Nestboxes for owls and kestrels
23 August 2006
Many larger species of birds that nest in cavities in trees or in older, undisturbed, buildings are having difficulty finding suitable nesting sites, as trees are felled or blown over and buildings are knocked down or converted.
Well designed and properly sited boxes undoubtedly help. About half the UK population of barn owls now nests in boxes provided by man.
Barn and tawny owls readily take to boxes in areas where they have suitable food to support themselves and their chicks. Little owls will use boxes but tend not to have as much difficulty in finding nesting sites as they need much smaller cavities than the larger species. Kestrels will take readily to boxes as do stock doves. Jackdaws can be a problem, as they may take over boxes intended for other species.
Can I buy a box or should I make my own?
Boxes are available commercially from a number of outlets. If you prefer to make your own, these pages will give you ideas on designs.
The main considerations in choice of materials are durability and weight. Choose exterior grade plywood or other robust timber that will weather well. Large boxes can be very heavy, and so thinner timber of 9-12mm is a suitable compromise thickness.
The exact measurements in the drawings are dependent on the thickness of the timber used, and may need to be slightly adjusted for different thicknesses.
Join the sections together with softwood battens fixed inside the box. Preservative can extend the life of the box, but only apply it to the outside of the box. Only use selected water-based preservatives, which are known to be safe for animals, such as Sadolin. Do not use CCA pressure-treated timber.
It is essential to drill several drainage holes to the bottom of the box. Owls and kestrels do not build their own nests inside a box and cannot nest on bare boards, so place a 2-3 cm layer of woodchips or similar material (but not straw) in the box.
Siting a box
Nestboxes should be installed by November to give the best chance of success the following year, although it may take several years before a new box is used.
Barn owl and kestrel boxes should be located either on isolated farmland trees or at the edge of a woodland, overlooking open land. Tawny owl boxes are best positioned within a woodland. All species require good visibility from the nest and a clear flight path to it. Ideally face the box south-east, but most importantly it needs to face away from prevailing wind direction.
Large nestboxes are heavy and great care must be taken when putting them up, especially when using ladders. The boxes only need to be sited at around 10 to 15 feet (3-5m) high to be successful, though in some circumstances, e.g. if there is a significant risk of vandalism, a higher position may be desirable even though it will make installation, monitoring and maintenance more difficult.
Make sure the box is firmly and securely fixed to its support. Fixing a box to a tree with nails may damage the tree or cause problems when it is ultimately felled. Galvanised nails and screws are preferable to wire ones, but nylon bolts are probably even better.
Alternatively, boxes can be fitted to trees with a length of wire inside a piece of hose or other protection around the trunk. Trees grow and fixings should be checked from time to time. Angle the box so that the floor slopes slightly away from the entrance. Should the eggs roll about, this ensures that they remain in the sheltered end of the box.
Boxes will need to be cleaned out periodically when debris has built up. This is easier if an ‘inspection hatch’ is added when the box is built. Hinge the hatch along the top edge, and secure firmly with a catch. Jackdaws and grey squirrels, which often use nestboxes, will unfortunately fill up a box with debris very quickly.
Great care should be taken when inspecting boxes or removing any material from them. The material may contain sharp objects such as thorns, and a squirrel or even a tawny owl may remain in the box until the last moment, bolting out only as you put a hand in the box. Do not breathe in the dust from the nesting material and dried droppings. Replace the floor panel if it shows signs of rotting.
We'd like to thank Peter Wilkinson for his extensive help with the revision of this information.