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The Barn Owl Trust

Conserving the Barn Owl and its Environment

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How to choose the best nestbox design

 
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The simplest way to choose a really good nestbox is to choose a Barn Owl Trust nestbox design! However, if you are thinking of buying a box from another supplier, building a box using someone else's design, or even designing your own, we strongly suggest that the nestbox meets the criteria that are set out below...

Just decide whether you are thinking of a box to go in a building, a box on a tree, or one on a pole, and click on one of the options below...

 
Nestbox design evaluation .
 

 

For nestboxes in buildings

  1. Entrance hole: minimum size 100mm x 100mm, optimum size 100mm x 130mm, maximum size 150mm x 150mm.
  2. Floor area of nest chamber: absolute minimum 0.16 m2
    Good size range 0.2 to 0.4 m2.
  3. Depth from bottom of entrance hole to nest must be not less than 460 mm.
  4. There must be an exercise/landing platform below the entrance hole that allows climbing/jumping young birds to get from the platform onto the roof of the box and (ideally) onto other nearby perching places. The platform must have a generous raised edge suitable for barn owls to grip easily.
  5. Human access for easy clearing-out of nest debris is essential.
  6. Measures aimed at reducing the chances of entry by other species (such as jackdaws and beech martens) are to be encouraged provided that they do not significantly reduce the box's suitability for barn owls.
  7. Should be substantially constructed yet light enough to permit safe erection using basic equipment. Normal indoor-box weight range is 10-15 kg. Total weight should not exceed 18 kg and an indoor-box under 8kg is probably not substantial enough.
  8. Should not be constructed from tropical hardwood unless the timber is certified as sustainably grown.
  9. Boxes should be supplied with information that specifies an erection height of not less than 3 metres above ground level and position selected where the box will be completely dry for many years. Information provided with boxes should also cover the following subjects: foraging habitat requirements, box positioning to maximise the chances of occupation (entrance hole visibility for birds flying into the building and building entrance visibility to birds flying outside), the need for clearing out debris so as to maintain internal depth, box erection and attachment methods, human safety issues. As a general rule, Barn Owl nestboxes should not be erected within 1km of any motorway, dual-carriageway or similar major road.

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For nestboxes in trees

  1. Entrance hole: minimum size 100mm x 100mm, optimum size 100mm x 130mm, maximum size 150mm x 150mm.
  2. Floor area of nest chamber: absolute minimum 0.16 m2.
    Good size range 0.2 to 0.4 m2.
  3. Depth from bottom of entrance hole to nest must be not less than 460 mm (NOTE: boxes with less depth may be acceptable if placed within the branches of a tree that a fallen nestling can climb, however, deep boxes are so much safer than we no longer recommend boxes with less depth).
  4. There must be an exercise/landing platform below the entrance hole that allows climbing/jumping young birds to get from the box into the tree and vice versa. The platform must have a generous raised edge suitable for barn owls to grip easily and should be positioned, and have sufficient shelter and drainage, to prevent rainwater being deflected into the box entrance.
  5. Interior must remain dry during prolonged heavy rain coming from any direction.
  6. All sides should overhang the floor and the floor should have adequate drainage. The installation of a (drier) false floor can be an advantage.
  7. There must be sufficient height difference between the nest and the external platform so as to prevent the accumulation of a continuous (internal/external) layer of pellet debris allowing rainwater to soak through the debris to the inside thereby chilling the nest contents.
  8. Roof should be covered in thick roofing felt guaranteed for not less than 10 years, Very steeply sloping roofs may not need covering but any apex join must be permanently waterproofed.
  9. Human access for easy clearing-out of nest debris is essential.
  10. Timber liable to decay within 20 years must be treated with long-lasting preservative: either pressure treated (CCA) or surface treated including all edges of all component parts.
  11. All screws/nails and any metal fittings used should be rust proof.
  12. Measures aimed at reducing the chances of entry by other species (such as jackdaws and beech martens) are to be encouraged provided that they do not significantly reduce the box's suitability for barn owls.
  13. Should be substantially constructed yet light enough to permit safe erection using basic equipment. Normal tree-box weight range is 13-18 kg. Total weight should not exceed 25 kg and a tree box under 10kg is probably not substantial enough.
  14. Should not be constructed from tropical hardwood unless the timber is certified as sustainably grown (FSC approved).
  15. Boxes should be supplied with information that specifies an erection height of not less than 3 metres above ground level and stresses the importance of positioning within the tree branches in the case of boxes that have less than 460 mm internal drop (see 1). Information provided with boxes should also cover the following subjects: foraging habitat requirements, box positioning to maximise the chances of occupation (entrance hole visibility), the need for clearing out debris so as to maintain internal depth, box erection and attachment methods, human safety issues. As a general rule, Barn Owl nestboxes should not be erected within 1km of any motorway, dual-carriageway or similar major road.

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For nestboxes on poles

  1. Entrance hole: minimum size 100mm x 100mm, optimum size 100mm x 130mm, maximum size 150mm x 150mm.
  2. Floor area of nest chamber: absolute minimum 0.2m2.
    Good size range 0.3 to 0.4 m2.
  3. Depth from bottom of entrance hole to nest must be not less than 460 mm.
  4. There must be an external platform below the entrance hole that allows ample room for an entire brood of young birds to exercise and await food deliveries; thus the danger of young birds falling (before fledging) must be minimised. External platform size should be approximately 0.125 m2 or larger.
  5. The platform must have a generous raised edge suitable for barn owls to grip easily and should be positioned, and have sufficient shelter and drainage, to prevent rainwater being deflected into the box entrance.
  6. Interior must remain dry during prolonged heavy rain coming from any direction.
  7. All sides should overhang the floor and the floor should have adequate drainage. The installation of a (drier) false floor can be an advantage.
  8. There should be sufficient height difference between the nest and the external platform so as to prevent the accumulation of a continuous (internal/external) layer of pellet debris allowing rainwater to soak through the debris to the inside thereby chilling the nest contents.
  9. Roof should be covered in thick roofing felt guaranteed for not less than 10 years applied by heat or adhesive (not nailed or pierced in any way). Very steeply sloping roofs may not need covering but any apex join must be permanently waterproofed.
  10. A flat or slightly sloping roof that provides additional exercise space for the young is advantageous.
  11. Human access for easy clearing-out of nest debris is essential.
  12. Timber liable to decay within 20 years must be treated with long-lasting preservative: either pressure treated (CCA) or surface treated including all edges of all component parts.
  13. All screws/nails and any metal fittings used should be rust proof.
  14. Should be substantially constructed yet light enough to permit safe erection using normal lifting equipment. Normal pole-box weight range is 18-30kg. Any pole box under 13kg is probably not substantial enough.
  15. Should not be constructed from tropical hardwood unless the timber is certified as sustainably grown.
  16. Within the box, a separate entrance hole into a small compartment so as to provide a secluded roosting space for an adult owl can be advantageous. (However, it should be designed so as to minimise the chances of this inferior cavity being used for nesting by barn owls).
  17. Measures aimed at reducing the chances of entry by other species (such as jackdaws) are to be encouraged provided that they do not significantly reduce the box's suitability for barn owls.
  18. Entry by beech martens should be prevented by positioning the pole away from buildings or trees and by wrapping a 1.5m section of the pole with thin aluminium or other very slippery material.
  19. Boxes should be supplied with information that specifies an erection height of not less than 4 metres above ground level and stresses the importance of using a substantial pole of not less that 150mm diameter (normally 6m long, 1.5m underground and 4.5m in height). Information provided with pole boxes should also cover the following subjects: foraging habitat requirements, box positioning to maximise the chances of occupation (entrance hole visibility), the need for clearing out debris so as to maintain internal depth, box erection and attachment methods, and human safety issues. As a general rule, Barn Owl nestboxes should not be erected within 1km of any motorway, dual-carriageway or similar major road.

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Permanent provision within roof spaces

If you're thinking of building-in a space for Barn Owls within the apex of a roof (such as a barn conversion) rather than erecting a pre-made box check out the other pages on the website, including essential design requirements for the creation of permanent provision within buildings here >

 

There is more information on this topic in the Barn Owl Conservation Handbook

 

 

 
 
 
The Barn Owl Trust is dedicated to conservation & education and does not operate a visitor centre.
Barn Owl Trust staff and volunteers
Waterleat, Ashburton, Devon TQ13 7HU
+44 (0) 1364 653026
info@barnowltrust.org.uk