Building bird nest-boxes

People often ask the question: "Do nestboxes in gardens really serve any useful purpose other than giving pleasure to us humans?" The answer to that question is definitely yes! Gardens are a most important habitat. The total area of all gardens in Britain exceeds that of all nature reserves, and as modern lifestyles destroy trees, hedges and old buildings, natural nesting sites are in decline. So nestboxes placed in gardens can make a real difference to the success or failure of a breeding species in a given area, especially when accompanied by the regular supply of suitable food and water.

Design

There is no standard, accurate design for a nestbox. Birds do not insist on their nest sites being mathematically precise! What they do require is a nest site which is secure and weatherproof, and as safe as possible from predators. Different bird species favour different types and locations of nesting sites, and so boxes must be constructed accordingly to meet these different needs. Two basic designs however will accommodate most common garden birds; either a partly open-fronted box, or a box with a circular hole at the front (of varying diameter for different species).

Materials

Undoubtedly wood is the best material to use; new or old wood, rough or planed, softwood or hardwood - it is really not important. Use what is readily available. A thickness of about 0.75 inches is ideal.

Manufactured board (plywood and chipboard) are not suitable for outdoor use (except for resin-bonded marine quality ply, which can be expensive), although they are perfectly acceptable for a nestbox placed under cover, for example a Barn Owl box placed in an old building.

After construction, the outside only of the box should be treated with a wood preservative (creosote alternative, ‘Enselle’, ‘Cuprinol’, ‘Sadolin’, etc.) to prolong its life and help repel water. If using planed timber, clear polyurethane may be used instead.  Roofing felt fixed to the roof is normally unnecessary if the wood is thick enough, and we find that bugs and fungi can rot the roof wood under felt.

Siting of nest-boxes

For nestboxes with open fronts and small entrance holes:

• Height
  Whether fixed to a tree or a wall, the height above ground is not critical to most species of bird, so long as the box is clear of inquisitive humans and prowling cats.

• Aspect
  If there is no natural shelter, it is best to mount a box facing somewhere between south-east and north, to avoid strong direct sunlight and the heaviest rain (normally from the southwest in Cornwall). The box should be tilted slightly forwards so that the roof may deflect the rain from the entrance.

• Predators
  It may be possible to deter predators by fixing the nestbox in a thorny bush or by placing chicken-wire around the entrance, but always ensure a direct flight-path to the entrance. If squirrels or woodpeckers are a serious threat, fix a metal plate around the entrance hole so that it can not be enlarged.

• Fixing
  Some authorities recommend nails to attach the box directly to a tree trunk or branch; others prefer the use of rope or wire right around the box and trunk (remembering to protect the trunk from wire cutting in by the use of a piece of rubber or the like). Both methods are satisfactory, but obviously annual maintenance is easier if the box is wired and can thus be taken down easily for cleaning.  If you are using nails on trees, consider buying a supply of aluminium nails that do not harm the tree or damage chainsaws that could be used should the tree require surgery later on.

• Number
  The number of nestboxes which can be placed in a garden depends on the species you wish to attract. Many species are fiercely territorial, such as Blue Tits, and will not tolerate another pair close by. About 2 to 3 pairs per acre is the normal density for Blue Tits. Other species such as Tree Sparrow, which are colonial nesters, will happily nest side by side.

Do not place any nestbox close to a bird-table or feeding area, as the regular comings and goings of many other birds are likely to prevent breeding in the box.

Annual cleaning of nestboxes

After the end of each breeding season, all nestboxes should be taken down and the old nesting materials removed (using gloves) and the box should be scolded with boiling water to kill any parasites. Do not use insecticides or flea-powders - boiling water is adequate. Annual cleaning is best carried out in October or November.

Under the terms of the Wildlife and Countryside Act 1981 (amended), if unhatched eggs are found in the box, they can only legally be removed from October to January inclusive, and they must be destroyed! It is illegal to keep them.

Winter

Leave the bird box up in winter as it will be useful as a roosting site for birds in bad weather.

Building a bird nest box

Small open-fronted nest-box & small-hole nesting birds

Small open-fronted nest-box

A variety of species may be attracted to an open-fronted nestbox placed in a garden, the commonest of which will be Robin and Wren, although it could also be used by Pied Wagtail, Spotted Flycatcher, Song Thrush and Black Redstart.

The size and construction is similar to that for a small-hole nestbox, except that the front will be a piece of wood 3.5 x 6 inches. This box too can be made from one 48 inch plank of timber, 6 inches wide, 0.75 inch thick. It is not necessary to hinge the roof, except for a Wren design, as the box can be cleaned through the entrance hole. The floor should have small drainage holes drilled in it.

The front opening should be approximately 1.5 inches for Wren, 5 inches for Spotted Flycatcher and Song Thrush, and 3.5 inches for other species listed (based on the design below).

Nestbox for smallhole-nesting birds
Many garden and woodland birds nest in holes and may be attracted to a nestbox.  The particular species attracted will depend on its local distribution and population, on the size of hole provided in the nestbox, and where you site the box.  An entrance hole of 28 millimetres in diameter will admit Blue Tit, Great Tit, Coal Tit, Tree Sparrow and Pied Flycatcher; whereas a slightly larger hole of 32 millimetres in diameter will also attract House Sparrow, Nuthatch, and Lesser Spotted Woodpecker.  Marsh Tits and Coal Tits prefer their boxes sited low down and close the ground, but if cats are a problem, you may have to be creative in choosing sites away from danger.   

The following design is for a basic small-hole nestbox. The roof should be hinged, for ease of cleaning out, either by a non-ferrous hinge and screws or by a rubber strip. The dimensions are only an approximate guide and are by no means critical; but if they are followed, this box can be made from one 48 inch piece of timber, 6 inches wide, and 0.75 inch thick.

Nest-box for small-hole nesting birds

Many garden and woodland birds nest in holes and may be attracted to a nest-box. The particular species attracted will depend on its local distribution and population, on the size of hole provided in the nest-box, and where you site the box. An entrance hole of 28 millimetres in diameter will admit Blue Tit, Great Tit, Coal Tit, Tree Sparrow and Pied Flycatcher; whereas a slightly larger hole of 32 millimetres in diameter will also attract House Sparrow, Nuthatch, and Lesser Spotted Woodpecker. Marsh Tits and Coal Tits prefer their boxes sited low down and close the ground, but if cats are a problem, you may have to be creative in choosing sites away from danger.

The following design is for a basic small-hole nest-box. The roof should be hinged, for ease of cleaning out, either by a non-ferrous hinge and screws or by a rubber strip. The dimensions are only an approximate guide and are by no means critical; but if they are followed, this box can be made from one 48 inch piece of timber, 6 inches wide, and 0.75 inch thick.The box may be screwed or nailed together, and small drainage holes should be drilled or cut in the floor.

Building a bird nest-box - large-hole nesting birds

Species such as Stock Dove, Jackdaw, Starling, Green Woodpecker and Great Spotted Woodpecker are all species which nest in holes, and a box of the following dimensions will meet their requirements.

The design shows a rectangular entrance hole 6 x 4 inches, which is acceptable to Stock Dove, Jackdaw and Starling. For Woodpeckers it is better instead to have a front hole of 2 inches diameter (50 millimetres) for Great Spotted Woodpecker, and 2.5 inches (60 millimetres) for Green Woodpecker. For all Woodpeckers, the box should be packed with a rotten log that is soft enough to cut with a knife, which they can excavate to make their nest.

The roof should be hinged, preferably with a strong brass hinge, so that new logs may be put in each year. Drainage holes should be drilled in the floor. The box should have a stout batten (approximately 24 inches long, or 3 x 1 inches timber) screwed firmly to the back of nestbox for mounting to a tree as high as possible in a secluded place.

Building a bird nest-box - Kestrel

Kestrels do not construct nests of their own, but lay their eggs either in the old nests of crows, or in cavities in trees, or ledges on cliffs and buildings.
A nestbox of the following design is best sited about 20 feet high on a solitary tree in farmland, using a stout batten, firmly screwed to the box, attached directly to the trunk by means of galvanised nails, 4 inches or longer. The entrance should face east, and the box tilted back slightly to keep the eggs and young to the rear.

It may be possible to wire the box to a branch, or to use a horizontal batten and wedge it into the fork of two branches. If there is dense foliage, prune away a few branches to give a clear flight-path to the box.
The nestbox should be constructed from 0.75 inch thick timber or resin-bonded exterior ply, and the dimensions below will produce a box of minimum acceptable size:

2 sides :- 12 x 20 inches
1 floor :- 12 x 21.5 inches
1 roof :- 12 x 26 inches
1 back :- 12 x 12 inches
1 front :- 12 x 4.5 inches

Thicker timber can be used, but adjust the sizes to give the same internal dimensions.

The box is assembled using waterproof glue and 2 inch screws or galvanised nails. The fixing batten should be about 28 inches long, or 4 x 1 inch timber.
A small branch about 1 inch diameter should be fixed at the front as a perch, since both young and adult Kestrels spend a large amount of time perching at the nest entrance.

About a dozen drainage holes should be drilled in the floor, about 0.25 inch diameter, and a layer of wood shavings or straw scattered over the floor.

This is a heavy nestbox and will require at least two people to erect it, using ropes and a ladder.

Building a bird nest-box - Barn Owl

'Nest' is not really the appropriate word to use in relation to Barn Owls, since they build no nest as such, but lay their eggs on a layer of pellets which has accumulated in their roosting site. Such sites are deep spacious cavities in trees (especially Elms), dark corners of barns, churches and old buildings, or even gaps in straw stacks. With the loss of so many elms in recent years through disease, and so many old buildings through modernisation, nestboxes provide a real opportunity for this declining species to re-establish its population.

Owls may accept a nestbox readily, but use it only for roosting for as long as two years before finally breeding. There is no real design for a Barn Owl nestbox - any large box is acceptable if it is at least 18 x 18 x 24 inches. For internal use (barns and old buildings) the traditional tea chest can be ideally adapted; but for external use, good quality, heavy, waterproof board such as marine ply must be used.
The ideal site for a box is a dark corner on a beam in an undisturbed building near farmland, away from busy roads, and where there is permanent access for the owls through doors, windows or other such holes.
An entrance hole 9 x 9 inches should be cut from one of the bottom corners, and a lipped tray fitted in front of the entrance to provide an exercise area for the young owls.

Building a bird nest-box - Tawny Owl

The natural nesting site for a Tawny Owl is a deep and dark cavity in a tree, such as the hollow in a broken branch, or sometimes they may use the old nest of a Magpie or a Grey Squirrel.

This nestbox is meant to represent a deep cavity and it should be sited high up on the edge of a stand of trees, and fixed at 45 degrees to the horizontal, either on the trunk or a major branch, and facing east. The support batten should be firmly fixed to the box and nailed into position, using 4 inch or more galvanised nails (fixing method 1 on the diagram); or it may be possible to fit a longer batten across the top of the box and wedge this into a fork between two branches (fixing method 2 on the diagram). It may also be possible to attach the box using wire only (fixing method 3 on the diagram), but ensure that heavy gauge plastic-covered wire is used and is stapled to the nestbox. About a dozen drainage holes (0.25 inch diameter) should be drilled in the base, and a covering of wood shavings put aside.

An ideal size for a Tawny Owl box is 9 x 9 x 31.25 inches internal dimensions, which requires the following pieces of timber of 0.75 inch thickness:

2 side pieces 9 x 31.25 inches
2 side pieces 10.5 x 32 inches
1 floor piece 9 x 10.5

Glue and screw together, using waterproof glue and about 2 inch screws. The fixing batten should be 1 inch thick, and approximately 28 x 4 inches for fixing method 1, or 36 x 4 inches for method 2. Thicker timber can be used, but adjust the measurements to give the same internal dimensions.

This will be a heavy nestbox and will need at least two people to erect it, using ropes and a ladder.

A word of warning! If a Tawny Owl box is occupied, it should NOT be visited in the breeding season! Apart from the obvious risk that disturbance may cause the parents to desert, there is a real danger of injury, as a Tawny Owl will attack any intruder and can inflict very serious injuries, especially to the face.

For further information you could call the Wildlife Information Service at the Cornwall Wildlife Trust.

Building bird nest boxes is available to download in Word format 571kb or PDF format 671kbs.

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