Nestboxes are a great way to provide habitat for 'garden' birds. Most people will just put up any old box in the hope that it will get used, regardless of what the birds may think. They can be made to be perfect homes for certain species, or death-traps. Aside from the interest of seeing more birdlife, there are more than a few general considerations to bear in mind.

Why have a nestbox?

It is estimated that around two million fledgling birds are raised in man-made nest boxes in Britain every year. Traditionally, many small garden birds have made their nests in holes in trees and rotten trunks, but nowadays these older and dying trees are cut down and cleared away as a matter of course. The slow decline in populations of many of Britain's small birds is, at least partly, due to a loss of breeding habitat. Nestboxes are a great way of providing birds with an easy home for rearing their young - and who wouldn't want the satisfaction of seeing a family of great tits being reared in a box they have made and sited themselves?

According to the RSPB¹, over 60 species have been known to use nestboxes; as well as blue tits and great tits (the greatest users), coal tits, nuthatches, robins, starlings, sparrows, and even tawny owls and kestrels have been known to use them². Much depends on the species found in your locality and the size and design of nestbox you use.

Key Considerations

There are two main types of nestbox that are used for general species. The 'open-fronted' nestbox has one-third of its front section cut away and is designed for larger birds such as robins, wagtails and flycatchers³ and the 'hole-fronted' has a front section intact apart from a hole to allow smaller birds access. The size of the hole in the second case is key; generally accepted sizes go from 25mm for a blue tit up to 32-38mm for a sparrow.

Location, location, location

If your nestbox is to be used, its siting is crucial. There should be a clear flight path to the box, preferably with plenty of perching opportunities nearby, and it should be at least 30 metres away from other bird boxes of the same type. Keep it a similar distance from any bird tables or feeders if you want to avoid unsightly, and occasionally dangerous, territorial squabbles. It will need to be between two and five metres above the ground, in order to be out of reach of cats, out of direct sunlight and facing north to north east to keep it out of the prevailing wind⁴. The box should also be angled slightly downwards to protect it from rain. If you can, avoid nailing your box to the tree as it will cause damage; the best way to attach it is some strong wire wrapped around the tree, placing a layer of old garden hose or old car tyre beneath the wire. If your nestbox is staying in place for a matter of years, remember that trees grow in width as well as height and you will need to adjust the length of wire annually.

Open-fronted nestboxes will be used more often if they can be located somewhere where they will be hidden by dense vegetation. They will still be sought out by birds but will be less accessable to predators.

Finally, sparrows and starlings will often use nestboxes placed under the eaves of a building, with the obvious benefits of deterring them from nesting in your roof. However, if you have housemartins⁵ nesting in the eaves of your house, you need to be a bit more careful. Siting a nestbox nearby may encourage house sparrows to move in, and unfortunately sparrows like to take over housemartin nests, with grisly results for the occupants⁶.

Maintenance and housekeeping

Aside from the point made above about tree growth, it is also very important to clean your nestbox out once a year. All kinds of bugs, fleas and parasites may have also moved in with your bird family! It is important to restrict the spread of diseases by having a good clear-out. If you have put the nestbox up to encourage young children to take an interest in nature, make sure you check the box alone before bringing them in to help you clean it. The sight of a dead baby bird or unhatched eggs can be quite traumatic for youngsters, and you might have the opposite impact to the one you initially intended. If, however, you think they are old and wise enough to handle it, it may be worth explaining this with some well-measured facts⁷.

This should be done a few weeks after the last fledglings have left the nest. Although this varies from species to species, sometime in October to November is generally accepted as a good time for most; it is easy enough to check the fledging times of the birds you have in residence either in the library or on the internet. Don't be tempted to use insecticides or flea powder as these will seep into the wood and potentially poison the following years' fledglings. Simply remove any old nesting material and use boiling water to sanitise the box. Any unhatched eggs should also be removed, and legally this can only be done between October and January. Once the box is clean, it's a kind thought to put some hay and/or wood shavings in the box to be used by hibernating mammals or roosting birds over the winter.

This is the only time of year you should disturb your nestbox. Approaches, however well intentioned, may lead to the adult abandoning the nest leaving a handful of tiny fledglings to starve alone. Enjoy from a distance.

Making your own versus pre-made boxes

The argument here is not just about the satisfaction of craftsmanship set against ease of purchase. Unfortunately, many nestboxes purchased from garden centres and the like are produced to be aesthetically pleasing and may not be appropriate for safe use. Whether you make your own or buy, here are some important points to consider:

• The floor space on the base should be at least 100mm square. It should have a liftable lid to facilitate cleaning, and the entrance hole should be at least 125mm from the base. Less than this and cats will find it easy to scoop the fledglings out.
• Wood used should be rough-cut softwood, unplaned and untreated. It should be at least 15mm thick. If the wood is smooth, it will not be possible for the fully-grown fledglings to leave the nest as they will just slide back down the inside of the box. Although it is possible to treat the box with water-based preservatives, it is preferable not to do this as the average box will last between five and ten years untreated. If you feel you must, however, treat only the outside and avoid treating the area around the entrance hole.
• Some boxes have a pretty piece of dowel sticking out beneath the entrance hole. Predators will thank you for the nice place to sit and relax while they feed on the occupants. Perching sticks are not the best thought-out of ideas.
• You can make a nestbox too well. If all the joins are perfect, you will need to drill some holes in the base to allow rain to drain out. Heavy storms can easily flood nestboxes, with catastrophic results.
• If you have a large local squirrel or woodpecker population near an open-fronted box, it may be worth fitting a metal plate in front of the hole. Both are capable of widening the hole.

Making your own nestbox

With a minimum of tools, effort and time, you can make your own box without being a trained carpenter. With a little research, you can design your box to accommodate species that you would like to see around your garden or encourage existing species to flourish. Even without wanting to tailor-make a box for a particular bird, (which is a bit hit-and-miss anyway most of the time), you can make a good generic box as described in this section.

It is quite difficult to explain how to construct a nestbox without reference to a diagram. Fortunately, there are excellent webpages at Beautiful Britain and 50Birds.com that will help you visualise how the box will look. What follows is a description of the RSPB-recommended method of making a nestbox⁸.

You will need....

• Plank of wood: 1500mm long, 150mm wide and 15mm thick.
• Approx. twenty 20mm galvanised nails.
• Tools: Wood saw, hammer, pencil and ruler, drill.
• Scrap rubber (eg, a bicycle inner tube, or some garden hose).

Mark out with the pencil and ruler...

All width dimensions are 150mm and depth measurements are 15mm. This makes it easy to mark out all the pieces on one plank. The biggest tip here is to make sure you write the name of each piece on as you draw it. This can save you hours of frustration.

• Two sides, each with a slope on one end. Measure 450mm along both sides of the wood and join these points together. Now, working from the end again, measure 200mm on one side and 250mm on the other. Join these points. You should now have two shapes marked on the wood, identical apart from the fact that one is reversed⁹.
• A base. Measure 120mm on and draw a line.
• A back. Measure 460mm on and draw a line.
• A front. For an open-fronted box, measure 130mm on; for a hole-fronted box measure 200mm on. Draw a line!
• A roof. Measure 220mm on and draw a line¹⁰.

Cut the pieces

Quite straightforward, this bit, once you've got the marking sorted out. Just saw along the lines! If possible, cut the join between the the front and the lid at an angle of 15 to 25 degrees to the vertical, so that one side of the front is slightly shorter than 200mm¹¹. When you place the roof on later, you will find that it makes a good join with the back and will lie flat on the sides and front.

Once you've done that, if you're making a hole-fronted box you need to create a hole. Measure 125mm into the centre of the 'front' and drill a hole of the appropriate size. As a guide, blue tits require a 25mm diameter, great tits 28mm and sparrows 32-38mm. If you're really not sure, the 28mm hole will suit a variety of small birds.

Put it together

Start by nailing the 'sides', longest edge onto the 'back', so there is a similar amount of 'back' showing above and below the 'side'. Then affix the 'base' to the bottom of the 'sides', then the 'front' so that the sloping top of the side-pieces is flush with the sloping top of the front¹². Now you just have the roof to put on.

This is where the rubber comes in handy. The aim is to use it as a hinge between the roof and back so that the roof is kept in place but remains openable. Place the roof on top of the box so that it is lying flat across the top of the nearly-completed box. Stretch the rubber tight across the sloping join and nail it in place. You now have your nestbox!

It is worth testing how well your box drains by pouring some water inside and seeing how quickly it runs out. If there is little drainage, drill some small holes in the bottom to allow better run-off.

More information

There is a whole series of related entries on Wildlife Gardening beginning with Getting Started.

The RSPB produce detailed guidelines and leaflets for nestbox makers, and it is well worth contacting them especially if you need information on particular species.

Wildlife Trusts exist in every area of Britain. They also produce handy guidelines, and most areas will occasionally put on events or workshops for nestbox makers. Find a local events programme or contact them directly for details.

The 'Make Space For Wildlife' campaign has an excellent article on the BBC's pages. It contains many excellent tips for making your garden a more wildlife-friendly place.

The British Trust for Ornithology, though largely involved in surveying, has some great tips regarding nestboxes and even produces its own guide. Details of this can be found on their website.