A nest box is not a decorative object for the house wall. It is a home, and like any home its success comes down to three sober details: the diameter of the entrance hole to the millimetre, the compass direction the hole faces, and whether anyone clears it out in autumn. Get these three right and next spring you have a real chance at great tits, nuthatches or sparrows. Skip them and you often hang an empty box in the tree for years and wonder why.
The good news: this is not science, it is care. And the best time to hang boxes and clean the old ones starts now in late summer and runs right through winter, from CW 38 into early spring CW 8. This piece walks you through it step by step.
The Entrance Hole to the Millimetre
The entrance hole is the single most important measurement on the whole box, and it is measured in millimetres, not in "roughly". The diameter works like a doorman: it lets in the species you built the box for and keeps out larger birds that would otherwise take the nest from the smaller ones.
These values have settled in over decades of field observation and are recommended this way by conservation bodies such as the German NABU and the Bavarian LBV:
- 26 to 28 mm: blue tit, coal tit, marsh tit
The smallest common hole. At 27 mm the stronger great tit stays out, so the dainty blue tit gets a fair shot. Ideal if you specifically want to help the small tits.
- 32 mm: great tit, nuthatch, tree and house sparrow
The all-rounder. If you are only hanging one box and you do not mind which species, choose 32 mm. It is the most reliably occupied.
- 45 mm: starling
For the starling, which is louder and larger. A 45 mm box hangs best a little higher and with plenty of clear approach.
- Open-fronted box: black redstart, wagtail, spotted flycatcher
Not a round hole but a half-open front wall. For species that like to brood with a good view. Belongs in a sheltered, covered spot under the eaves.
Measure with callipers if in doubt. A hole drilled to "somewhere around 30" can in practice be 34 or 35 mm, and then a sparrow moves in instead of the tit, or a woodpecker enlarges it. A cleanly drilled 32 mm is a cleanly drilled 32 mm.
Where the Hole Should Face: Aspect and Location
The second big lever is the compass direction. In Central Europe the weather comes mostly from the west to south-west. An entrance hole that faces that way catches every driving rain, and a soaked nest is life-threatening for a brood.
Hence the simple rule: face the hole east to south-east. That way the box gets the mild morning sun but stays sheltered from the harsh afternoon heat and from the weather. North is too cold and dark; south turns into an oven at the height of summer.
Choose the aspect
Aim the entrance hole east or south-east. Use the compass app on your phone if unsure. Facing away from the main weather side matters more than the perfect number of degrees.
Tilt the box slightly forward
Hang the box so the hole points minimally downward, just a few degrees. That way rain runs off the front and does not drip inside. Never tilt it backward.
Height of 2 to 3 metres
This height suits most cavity nesters and sits outside the comfortable jumping range of cats. Higher is fine but not required.
Keep the approach clear
No branch should be directly in the way of the hole. Birds like to fly the hole freely. A twig or two nearby for cover is good; a lattice of twigs right in front of the opening is not.
Also keep distance between several boxes of the same species. Two tit boxes two metres apart lead to territorial squabbles, and in the end one stays empty. Ten metres apart is a good guideline; open-fronted boxes and different species may hang closer together.
Fixing It Tree-Friendly
A detail many overlook: how the box hangs on the tree decides the health of the tree. Wire pulled tight around the trunk cuts in over the years as the trunk grows thicker. It grows into the bark and can ring-bark the tree.
To hang it, use aluminium nails (rust-free, and they will not damage a chainsaw later) or a sheathed hanging bracket slung over a branch rather than tensioned around the trunk. Alternatively, hold the wire off the trunk with a piece of hose or wood so air remains and the tree can keep growing. For house walls and fences there are boxes with a mounting rail. Check the fixing once a year when you clean: is the bracket still seated, is the nail firm, has the wind loosened anything.
Cleaning in Autumn: the Annual Clear-Out
Now comes the part almost everyone forgets and that nonetheless decides the next season. An old nest stays in the box after the young have fledged. It harbours bird fleas, mites, feather lice and other parasites that pounce on the new brood in spring. A clean box is healthier, and many birds avoid a box stuffed full to the top anyway.
The best time is October, CW 40 to 44. By then the breeding season is safely over, and the birds still have all winter to discover the freshly cleaned box as a roost. In mild winters, tits huddle together in a box overnight.
Open carefully and take a quick look inside
Knock first, then open slowly. Nest boxes sometimes host dormice, bumblebee queens, earwigs or lacewings over winter. If someone is home, close up and come back later.
Remove the old nest completely
Take out all the old nest material and throw it on the compost or into the hedge, not directly under the box. That way the parasites do not simply crawl straight back up.
Sweep it out dry
Sweep the box out dry with a hand brush or a wooden scraper. If there is a heavy parasite load, rinse with hot water afterwards and let it dry well. No washing-up liquid, no chemicals, no insect sprays.
Check the fit and that it is watertight
Look at whether the wood is still sound, whether cracks have opened up, whether the roof still protects. A small hole in the floor is good: it lets residual moisture out. If it is missing, drill two or three drainage holes of 5 to 6 mm.
Close it and leave it hanging
Close the box and leave it up. It stays outside all year and serves as a roost in winter.
If you missed the autumn window, it is no disaster. Clean at the latest in late winter through to early February, CW 6 to 8, before the first birds seriously start scouting for quarters. After that, leave the box in peace until the season is over.
Hanging It Up: the Right Time Is Now
Many people only hang the new box in March, just before things kick off. That works, but it is late. Birds scout potential nest cavities as early as autumn and winter, and a box that has hung since CW 38 is long since a familiar part of the territory come spring.
So: hang it up as soon as you have the box. The window from CW 38 (mid-September) to CW 8 (late February) is perfect. A box that spends the winter outside gets tested as a roost and is taken up all the more readily in March. Everything should be up by late February at the latest, because great and blue tits begin their territory search from early March.
Hanging it fits neatly into the garden jobs for September, when there is time to prepare for the cold season anyway. And if you want to support the birds through winter with a reliable offering, you will find the matching companion in the piece on feeding birds year-round and supporting them naturally.
Material and Build: What Matters
Not every pretty box from the hardware store is a good box. A few features separate the home from the decoration:
- Untreated wood or wood-concrete
Solid, untreated wood at least 18 to 20 mm thick insulates well and breathes. Wood-concrete (a mix of sawdust and cement) is even more durable and is barely pecked open by woodpeckers. Both materials are proven.
- No perch in front of the hole
An approach perch looks cute but mainly helps nest raiders like magpies and cats that grab hold of it. Cavity nesters need no perch. Leave it off or saw it away.
- Removable roof or hinged front
Without an opening for cleaning, the box is useless after two years. Look for a hinged front wall or a removable roof with a securing hook.
- Overhanging roof and drainage holes
A roof that projects beyond the hole keeps rain off. Small holes in the floor let moisture out. Together they keep the interior dry.
A nest box is a promise in its first year and a habit from the third year on. Hang it up, tend it, wait.
An old gardener's rule
Who Moves In at Which Hole?
The great tit is the most common and most rewarding box tenant. It reliably takes 32 mm boxes, broods from April and often raises two clutches. In winter it stays in the garden and uses the box for roosting. If you hang only one box, hang it for the great tit.
The blue tit needs the smaller 26 to 28 mm hole and is a touch fussier about the location. The nuthatch will wall a too-large opening down to its preferred size with mud, a fascinating behaviour. House and tree sparrows happily take 32 mm and even brood sociably in a row if you hang several boxes side by side. Starling and black redstart need the larger or open-fronted designs respectively.
Common Mistakes That Leave the Box Empty
When a box stays empty year after year, it usually comes down to one of these: the hole faces into the weather, the box hangs too low and within reach of cats, a branch blocks the approach, the old nest was never removed, or it hangs in far too busy a spot. When in doubt, move it and give it a fresh season. Sometimes two metres of offset and a different aspect are all it takes.
Häufige Fragen
Which entrance hole size for which bird in a nest box?
The hole size determines the tenant: 26 to 28 mm for the blue tit, coal tit and marsh tit; 32 mm for the great tit, nuthatch and sparrow; 45 mm for the starling. For the black redstart, wagtail and spotted flycatcher you do not use a round box but an open-fronted one. Measure the hole with callipers, because just 3 mm more can let in a larger bird instead of the tit. A cleanly drilled 32 mm hole is the best all-round size if you are only hanging one box.
Which direction should a nest box face?
The entrance hole should face east to south-east. That way the box gets mild morning sun but stays sheltered from weather out of the west to south-west and from the harsh afternoon heat. Tilt the box minimally forward so the hole points a few degrees downward and rain runs off the front rather than into it. North gets too cold and dark; south gets too hot at the height of summer.
When do you clean a nest box?
The best time is October, CW 40 to 44, when the breeding season is safely over. Take the old nest out completely, sweep the box out dry and skip washing-up liquid or chemicals. If there is a heavy parasite load, rinse with hot water afterwards and let it dry well. If you miss autumn, clean at the latest in late winter through to early February, CW 6 to 8, before the birds start scouting for quarters. Wear gloves, because parasites lurk in the old nest material.
When is the best time to put up a nest box?
The best time to hang the box is from CW 38 (mid-September) to CW 8 (late February), so in autumn or winter rather than only in spring. Birds scout potential nest cavities as early as winter and use the box as a roost on cold nights. A box that hangs over winter is a familiar part of the territory by March and gets taken up more readily. Everything should be up by late February at the latest, because great and blue tits begin their territory search from early March.
How do you fix a nest box without harming the tree?
Use rust-free aluminium nails or a sheathed hanging bracket that you sling over a branch rather than tensioning tight around the trunk. Bare wire cuts into the trunk over the years as the tree grows thicker and can ring-bark it. If you do use wire, hold it off the trunk with a piece of hose or wood so air remains. Hang the box at 2 to 3 m with a clear approach and check the fixing once a year when you clean.
Spotted a mistake?
