Birdboxes and other nest sites

Nestboxes and other artificial homes for birds can be very important for nesting success, especially in habitats where food is plentiful but nesting sites are scarce. Such habitats may include:

• Managed woodlands which have an absence of mature trees, deadwood and undergrowth.
• Newly planted or young woodlands, which have no large or mature trees, or deadwood.
• Gardens, parks and urban areas, where requirements for safety, neatness and other qualities result in a lack of nesting sites.

For comprehensive details on nestboxes and other structures for a wide range of bird species see Du Feu (1993), or for details on structures for birds of prey see Dewar and Shawyer (1996). A wide range of nestboxes for birds, invertebrates and mammals, together with anti-predator devices and other accessories are available from Jacobi Jayne & Company.

Habitat improvement, including retention of old trees for nesting sites and planting new trees and shrubs to form thickets will provide nesting sites. Good woodland edge habitats, ponds and grassy areas provide the insect life on which many birds, bats and other creatures depend.

Boxes, platforms, bundles of brush, artificial cavities and other structures can be used to encourage birds to roost and nest. All sorts of materials and designs can be used, as long as they fulfil the following criteria:

• Nestboxes and other structures must be attractive to nesting birds, and be designed and sited so that a brood of young birds can be successfully raised. Boxes which are flimsy, badly sited or not securely attached may attract birds to nest, only to result in failure. Boxes of thin material such as plastic, thin wood or metal will not provide sufficient insulation.
• Nestboxes must be safe from predators, including other birds, mammals and people. Various design details and devices can be used to deter magpies, squirrels and other pests. Nestboxes which are hidden and/or difficult to reach are less likely to attract unwanted attention from people. A variety of nestboxes of different types in any one area may help deter predators, which may learn to recognise and raid a ‘standard’ design.
• Nestboxes and other structures should be simple and cheap to construct, and be easy to inspect and clean.
• Nestboxes must be of durable material, both to repay the effort put into construction and fixing, and because birds may take some years to accept and use them. Boxes may be used for roosting, even if they are not used for nesting.

Materials and designs

Wood

Wood for nestboxes should be over 15mm thick to provide sufficient insulation. Other dimensions are not critical, apart from the size of the entrance hole. Recovered materials including pallets, packing cases, floorboards and other scrap wood is suitable provided care is taken in dismantling pallets and removing nails as necessary. Softwood is easier to work, but hardwood will last much longer. Exterior plywood is suitable, but chipboard and other boards for internal use are not.

Basic woodworking tools are sufficient for construction purposes, with a 28mm drill bit useful for making entrance holes.

Note the following:

• Keep the wood grain vertical to help shed water quickly.
• Avoid exposed horizontal end grain.
• Fix the floor just above the lowest point of the side panels, so that water drips off the bottom rather than seeping round into the joint with the floor (see diagram).
• Unless there is a sufficiently wide crack between the floor and the sides, drill drainage holes in the floor to allow any moisture to drain out.
• Construct using galvanised nails or brass screws. Bright wire nails will quickly rust.
• Seal side joints with waterproof glue, to keep the box dry and free of draughts.
  
• Perches are not necessary on small or medium size boxes, up to 130 x 150mm base area. Nesting birds do not need perches, which will merely encourage predators.
• Hole entrance boxes must have a hinged or removable lid for inspection and cleaning. A simple hinge made of a piece of inner tube will waterproof the joint as well as allow easy opening. Brass hinges can also be used, but steel hinges will quickly rust solid. The lid should slope sufficiently to shed water, and can be further waterproofed with a layer of vinyl flooring or similar. A hook and eye or nail with piece of soft wire can be used to fasten the lid closed. Fully removable lids should have a lip all round to hold the lid in place.
• The lid or roof should have a generous overhang to prevent rain blowing into the entrance hole, and also to discourage predators.
• The outside of the box can be treated with wood preservative for increased durability, but leave the inside of the box untreated. Preservative is lethal to bats, who may use the box.

Table 12a: Nestboxes

Sawdust and cement

Nestboxes of sawdust and cement (‘woodcrete’) have been commercially available for some years, and make very durable and well insulated boxes which have a high rate of nesting success. As placing and fixing nestboxes can be awkward and time-consuming, investing in these very durable boxes can be worthwhile. Boxes for hanging from branches and other places are also available, and have the advantage of being very difficult for predators to reach.

Du Feu (1993) gives details on DIY methods of using sawdust and cement to make nesting boxes.

Filled boxes

Birds which excavate nesting holes, such as woodpeckers and willow tits, can be attracted with boxes filled with soft, inert material, such as expanded polystyrene, balsa wood, woodshavings or sawdust, which the birds will excavate. Boxes will need refilling after use, so a fresh nest can be excavated the following year.

Chimney boxes

Chimney boxes were originally designed for tawny owls, but may be used by other species. The box is made from 4 x 25mm thick planks, about 750mm long and 250mm wide, with a base of perforated zinc. A hatch near the base can be included for inspection of the nest. Fix the chimney above or underneath a branch as shown, at an angle of about 45˚. Cover the floor with bark chippings.

Basket

Long eared owls may nest in baskets fixed to the branches of trees in open woodlands, copses and hedgerows. Use an ordinary 450mm (18”) diameter plastic-coated wire hanging flower basket. Weave willow wands or other pliable shoots to thicken the basket and place an upturned turf in the bottom, and then partly fill with twigs and dry leaves. Fix the basket in the cleft of a tree, using plastic cable-clips or tree-ties, loosely fastened to allow for growth.

Scrape

Nightjars nest on bare ground in a scrape, especially in sandy soil in woodland clearings and heathland with isolated trees. Clear a patch of vegetation on the north side of a large shrub to leave a bare patch about 1m in diameter. See here for details of constructing ‘doughnuts’ of brash to encourage woodcocks.

Siting

• Site boxes so they are sheltered from prevailing wind, rain and strong sunlight, normally from facing north through to south-east. Avoid points where water runs down tree trunks.
• If possible angle tit boxes slightly as shown, to increase the shelter to the entrance.
  
• Apart from tree creeper boxes, nestboxes must be sited with a clear and convenient flight path into the box.
• See Table 12a for guide to height above ground for different species. Height above ground is generally less important than security from predators.
• The density of boxes depends on the type of habitat and the species involved. Generally start with a selection of boxes at fairly evenly spacing, and keep adding boxes until the occupancy rates indicate that lack of nest sites is no longer limiting population. 100% occupancy is unusual however, with 50% a good success rate. Du Feu (1993) suggests about ten assorted small boxes to the hectare, but denser spacings are possible for some species. Some species nest in colonies.
• Favoured sites are at the edges of woodlands, glades and rides, and other points where two habitats meet. Small copses, hedgerows and isolated trees are also suitable.
• Nestboxes can be put up at any time of year, as birds may use them for roosting as well as nesting, and it may be some time before the box is accepted for nesting. If boxes are not used after three or four years, take them down and re-site them.

Fixing

Boxes must be securely fastened to the support. Where boxes are to be inspected and cleaned annually, location and method of fixing should allow this. Where boxes are to be permanently fixed to living trees, the method must allow the tree to grow. Fixing to man-made structures avoids this problem, and such boxes are often less accessible to predators, but permission must be sought from the owner of the structure.

Safety

• Fixing, inspecting and cleaning nestboxes nearly always involves working from a ladder, which is potentially hazardous. Never work on your own. Preferably use an alloy ladder, with a splayed base for greater stability. If working within 10m of overhead lines, a wooden or non-conducting ladder must be used. Take great care in siting the ladder securely at the base and top, and ensure that a second person holds the ladder steady at all times.
• Take care when using hand tools at a height. Do as much as possible at ground level, for example knocking nails part-way through battens, to avoid any procedures which require using both hands when up a ladder.
• Large or heavy boxes and nesting structures should only be fitted by persons suitably trained to work aloft.
• A 3m long nest box hanging pole is available, which allows nest boxes to be hung or removed without needing a ladder. Similar devices can be improvised. Operators are advised to wear helmets in case the work dislodges anything, or the box itself falls down.

Tying

Various methods of tying can be used to avoid the problems caused by nailing into trees.

• Using screws, securely attach a batten, preferably of hardwood, to the back of the box.
• Use a length of wire, thick polypropylene twine or thin rope, or bands cut from old inner tubes. Wire, twine and rope must be checked annually to make sure they are not strangling the tree. Old inner tubes will stretch, but must be checked annually and replaced if they show signs of perishing.
• If the wire or twine is looped around the batten and loosely hung as shown, the box can be simply edged up a little each year to make allowance for expansion of the trunk.

Nailing

• Any nails or screws damage living trees, and should not be used on wood of commercial value.
• Steel screws or nails should never be used as they can cause damage or injury if the timber is cut with a chain saw at some time in the future. Steel screws or nails will quickly rust, making them difficult to remove.
• Screws, nails and pieces of wire can quickly get embedded as the tree grows, and will not be visible from the surface.
• Nails and screws can also be pushed out as the tree grows, causing the nestbox to fall.
• Nylon bolts, or copper, aluminium or brass nails can be used in trees which are not of commercial value, and will not cause damage or injury if the wood is cut with a chain saw at some future date.
• The safest method of securing is to first screw a hardwood batten to the back of the box, fixed either vertically or horizontally. Then attach the batten with large-headed non-steel nails. As the tree grows, the nails should move out with the batten rather than being pulled through.
• Alternatively, use a 100mm long x 15mm nylon bolt which is screwed halfway into the tree. Then using a hacksaw, cut off the head of the bolt. Hang the box on the bolt, and then secure it with a washer and nut. This can then be slackened each year as the tree grows.
• Larger boxes should be fastened to a backing board, which is fixed to the tree with a nylon bolt and a length of webbing. For method see Dewar and Shawyer (1996).

Hanging

Birds are not deterred from using boxes which are hanging freely from branches or other locations. These are also difficult for predators to reach, but may be vulnerable to interference by people as they are more obvious and easy to disturb than boxes which are attached to trunks. Hang the box on a length of galvanised wire securely attached to the box, and looped around a branch or similar and twisted to secure.

Deterring predators

Predators of woodland nesting birds may include other birds, squirrels, rats, cats and people. Various methods can be used to deter them.

• Siting the box where it is not easily accessible to people.
• Covering the lid with a flexible overhanging piece of vinyl flooring or similar, so squirrels or other animals slide off if they try and approach from the lid. This also helps waterproof the box.
• Gorse or other thorny material stapled to the lid, and attached around the trunk above and below the box will deter cats, rats and squirrels.
• Hanging boxes (see above). A cone of stiff plastic, open end down, threaded on the wire above the box will be a further deterrent.
• Hexagonal mesh, 40mm mesh size, can be used to cover the front of open boxes to deter crows, jays, cats and squirrels, but not weasels. Alternatively, the smaller 31mm mesh can be used, depending on the species of bird the nestbox is designed for.
• Greater spotted woodpeckers may hammer through wooden boxes to take young birds, but may be deterred by covering the box in rubber sheeting.

Cleaning and maintenance

• Boxes should be cleaned annually to remove any parasites which have built up. Don’t remove old nests for at least 3 weeks after a brood has fledged, in case a second brood is raised.
• Discard old nesting material at some distance from the nestbox site. Wear a mask to avoid inhalation of fungal spores.
• Use a pyrethrum based insecticide to kill off fleas or other infestations if necessary.
• Place a layer of wood shavings, bark chippings or polystyrene packing chips in the base of large boxes. Do not use sawdust, which sets hard if it gets wet.

Other management work

Work can be done to existing trees and shrubs to make them more attractive to nesting birds and bats. Great care must be taken when working from a ladder. Any work which involves tree climbing must only be done by specialists.

Before doing any work, especially to existing holes or cavities, make sure you are not damaging a habitat which is already valuable, for example for invertebrates.

Use natural materials wherever possible, to be less conspicuous to predators. Bark, slabwood with bark attached, and old weathered timber is more suitable than new timber.

• Existing holes which are too open for bird nesting use can be partially boarded up, leaving a small hole at the top.
• In trees with soft wood such as willow or birch, small cavities can be made at points where branches have dropped, at pollards or other pruning cuts.
• Bark or slab wood can be attached at the side of the trunk to make a ‘cavity’ for treecreepers. Thick barked trees such as oak, alder, poplar and pine are preferred. A deep fork can be converted to a ‘cavity’ with sides and a roof of bark.
  
• Rotting birch or willow logs can be tied to the trunks of living trees, and may attract willow tits or other birds which excavate cavities in rotting wood.
• Bundles or ‘faggots’ of twigs and small branches can be attached to the trunks of trees using twine or similar. Thorny material is particularly good against predators, but handle with care. Bundles of conifer branches, for example from discarded Christmas trees, make a useful early season nest site before shrubs are in full leaf.
• Pruning shrubs to create a mass of dense growth makes useful nest sites, as in a regularly trimmed hawthorn hedge or similar.