If you put up a box and hope for the best, you are leaving the outcome to chance. If you choose the entrance diameter deliberately, you are largely choosing which species can use it, because the hole excludes everything bigger than itself.
These figures are well established and worth getting right to the millimetre. The RSPB and the British Trust for Ornithology both publish guidance and it is worth checking their pages alongside this one.
Hole diameters for British garden birds
| Diameter | Species it suits | Notes |
|---|---|---|
| 25mm | Blue tit, coal tit, marsh tit | Excludes house sparrows and great tits, which is the point if blue tits are what you want |
| 28mm | Great tit, pied flycatcher | Also usable by blue tits, so you may get either |
| 32mm | House sparrow, nuthatch | House sparrows are in serious decline and worth encouraging |
| 45mm | Starling | Also declining sharply, and a box for them is a genuinely useful thing |
| Open front, around 60mm high | Robin, wren, pied wagtail | Needs dense cover, not an open wall |
The reason the small sizes matter is competition. Great tits and house sparrows will take a box with a larger hole and are more assertive than blue tits, so a 25mm entrance is how you reserve a box for the smaller species.
Why perches are actively harmful
A great many decorative bird boxes have a little dowel perch below the hole, and it is the clearest sign a box was designed for a shelf rather than a garden.
British hole nesting birds do not need a perch. They land at the entrance and go straight in. What the perch does is give predators something to hold onto: magpies, jays, grey squirrels and cats all find a perch extremely helpful while reaching into a nest.
If a box you own has a perch, removing it improves the box. That is genuinely all there is to it.
Depth below the hole
Less discussed and nearly as important. The floor should sit at least 125mm below the entrance hole, so that a cat or a corvid reaching in cannot get to the chicks.
Shallow decorative boxes fail this test more often than they fail on hole size, and it is worth measuring inside a box before you hang it.
Open fronted boxes
Robins, wrens and spotted flycatchers use open fronted boxes rather than hole entrances, and the requirements are different. They want an open front around 60mm high, and crucially they want dense cover: a hedge, ivy, a climber on a wall. An open fronted box on a bare fence panel is a feeding station for predators and will usually stay empty.
What our boxes actually are
Worth being straightforward here. Our seagrass bird boxes are decorative garden pieces that provide shelter and roosting spots, and the wise owl and grumpy old man tree trunk faces are hand carved from solid wood in the same spirit.
They are not certified timber nest boxes built to BTO dimensions, and we would rather say so than imply otherwise. Small birds do use shelters of this kind for roosting, particularly in cold weather, and they add real character to a garden. If your aim is specifically to have blue tits raise a brood, buy a conventional timber box built to the dimensions above and put one of ours somewhere else in the garden.
Both things can be true, and a garden with a good nest box and a beautiful shelter is better than a garden with only one of them.
When to have boxes up
By late February at the latest. British birds prospect for nest sites from late winter, and a box that appears in April has missed most of the decision making. National Nest Box Week runs from 14 February each year, which is a useful annual prompt.
A box put up in autumn is better still, because it weathers over winter and gets used for roosting, which is how birds discover it.
Our guide to where to put a bird box covers siting, which matters nearly as much as the hole.
Questions people ask
What size hole does a blue tit need?
25mm. That diameter also excludes house sparrows and great tits, which is how you reserve a box for the smaller species rather than losing it to a more assertive one.
What hole size suits house sparrows?
32mm. House sparrows have declined sharply in Britain and boxes for them are genuinely worth putting up.
Should a bird box have a perch?
No. British hole nesting birds do not need one, and a perch gives magpies, squirrels and cats something to hold while reaching into the nest. Removing a perch improves a box.
How deep should a nest box be?
The floor should sit at least 125mm below the entrance hole, so a predator reaching in cannot reach the chicks. Shallow decorative boxes commonly fail this.
Are seagrass bird boxes real nest boxes?
They are decorative shelters that birds use for roosting rather than certified timber nest boxes. If you specifically want a brood raised, use a conventional timber box built to standard dimensions.
