Wild Owl Nest Boxes
If you have the facilities and the environment is right for the Owls, erecting a nesting box for those species that are known to use them - Barn Owls, Tawny Owls and Little Owls - will serve a very useful function in providing suitable accommodation in which Owls can raise their young safely and securely.
When built from the right materials, artificial nestboxes are likely to be better designed, better built and able to withstand the rigours of weather, as well if not better than their natural counterparts. They can also be sited close to the all-important recognised prey sources - this will prove especially beneficial in winter, when any resurgence of the owl population is most likely to suffer due to the lack of food.
Regarding materials, for all exterior boxes it is necessary to choose marine grade plywood or other robust timber that will weather well. Do not use CCA pressure-treated timber or tropical hardwoods. 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 and only use selected water-based preservatives which are known to be safe for animals. It is essential to drill several drainage holes to the bottom of each box to enable it to remain dry inside.
As owls do not build their own nests inside a box and cannot nest on bare boards, placing a 1” (2-3 cm) layer of woodchips or similar material (but not straw) in the box is a preferredl option, though owls will lay eggs on top of their own absorbent pellet material.
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.
To find out more about building and siting owl nest boxes, download our free booklet "Saving Britain's Owls":
Barn Owl Nest Boxes
The preferred “natural” nest sites of Barn Owls include hollows in high trees, tall rural buildings (old and new), rock crevices & holes in cliffs and though Barn Owls like their privacy, successful boxes can be sited near to quite busy locations such as farmyards. ln all cases, however, the predominant requirement is shelter from rain, due largely to the Barn Owl’s lightly oiled plumage, which makes them more prone to soaking than other species.
So even when contemplating an indoor box - best when there is little risk of immediate disturbance and a permanent & visible means of access (Barn Owls look for holes, not boxes) - the idea is to create a cavity space that is enclosed on at least three sides and has at least one internal baffle to allow the inhabitants to hide away from view.
An important design element of Barn Owl nestboxes is the provision of a front shelf or tray, which will allow fledglings to exercise their wings before first flight - Barn Owl chicks take a long time to fledge (about 8 weeks on average) and, once fledged, remain dependent on their parents for a further three to five weeks.
The ideal size of the Barn Owl box is large - a minimum of 15” x 13” x 25” is recommended, with 6” ledge and 8” x 6” entrance (36 x 32 x 60cms with 15cms ledge & 19 x 15cms entrance). A removable lid or panel should be included for qualified inspection and occasional clearing out between occupants. A converted tea-chest also makes an ideal indoor nesting box for a Barn Owl.
An indoor box should be securely nailed to beams, ensuring that no nails, screws or other sharp edges or points are left to protrude into the box or any part on which the birds might perch. It should be positioned as high as possible - at least 10 feet (3m) above ground level, but in the case of unventilated modern farm buildings, below the apex of the roof, where it can get very hot in summertime. It is best to have the front opening of the box directly facing the owl’s point of access to the building. Alternatively Barn Owl boxes can be fixed under the eaves of remote buildings and are likely to be equally as successful if they provide privacy, shelter from the rain, direct access and are spacious enough.
Exterior Barn Owl boxes are best positioned in trees well out of reach of the casually curious, (but not so high that any stray owlet might be killed in the fall to earth) either at the edge of woodland overlooking open land, or in large isolated trees or on purpose-built poles located on open meadow or grassland at a location at least half-a-mile (1km) from the nearest busy road. The box needs to be sited in or close to habitat that has an abundance of suitable prey - small mammals, etc. Visibility from the nest and a clear flight path to it are essential. Equally important, the box needs to face away from the prevailing wind & rain direction, usually towards the south east.
Tree-sited nestboxes are more likely to be used by other species than those in buildings. The box should be firmly and securely fixed to its support. Fixing a box to a tree with nylon cable ties or bolts is certainly the most conservationally correct. If possible, the box should be angled so that the floor slopes slightly away from the entrance, so that eggs remain dry & protected.
Tawny Owl Nest Boxes
The Tawny Owl is a woodland bird and as one of the most adaptable of owl species, can be found even in urban and garden environs. Tawny Owls are content to nest in smaller cavities than Barn Owls and the designs for suitable nest-boxes reflect this.
The upright kind can be fixed to tree trunks or larger branches and measure up to 36” (90cms) in length; chimney nestboxes - which replicate the hollow ends of large broken branches - can be longer, up to 60” (150cms) in length and can be strapped to the underside of sloping branches with an angle of c. 45 degrees. Both types of box need an entrance of not less than 10” (25cms) square which should face away from prevailing wind and rain; should have several drainage holes at their base and need to be sited at least 12 feet (3.7m) from ground level.
Because, unlike Barn Owls, Tawny Owl chicks start to explore their surroundings well before fledging, the box needs to have nearby branches they can climb over. As a possible garden dweller, it should be appreciated that Tawny Owls are sensitive to disturbance when on eggs, and can also be aggressive, dangerous even, if they or their young are approached. So it is best to leave them well alone while they are nesting and you need to consider the likely proximity of young children in the garden before putting up the box.
Little Owl Boxes
Little Owls don’t build nests but rely on ready-made natural nesting sites including hollow branches and rabbit burrows. They will also nest in holes in buildings, cliffs, haystacks and nestboxes which are sited on farmland, orchards, parkland, sand dunes and industrial wasteland where their preferred prey of invertebrates (worms, beetles, slugs, earwigs & moths) supplemented with small mammals, is prevalent.
Either box-type or tube-type designs are suitable, the ideal size of which is 8” x 8” x 15” (37cm x 19cm x 19cms). A critical measurement is the size of the entrance hole - a 3” (7cm) aperture allows the Little Owls easy access but prevents the much larger Tawny Owl from entering. Depending on design, they can be attached to tree trunks with cable ties, or wired to the underside of branches. Ensure that no nails, screws or other sharp edges or points are left to protrude into the box or any part on which the birds might perch.
Young Little Owls begin to leave the nest site at about 3 weeks and like to hide in surrounding vegetation and branches to minimise the risk of the entire clutch being taken by a predator, so bear this in mind when siting your box. Young Little Owls fledge at about 5 weeks, but the parents continue to feed them for another month, at which point they are independent and Ieave the nest to establish their own territory.