and direction also matter

Even a perfectly sized box may stay vacant if the timing or location is unsuitable. Tits begin looking for suitable cavities early, and many pairs are already inspecting potential sites in late winter.

This means:

• Install it in autumn or early winter, and no later than the end of February
• Place it between 2 and 4 metres high on a tree, wall or post
• Face the entrance north-east or east to avoid strong winds and midday sun
• Leave at least 10 metres between two tit boxes, as these birds are highly territorial

The box should be firmly secured without damaging the tree. Many people use wire threaded through a piece of old garden hose, which prevents it from cutting into the bark. Feeding stations immediately beside the box are not a good idea: too much activity at the front door causes considerable stress for breeding birds.

Care after breeding: a clean “rental property” for next season

Once the young birds have fledged, it is worth checking inside. Old nests may be heavily soiled and contain parasites. Opening the box in late summer or autumn and removing the old material makes room for the next breeding season.

A basic clean with gloves and a brush is sufficient. Chemical cleaners and scented products are strictly unsuitable, as they alter the natural smell and can make the box less appealing to birds.

When several nest box types are worthwhile in a garden

Slightly larger gardens benefit from a mixed arrangement: several smaller boxes with 28-mm holes for blue tits and similar species, alongside some 32-millimetre boxes for great tits. This helps distribute species more evenly and reduces conflict.

Adding open-fronted nest boxes can also attract other species, including robins and pied wagtails. However, these should be hung well away from the tit boxes so that the different bird species do not interfere with one another.

Why an empty nest box often comes down to one incorrect number

Many garden owners question themselves when a nest box remains vacant: is the position wrong, is there too much noise, or are there too few trees? In many cases, the issue is simply the drill bit - or, more accurately, the chosen size.

Correcting the diameter to 28 or 32 millimetres, creating a rough and natural interior surface, and putting the box up at the right time of year greatly improves the chances of attracting feathered tenants. Then, come spring, blue tits or great tits may be perched at the entrance, feeding their young and turning a simple wooden box into the liveliest spot in the entire garden.