The best materials for bird nest boxes are harder types of wood, such as cedar, oak or beech, at least 15mm thick, or woodcrete (a waterproof mixture of wood fibres and concrete). These provide good insulation and are difficult for predators to gnaw through to get to a nest.
Avoid metal, ceramic, plastic or resin boxes with thin walls that don’t provide insulation and can become too hot or too cold for chicks to survive. These impermeable materials can also cause condensation inside a box, creating damp and unhealthy conditions.
Caption: Harder types of wood work well for nest boxes
Birds will stay cosy in a box with good wind- and waterproof joints between the sides. Look for a box with the wood grain running vertically down the length of the sides, rather than horizontally across. This encourages water to run off the wood and reduces the chance of it penetrating the end grain, which can lead to dampness and rotting. It’s helpful to avoid the back of the box sitting fully against a tree trunk where it can be dampened by rain running down the surface. Some boxes have wood or rubber battens for mounting the box to help you avoid this.
Caption: Battens hold the box proud of the tree to keep it drier
The size of the hole determines which species of birds are able to use a nest box, with small holes excluding larger birds from nesting. Choose a hole 32mm across to give access to all our common small garden birds or a 25mm hole, if you prefer to restrict access to the smaller tit species and possibly tree sparrows.
Look for a hole that is at least 120mm above the floor of the box. This is something that is often missing from the smaller, commercially available bird boxes. A hole that is too low will allow predators, such as pet cats, to reach in and take the nestlings.
A metal plate around the hole can prevent woodpeckers and squirrels from enlarging it so that they can get inside.
Caption: A metal ring around the entrance hole will stop predators getting in
A sloped roof can help rainwater to run off and discourage predators, such as pet cats, from sitting on top of the box, waiting for fledglings to emerge.
Roofs that project over the sides will help to keep the whole box drier, and a projecting front can also provide useful shelter from the elements over the entrance hole. It will also help to prevent predators from reaching down off the roof into the nest box.
PERCHES AND PURCHASE POINTS
A perch isn’t necessary for small birds, and can provide squirrels and weasels with a foothold to reach into the box to grab eggs or chicks. Avoid protruding bases or other decorative features that could give a predator something to hang onto if they leap to reach your box.
Many bird-watchers like to keep an occasional eye on the progress of nests and this is easier to do if there is an access hatch in the roof, front or side of the box.
It’s useful if you can get your whole hand into the box to pull out any old nesting material and this is usually viewed as a sign that the box is large enough for birds to nest in.
It is preferable if the floor isn’t flush with the bottom of the box, to prevent drips of water from the side of the box entering the end grain of the wooden base and making it damp. However, this is a rare feature on commercially produced boxes. Drainage holes will prevent a nest becoming waterlogged if water does manage to enter the box.
Caption: Drainage holes will prevent the nest getting waterlogged