Why the Size of the Hole Decides Who Moves In

Walk into any garden centre and you will find nest boxes in every shape, colour and size. The single most important detail, though, is often the least glamorous one: the diameter of the entrance hole. Get that right and you are effectively writing an invitation addressed to a particular species. Get it wrong and you will either attract nothing at all or, worse, hand a box over to the wrong tenant.

As a rule, small holes suit small birds. A hole of 25mm will admit a blue tit but keep out the larger great tit, while a 32mm entrance opens the door to house sparrows. Boxes with wide openings or no front at all appeal to birds that evolved to nest in sheltered ledges and hollows rather than tree cavities. Choose the box to match the bird you want, and place it where that species naturally looks for a home.

Small-Hole Boxes for Tits

Blue tits are the most willing garden nester in Britain, and they are remarkably unfussy apart from that hole. Go for 25mm and you will almost certainly get blue tits, with coal tits and even marsh tits showing interest in wooded areas. Bump it up to 28mm and great tits will take over; they are bigger, bolder and quite capable of ousting a blue tit pair if the entrance allows it.

Useful detail when buying or building:

• Internal floor around 100–120mm square, with the hole roughly 125–150mm above the floor so chicks cannot spill out.
• Timber at least 15–20mm thick for insulation, untreated inside, with drainage holes in the base and a small gap at the top for ventilation.
• No perch below the hole. Perches help predators, particularly great spotted woodpeckers, and give nothing to tits.
• A side or roof panel that opens easily, so you can clean the box out each autumn.

Hang tit boxes 1.5–3 metres up, ideally facing between north and east so the box does not bake in afternoon sun. A slight forward tilt keeps rain off the entrance.

Open-Fronted Boxes for Robins, Wrens and Spotted Flycatchers

Robins do not nest in cavities, so a standard box with a hole is useless to them. They want a shelf with three sides and a roof, open across the front, tucked into ivy, a climbing rose, a shed wall or a dense hedge. Position it just 1–2 metres from the ground and partly shaded, and keep it well hidden; robins abandon boxes that feel exposed.

Wrens are even more secretive and will use a similar open-fronted design, though they prefer it lower and buried deep in vegetation. Spotted flycatchers take an open box fixed to a wall or tree trunk, typically 2–4 metres up, often close to a branch where they can watch for insects. If you want to try for a flycatcher, offer several boxes around the garden and be patient — this is a declining species that will not be rushed into a decision.

Nesting in Numbers: House Sparrows and Starlings

House sparrows are colonial by nature, and a single box on a lonely pole is not what they are looking for. Fit a group of boxes — three, four or more — close together under the eaves or high on a sheltered wall, with 32mm entrances. Keep the gaps between them small, perhaps 30–60cm, because sparrows like neighbours.

Starlings need something considerably more substantial: a box with a 45mm hole, deep enough for a bulky nest, fixed high under the eaves at 3 metres or more. Starlings are noisy, messy tenants and wonderfully entertaining to watch, so place the box where droppings will not land on a patio or a washing line.

Boxes for Swifts and Other Specialist Nesters

Swifts are a different project altogether. They need a box with an oval entrance roughly 30mm high by 65mm wide, mounted at least 5 metres up, usually under the eaves of a house, with a clear flight path in front. Once swifts find a site they return to it year after year, so a little planning pays off handsomely.

House martins build their own nests, but you can help by providing artificial cups under the eaves, ideally in groups, since they nest in colonies too. Whatever you choose, avoid boxes with a clear plastic window or a metal perch — the first overheats and the second invites trouble.

Siting, Spacing and Looking After Your Boxes

Once you have matched the box to the bird, placement does the rest. A few practical points:

• Face boxes away from the prevailing wind and strong sun; north-east is the safest general choice.
• Space tit boxes at least 5 metres apart to reduce squabbles, and use different hole sizes around the garden to cater for several species.
• Fix boxes securely with a galvanised screw or nail, not wire that can cut into bark, and never disturb a box in use — once a bird has chosen it, leave it alone until the young have gone.
• Clean out old nests each September to November, wearing gloves, and never reuse nesting material.
• Adjustable boxes are worth the extra cost if you fancy switching hole sizes between seasons.

Put up more boxes than you think you need, in more places than you think sensible. Most gardens lose one box to the weather, one to a predator and one to a wasp nest before the season is out — and a garden with four well-sited, species-matched boxes is a garden that will be full of birdsong by spring.