Materials

Nest boxes should be made from wood (or woodcrete - a unique combination of sawdust and concrete). Metal and plastic are unsuitable materials as they may cause the contents of the nest to overheat or allow condensation to build up inside the box, wetting eggs and chicks. The type of wood used is not critical but a box made from hardwood, such as cedar, oak or beech, will outlive one made of pine (softwood). Rather more critical is the thickness of the wood which should be at least 15mm to provide sufficient insulation and to prevent warping. To extend the life of your nest box the outside can be treated with a non-toxic, water-based wood preservative but the inside must be left untreated. It is better to nail your box together rather than gluing it. Do not put a perch on the front of the nest box; this is not necessary for the birds and may give predators a foothold.

Hole sizes

Different size holes are suitable for different species:
choose 25mm or larger for Blue, Coal and Marsh Tit;
28mm or larger for Great Tit and Tree Sparrow;
32mm for House Sparrow.

Access

Nest boxes should have a means of easy access for both inspection and cleaning, and to enable you to record the contents of your box. This can be achieved by using a length of car inner tube or Butyl rubber. Cut to the width of the box and nail it along the back of the box and the roof to provide a waterproof hinge. Make sure you drill a couple of drain holes in the base of your box to ensure that any rain that does get in can drain out quickly.