Oliver Radcliffe
Start with the hole, not the box
The single most important decision you will make when buying or building a nest box is the size of the entrance hole. Get that wrong and you will either exclude the very birds you hoped to attract or roll out the welcome mat for larger, more aggressive species that will bully smaller ones away. British garden birds have evolved very specific requirements, and a box with a 32mm hole is a completely different proposition to one with a 25mm opening. Before you spend a penny, decide which bird you want to help – then match the hole to the species.
Which box for which bird?
Our common garden hole-nesters fall into a few clear categories. Use this as your starting guide:
- Blue tit: 25–28mm hole. They are our smallest common hole-nester and will happily use a box with a tiny entrance. A 25mm hole also keeps out great tits and sparrows.
- Great tit: 28–32mm hole. Slightly larger and more assertive than blue tits, so give them a box with a 32mm entrance and a roomier interior.
- House sparrow: 32mm hole. House sparrows are social nesters, so hang two or three boxes close together – ideally under the eaves – to mimic their natural colonial habits.
- Tree sparrow: 28mm hole. Less common in gardens, but if you are lucky enough to have them, a smaller hole and a box mounted on a mature tree is ideal.
- Starling: 45mm hole. Starlings need a deep box with a large entrance. They are declining sharply in the UK, so a starling box is a genuinely useful contribution.
- Robin: open-fronted box. Robins do not use holes. They want a box with a wide, open front – roughly 100mm high – tucked into ivy, a shed wall or a dense shrub, low down and well hidden.
- Wren: open-fronted box. Similar to a robin box but smaller and even more concealed. Wrens prefer to nest deep inside hedges or log piles, so site it low and sheltered.
Placement: height, aspect and shelter
Where you hang the box matters almost as much as the hole size. For hole-nesting species, aim for a height of between 2 and 4 metres, clear of easy access for cats. Face the box between north and east – this avoids the hottest midday sun and the wettest prevailing south-westerly winds. A slight downward tilt (about 5 degrees) will keep rain out. Open-fronted boxes for robins and wrens should be much lower, between 1 and 2 metres, and hidden behind foliage or against a wall. Avoid placing boxes directly beside bird feeders, where traffic and predator attention are high. And do not cram boxes together unless you are targeting sparrows; most tits are territorial and will squabble over prime real estate.
Materials and design details that matter
A good nest box is simple, robust and made from untreated timber at least 15mm thick. Treated wood, plywood and soft plastics can overheat or leach chemicals, so stick to untreated hardwood or thick softwood. Look for these features:
- Drainage holes in the base to stop water collecting.
- Ventilation slots near the top to prevent condensation.
- No perch below the hole. Perches help predators such as grey squirrels and woodpeckers reach inside.
- A removable roof or side panel so you can clean the box out each year.
- A metal plate around the entrance hole if squirrels or woodpeckers are a problem – they will otherwise enlarge the hole and ruin the box.
If you are building your own, use galvanised nails or stainless steel screws, and make the floor slightly recessed to keep the nest dry. A small groove or kerf cut into the inside of the front panel below the hole will give fledglings a foothold when they climb out.
Cleaning, maintenance and timing
Nest boxes are not permanent fixtures. Clean them out once the breeding season is over, usually from September onwards. Remove old nesting material, scrape off any droppings and scrub the inside with hot water – never use chemicals or bleach, which can harm future occupants. Let it dry thoroughly before replacing the lid. Repair any damage and check that the fixings are still secure. Put your box up by February if you want it used for the coming spring, though autumn placement allows birds to discover it as a winter roost. Do not open the box between March and August, as you risk disturbing nesting birds and may be breaking the law.
A few final touches for a wildlife-friendly garden
The best nest box in the world will not help if the surrounding garden is hostile. Leave a patch of long grass, plant native hedging such as hawthorn, blackthorn or hazel, and avoid pesticides and slug pellets that reduce insect prey. Provide a shallow dish of water for drinking and bathing. Keep cats indoors at dawn and dusk during the nesting season. And be patient: it can take a year or two for birds to find and trust a new box. Once they do, you will be rewarded with one of the most intimate and fascinating spectacles British wildlife has to offer.
