There are two basic types of nest boxes for Tawny Owls. The upright design as shown in Fig. 1 is normally strapped or nailed to the side of a vertical tree trunk or major branch. The sloping type as shown in Fig. 2 is designed to be strapped or tied to the underside of a sloping side branch at an angle of approximately 45°. Exact dimensions are not too critical provided that the box is big enough with an entrance hole not less than 200mm square. The upright type box should be fitted with a thick roofing felt top. Both types must have several large drainage holes in the bottom not less than 12mm diameter.
Use pressure treated (tanalised) 18mm planks, or 18mm softwood ply (known in the building trade as CDX). Please avoid using hardwood ply unless it is stamped ‘FSC Approved’. If pressure treated timber is not available, use a brush-on preservative and ensure that all the edges are treated before assembly. Whichever timber treatment you use be sure to follow the manufacturers’ instructions and make sure that the box is completely dry before erection. Assemble the box using galvanised 50mm (2″) nails and/or brass or plated screws. We recommend the use of ratchet straps or polypropylene rope to fix the box to the tree as these cause minimal damage. Nails cause greater damage, but are often the most practical option.
The most common nest site is usually a mature tree in a copse or larger stand of trees. Moist deciduous
woodland seems to be the preferred choice in Northern Europe though nesting sites in coniferous forests are not uncommon. Nestboxes need to be sited in one of the most mature trees in the woodland, preferably within the woodland itself rather than on the fringes. Fix your nestbox securely to a large tree as shown in Fig. 3. In a remote undisturbed area the box can be placed as low as 3 metres (10’) above the ground. In more populated areas or places where vandalism may occur, the box should be secured not less than 3.7 metres (12’) up or higher if possible. Try to face the box entrance away from the prevailing wind (generally this means avoiding the west or south-west).
You are responsible for your own safety – assess the risks and be careful.
Clearing out the nestbox
Tawny Owls will use a wide variety of nest site types, including old crow or woodpigeon nests where cavities are in short supply. Irrespective of nest type, young Tawnies usually leave the nest for good at around 3-4 weeks of age, a phase known as ‘branching’. It’s therefore much less important to maintain the internal depth of a Tawny Owl box compared with a Barn Owl space. Nevertheless, the majority of the contents can be removed. In addition, nestboxes should be regularly checked to ensure they’re still well-fixed, in good condition and dry inside. If there is any doubt that the box will still be up by the next time you check, strengthen its attachment or take it down and re-hang it. In the case of tree-mounted nestboxes, be aware that trees expand as they grow and nestboxes can literally be pushed off as the exposed parts of screws and nails gradually disappear into the tree. Similarly, anything wrapped around a trunk or branch may need to be slackened off.
As far as timing is concerned, Tawny Owls breed about a month earlier than Barn Owls so first eggs are typically laid around mid-March. However, eggs have been reported as early as December. Early autumn is therefore usually the best time for any works.