Nestboxes for small garden birds

Watching birds nesting in your garden can be very rewarding, especially if they are in a purpose-built nestbox. Tits are particularly happy to use boxes and are always a delight.

Why build nestboxes at all?

Nestboxes are excellent substitutes for the holes found in old trees. In many parks and gardens there may be plenty of food for small birds but nowhere for them to nest.

Which birds will use a nestbox?

Over 60 species are known to have used nestboxes, including of course blue and great tits. Among the others are coal tits, nuthatches, house and tree sparrows, starlings, spotted flycatchers, robins, house martins, kestrels and tawny owls. Much depends on where the box is and on your surroundings. For comprehensive information on nestbox design, see Nestboxes by Chris du Feu, BTO Guide 23, obtainable from the National Centre for Ornithology, The Nunnery, Thetford, Norfolk 1P24 2PU. Tel 01842 750050.

How do I make a nestbox?

Natural nest holes do not come in standard sizes, so use the following dimensions only as a guide. Use a plank about 150 mm wide and 15 mm thick. The size of each section to be cut is shown on the diagram. The inside of the box must be at least 100 mm square and the bottom of the entrance hole must be at least 125 mm from the floor. If it is less, young birds might be scooped out by a cat. Use galvanised nails or screws. The inside front surface should be rough to allow the young birds to clamber up. A drainage hole should be drilled in the base. The entrance hole size depends on the species you hope to attract: 25 mm for coal, marsh and blue tits, 28 mm for great tits and 32 mm for nuthatches, house and tree sparrows. A starling box needs to be 25-30% larger with an entrance 45 mm across. Hinge the lid with a strip of leather or rubber (an old piece of bicycle inner tube would do). Do not nail the lid down but use a good catch to fasten it. You will want to clean out the box in the autumn. The same box with the upper half of the front taken away altogether may attract the occasional robin, pied wagtail or wren to nest. Spotted flycatchers prefer an even shallower, open-fronted box.

What can I use to treat the wood?

Softwood boxes may be treated with water-based preservatives such as Fenceguard: apply only to the outside of the box, and not around the entrance hole. Whatever you use, make sure the box dries and airs thoroughly before putting it up.

Where should the box he placed?

Boxes for tits, sparrows or starlings should be fixed two to five metres up a tree or wall, out of the reach of cats and curious humans! Unless there are trees or buildings which give permanent shelter, it is best facing between north and east, thus avoiding strong sunlight and the wettest winds. Tilt the box forward slightly so that any driving rain will hit the roof and bounce clear. House sparrows and starlings will readily use nestboxes placed high up under the eaves and these will often deter the birds from nesting in your roof! Keep these away from areas where house martins normally nest. Open-fronted boxes for robins and wrens need to be low down, well hidden in vegetation.

How should I attach my nestbox to a tree?

Fixing your nestbox with nail's may damage the tree. It is better to attach it with wire around the trunk or branch. Use a piece of hose or section of car tyre around the wire to prevent damage to the tree. Remember that trees grow in girth as well as height, and check any fixing every two or three years.

How close together can nestboxes be positioned?

Two boxes of the same kind may both be occupied if they are at the edge of adjoining territories and if there is plenty of natural food. Tits can, however, be very aggressive and seldom nest at densities greater than two or three pairs per acre. By putting up different boxes, several species can be attracted.

Should the nestbox be cleaned out each year?

The nests of most birds harbour fleas and other parasites which remain to infest young birds that hatch the following year. We recommend that old nests are removed in October or November. Use boiling water to kill any remaining parasites. Insecticides and flea powders must not be used. If there are unhatched eggs in the box, these may be removed legally only between October and January, and must be disposed of. If you place a small handful of clean hay or wood shavings (not straw) in the box once it is thoroughly dry after cleaning, it may be used during the winter by small mammals or birds for hibernating or roosting respectively.

Why were there dead young or unhatched eggs in the box?

It is quite normal for a few eggs to fail to hatch, or for some young to die. Blue and great tits lay up to 14 eggs to allow for such losses. Cold weather and food shortage may lead to nest desertion, or to only the strongest young surviving. The death of one parent or interference from animals or humans may also cause desertion. We recommend that nestboxes in use are not inspected. Simply watch and enjoy from a distance.

How can I keep predators away from my nestbox?

Nestbox predators include cats, squirrels, rats, mice, stoats, weasels, woodpeckers and members of the crow family. As predators mainly hunt early in the morning, most people are unaware of their presence. A metal plate fixed around the entrance hole may deter woodpeckers and squirrels, while barbed wire, gorse or rose clippings above and below the box will give some protection against most mammals. Various commercially available deterrents may help reduce predation. The BTO guide already mentioned contains further ideas.

Conflicts between species

Birds such as sparrows and starlings often take over nesting holes used by tits. Most tits are able to defend a box successfully, provided that the intruder cannot get inside. A hole size of 25 mm will exclude larger species. Do not fix a perch on the front of your box as this will encourage intruders. Tits do not need a perch. By putting up boxes with different sized holes you will cater for all species. Please remember that sparrows and starlings are in serious decline and may need help even more than the tits. Do not place sparrow boxes close to ones intended for other birds.

Why do tits hammer away at the entrance hole?

This is probably a form of display by the male, rather than an attempt to enlarge the hole. Later, the female will also peck vigorously: natural holes may have all the surrounding bark chipped away. This may help her to judge how soft the wood is and whether the hole will provide a safe, predator-proof home in which to raise her brood. Blue and great tits will also hammer at the inside of a box or nest hole, perhaps as a form of display.

Why do tits enter nestboxes in autumn and winter?

They may be looking for a suitable place to sleep or perhaps feed. Roosting boxes are often subsequently used for nesting. Tits will not seriously inspect potential nest sites until February or March.

How can I prevent insects from taking over a nestbox?

Bees, wasps or earwigs will, an occasions, take over nestboxes and there is little one can do to prevent it - apart from using insect sprays. As many of,the insects are useful food for birds, it is best to leave well alone. Insects often move in after birds have finished nesting. Any young found dead are likely to have died of other natural causes. Despite thorough cleaning, it is not unusual for the same type of insect to return to the box in subsequent years. So leave that box in situ and put up another one a few feet away. It is rare indeed for both to be lost to insect invasion.