Bird box material gets discussed as a style question and it is really a welfare one. What a box is made of determines how warm it stays, how long it lasts and whether it is safe for a brood.
Wall thickness, which matters most
The single most important specification and the one decorative boxes fail. Walls should be at least 15mm thick, and closer to 20mm is better.
The reason is thermal. A thin walled box swings rapidly with the outside temperature, overheating on a summer afternoon and losing heat on a cold night. Chicks cannot regulate their own temperature well, and a thin box puts them through the whole range of a British spring day. Thicker timber buffers that considerably.
This is why a solidly built box outperforms a prettier thin one every time.
Which timbers work
Untreated softwood is entirely adequate and what most conventional boxes are made from. It will last several seasons and can be replaced without much cost.
Hardwoods such as oak last far longer and are heavier and more expensive. Cedar is naturally durable and resists rot without treatment, which makes it a good choice if you want a box that lasts a decade.
What to avoid is thin plywood, which delaminates outdoors within a couple of seasons, and any timber that has been pressure treated with older preservatives.
The interior must stay untreated
Non negotiable. Birds are in direct and prolonged contact with the inside of a box, and chicks spend weeks against those surfaces.
No paint, no varnish, no wood preservative, no oil. If the outside is treated, use a water based, animal safe preservative and keep it entirely to the exterior, applying it well before the season so any residual fumes have long dispersed.
Paint on the outside
Fine in principle, with two caveats. Keep it to the exterior, and avoid dark colours, which absorb heat and raise the internal temperature significantly in summer sun. A pale or natural finish is safer, and it is another reason for the north east facing convention.
Roof, drainage and ventilation
Three details that separate a functional box from an ornament.
The roof should overhang the entrance so rain does not run in, and ideally be hinged or removable for cleaning. A sloping roof sheds water; a flat one holds it.
The floor needs small drainage holes so any water that does get in can leave. Without them a box becomes a bowl.
Small ventilation gaps near the top of the sides prevent the box overheating and reduce condensation. Both drainage and ventilation are easily added to a box that lacks them, with a drill and five minutes.
Metal, plastic and woodcrete
Metal boxes are a poor idea, heating and cooling far too quickly. Plastic has the same problem, alongside condensation.
Woodcrete, a mix of wood fibre and concrete, is genuinely excellent and used widely by conservation organisations. It is heavy, extremely durable, thermally stable and squirrel proof. It is also expensive and looks industrial, which is why most gardeners do not buy it.
What ours are, plainly
Our bird houses fall into two groups, and it is worth being clear about both.
The seagrass boxes, such as the small round box and the larger round box with chimes, are woven natural fibre. They are decorative garden pieces that provide shelter and roosting spots, and they will weather over time as any natural material outdoors does. They are not built to the thickness and drainage specifications a nest box needs.
The tree trunk face houses, the wise owl and the grumpy old man, are hand carved from solid wood with an untreated natural finish, which puts them closer to a functional box in material terms though they remain decorative pieces first.
If your aim is specifically to have a brood raised in your garden, buy a conventional timber box built to standard dimensions and hang one of ours elsewhere. A garden can happily have both, and most of the ones we like best do.
Questions people ask
How thick should a bird box be?
At least 15mm, and closer to 20mm is better. Thin walls swing with the outside temperature, overheating in sun and losing heat at night, which chicks cannot cope with well.
Can I paint a bird box?
The outside only, in a pale or natural colour, using a water based animal safe product applied well before the season. The interior must stay completely untreated, since birds are in direct contact with it.
What is the best wood for a bird box?
Untreated softwood is adequate and inexpensive. Cedar resists rot naturally and lasts far longer. Avoid thin plywood, which delaminates outdoors within a couple of seasons.
Do bird boxes need drainage holes?
Yes, small holes in the floor so water can escape, plus small ventilation gaps near the top of the sides. Both can be drilled into an existing box in a few minutes.
Are seagrass bird boxes suitable for nesting?
They are decorative shelters used for roosting rather than boxes built to nesting specifications. For a brood, use a conventional timber box and keep a decorative one elsewhere in the garden.
