Background

The Barn Owl is one of the most loved and distinctive birds in Britain, but the population has plummeted since the 1930s due to the changes in farming and country life in the UK. There are many people in the countryside attempting to reverse this decline which involves a great deal of time, effort and funding. Fortunately for Barn Owls they are assisted by British and European legislation that protects both the species and its habitats.

The majority of Barn Owls in Britain can be found breeding at low altitudes. This is largely due to a wider range of available prey items, milder winter temperatures and reduced snow cover. In Britain, Barn Owls are almost always associated with open grassland, woodland edge and extensive networks of rough-grassland field margins. Habitat types such as these provide the essential habitat requirements for the Barn Owl’s favoured prey, the Short-tailed Vole, Wood Mouse and Common Shrew. Ideal grassland is permanent and tussocky, with a thick sward and deep thatch or litter layer composed of dried stalks.

The Barn Owl normally requires a large hole in which to nest and rear young, which often is dictated by climate, in particular rainfall. Barn Owls suffer during prolonged periods of rainfall; hunting becomes difficult, breeding success is reduced, and fledgling mortality is increased. Nest sites that are susceptible to heavy rainfall will result in the rapid death of owlets. To be a successful breeding site, a consistently dry location is imperative. Natural breeding sites for Barn Owls are typically old buildings, agricultural buildings, tree cavities and rock fissures. Suitable sites are, however, at a premium, which results in large areas of suitable habitat unused by Barn Owls. This is where nest boxes become invaluable. Nest boxes can be placed in farm buildings, barns and trees, providing adequate artificial nest holes. Breeding Barn Owls show a high level of fidelity to a successful nest site, and will repeatedly use this site provided resources and disturbance remain little unchanged. Additional and specific roost sites are also commonplace, situated close to the nest site, and often in view of the nest (for reasons of defending the nest). It is therefore desirable to provide Barn Owls with a choice of two nest boxes.

Choosing a site

It is worth emphasizing that it is the lack of suitable foraging habitat, and not the absence of nest sites that is the major population limiting factor for Barn Owls. Provision of boxes without suitable prey-rich habitats can be a waste of resources and effort. Where suitable habitat is available, nest box provision is becoming increasingly important, particularly where existing sites are threatened by destruction or become subject to human disturbance.

Tree boxes

Barn Owls are not woodland birds. Choose a lone, mature tree in the middle of rough grassland, or a large tree dominating a hedgerow. Ideally the tree will have large branches allowing easy access to the box from a range of directions; mature oak trees are ideal and will often offer additional roosting hollows. Position the box away from the prevailing weather but facing open land so that the bird can locate the box easily and have a clear flight path into the box. The box should be sited within the body of the tree; young owls must be able to walk into the tree/ along a branch and return safely to the box. If the entrance of the box protrudes beyond the trunk owlets will inevitably fall out of the box. Allowing young owls a safe access to and from the tree is the single most important factor when positioning a tree box; failure to comply will result in high infant mortality and nullifies the provision of an artificial nest site.

Large and exposed tree boxes can attract unwanted human attention and interference from animals. Boxes are best sited on private farms and estates rather than near public footpaths. For nesting Barn Owls the height of the box is not important, however it is usually best to position the box so that it can only be reached by ladder.

Exterior box design:

The Hawk Conservancy Trust recommends the Major Nigel Lewis ‘Mark VII’ box (see below for details). Major Lewis has designed his box to deter competitive interference from Jackdaw. The box is built to last and is very weighty; to install the box requires strong assistants and suitable lifting gear.

The box is built using 18mm marine or tanalised plywood, stained with an outdoor wood preservative and the roof is felted.

Interior boxes

Old farm buildings are a traditional nesting place. Boxes are most commonly accepted if they are placed at or near to known roosting sites. Many roosting sites in buildings do not provide an appropriate nesting cavity and so the introduction of a box can prove successful, particularly if a natural breeding site becomes unsuitable. Boxes installed in buildings with no previous evidence of roosting Barn Owls are seldom used.

Boxes can be simple (a tea chest conversion where an extended board provides a strong base and a wide exercise ledge) provided there is no interference from Jackdaw.

The box should be positioned so that young owls can leave the box and move along rafters/roof beams, and safely return to the box.

Points to remember

• Permanent, rough grassland is ESSENTIAL
• Avoid siting boxes within 1.5 miles of a busy road, or 3 miles of a motorway (they are responsible for 3000-5000 Barn Owl deaths annually). Ensure high road side hedges are present to allow safe crossing.
• Choose sheltered areas at low altitudes – avoid exposed areas
• Avoid facing the nest box entrance into the prevailing weather
• Avoid instances of drowning by floating a plastic tray in nearby cattle troughs
• DO NOT disturb Barn Owls - during March, April and May the owls are particularly sensitive and prone to desertion.

The Lewis mark VII design for Barn Owl nest boxes:

This is a box designed by the Imber Conservation Group, who we work with putting up nest boxes on Salisbury Plain. The top corridor, with the outer door at one end and the inner door at the other end, is designed to prevent other birds (eg jackdaws) from using the box as they are unable to get nesting material round the corners and into the main chamber. The dead end lower corridor is a bolt hole for owlets but not accessed from the main chamber; owlets can only leave the main chamber by the high door so are unlikely to be able to get out before they are ready to fledge.