Why nest boxes need a clean at all

Watching a brood of blue tits fledge from a box on the shed wall is one of the genuine pleasures of a British garden. But once the last chick has gone, the box is left holding a thick, matted layer of moss, feathers, animal hair and droppings — and, rather more importantly, the parasites that lived alongside the brood. Bird fleas, biting lice and, above all, red mite will happily sit tight in the joints and crevices of the wood, waiting for the next family to move in.

Old nesting material also holds damp, which encourages mould and speeds up rot in a wooden box. Left for a few seasons, a neglected nest can reduce the box to something no bird will use. Chicks reared in heavily infested boxes tend to be smaller and less likely to survive to fledging, so emptying and scrubbing out your boxes each year is one of the cheapest, kindest bits of bird care there is.

When to clean the box

Timing matters more than most people realise. Wait until you are certain the box is empty. Blue and great tits usually raise a single brood and are finished by July, but house sparrows and starlings can squeeze in two or three broods and may still be in residence well into August. Swallows and house martins nest later still. So start checking from late August, and be prepared to wait a few weeks longer.

If you open a box and find eggs, small chicks or an adult sitting tight, close it quietly and try again in a fortnight. Never disturb an active nest.

The ideal window in most of the UK is September to October. That gives the wood time to dry thoroughly before the cold sets in, and it comes before many birds start using boxes again as winter roosts — great tits, blue tits and especially house sparrows will pack into a box on a frosty night. It is also well before any bat is likely to take up residence. If you open a box and find a bat, close it at once and leave it alone; all British bats and their roosts are protected by law.

What to take out with you

• A pair of disposable gloves, or thick gardening gloves
• A stiff brush, ideally with a small wire or nylon head for the corners
• A paint scraper or an old blunt knife
• A bucket of hot water, straight from the kettle and left a few minutes to cool
• A clean cloth or sponge
• A bag or bin liner for the old nest material
• A screwdriver, if the lid or access panel is screwed shut

That is the whole kit. No insecticides, no flea powders, no bleach, no creosote and no wood preservative inside the box. Hot water does the job perfectly well, and anything harsh will leave residues that can harm the next brood of chicks.

How to clean a nest box properly

• Put your gloves on before you start, and keep them on until you have binned the old material and washed your hands.
• Open the box and lift the old nest out in one piece if you can. Bag it and put it in the bin — do not add it to the compost heap or tuck it under a hedge, as the parasites will simply find new hosts nearby.
• Scrape out anything caked into the corners and along the joins with your scraper or blunt knife.
• Scrub the inside surfaces, the corners and the underside of the lid with hot water. Red mite hide in exactly those seams, so be thorough.
• Pour hot water through the box to flush it out, then let it drain. Do not soak it to the point of saturation; wood needs to dry, not swell.
• Leave the box open in a sunny, airy spot for a day or two until it is completely dry, then refit the lid or panel.

Do not add fresh nesting material. Birds prefer to build their own, and anything you put in can hold damp.

Repairs, drainage and a few sensible extras

While the box is open and empty, it is a good moment for a quick service. Check that it is still firmly fixed, and that any screws or brackets have not rusted through. Clear the drainage holes in the base and make sure the ventilation gaps near the top are open.

If the entrance hole has been enlarged — often by wear or by a greater spotted woodpecker — fit a metal plate, which also stops squirrels and woodpeckers widening it further. Treat the outside of a wooden box with a water-based preservative if it needs it, but never the inside, and keep it away from the entrance hole. Replace any part that has gone soft with rot. Ideally the box should tilt very slightly forward so rain runs off the roof rather than into the hole.

Which boxes to leave alone

Not every nest should be cleared. Swifts reuse the same nest site year after year, and their nests should be left undisturbed. House martins repair and reuse their mud cups each season, so do not knock them down. Open-fronted boxes used by robins and wrens are worth cleaning if a nest was built in them, but wrens also use them as winter roosts, so finish the job before the first hard frost.

If you have more boxes than you have time for, clean half this autumn and half next. Even doing a few each year makes a real difference — and come spring, a dry, clean, parasite-free box is exactly the sort of des res that will tempt a pair of tits to move straight back in.