Modifying a chimney box

to make more room for

your tawnies

(UK interest only, and I don't even recommend trying it! I just had the spare box)

THE CHIMNEY-STYLE BOX for Tawny Owls (below right) may be one of the more abominable abodes inflicted on a bird anywhere, but it has one virtue. It can be quite easily converted into a pleasant nesting box with plenty of space for a mother and several owlets. It'll even take a camera as well. Conversion is reasonably simple and quick -- probably quicker than running up a box from scratch -- so as the chimney box can be had for £25 it's an option to consider. Tool requirements are minimal.

This page mainly applies here in the UK as it seems no other European country is as dominated by these unfortunate boxes. If you do modify one, make sure the box you buy has minimum side dimensions of 10 inches. Several retailers sell chimney boxes with sides of 8.5 x 8 in; these should be avoided. UK retailers and boxes are listed on page 9.

The conversion described here does not include the windows shown in the photo above; these are to provide light for a daytime camera and are not a required feature.

You don't have to follow the plans set out here in every exact detail. Unlike my other nestbox, I did this one on the fly and in a hurry, and in places it shows! The plans are given as a guide to what can be done. It's probably better to take active charge of your own plans rather than follow someone else's mindlessly. But check this page through as there are several things that mustn't be overlooked in modifying this box. Sketching things out on paper beforehand is highly advisable.

Photos below right.Top: the original chimney box with its cleaning hatch off. The owls enter from the top. Bottom: the modified box goes up at last . . . and will have to come down again because I can't get the tree attachment right -- it's too high up.

What you need, apart from the box

TOOLS

1. A sharp saw.

2. Screwdrivers (a short-handled screwdriver is useful to work in the limited space inside the box).

3. Power drill, set of sharp bits and sanding disk attachment (or plane).

4. Set square, tape measure, pencil, rule.

MATERIALS

1. Enough exterior plywood (9 mm if box is made of this) for ledge and to close off end of box.

2. Wood pieces for ledge support, internal and back battens and some other small items.

3. Exterior (waterproof) glue.

4. Wood screws (galvanised, various sizes).

5. Medium-grade emery or sand paper.

7. Yacht varnish (plus paint brush and white spirit thinner/cleaner).

8. Wood stain (use on exterior only).

Optional

1. A set of clamps (one table or bench type for sawing and several small "portable" type for holding pieces of wood in place while glue sets).

2. Small wood plane -- used for shaving cambers on internal battens and the ledge support.

Converting the box

Step 1: Remove the latch

Remove the hatch latch (photo at right). This may result in some of the top layer of plywood beneath being pulled off, and you may need to fill the depression later with wood filler.

Step 2: Saw off top end of box

Mark and saw off the top of the box above the top opening (cut is shown by red line in the diagram below). After this you should have an opening with all four sides level. It's not necessary to remove the projecting part of the back panel at the other end.

Step 3: Saw off front panel (optional)

It can be an advantage to have access to the inside of the box by removing the front panel. This is only essential if a camera is being installed. If not installing a camera, it's better not to remove the front as the box will be more weatherproof.

Warning: If the front panel is removed it's advisable to seal the top and sides of the join by rubbing mastic in before the box is put up in the tree. This forms a temporary seal that'll allow the front panel to be removed again if necessary. The camera compartment is reached through a door at the right end -- see later.

Mark off with a pencil line as shown in the diagram above right by the red line. The line should be about 1 in back from the front of the box. Mark and saw carefully as the box must fit back together neatly. Sand off any roughness and splinters. The front panel will be secured to the rest of the box by screwing into four corner battens, shown later. If you are putting in a camera, check out the diagram at the bottom right of page 8: Addng a camera, before continuing. Any extra instructions for camera installers are flagged in red below.

Step 4: Modifying the new entrance door

The entrance (formerly the hatch) needs to be made smaller by adding a 2-inch strip of ply on the left side and a 4-in high strip across the bottom. The bottom piece is a "doorstep" to keep the chicks safe. This is an essential feature and must not be left out. These pieces of plywood can be scavenged from what's been cut off the top in Step 1. Aim to end up with an entrance about 6 in x 6 in.

Fix the side piece first as it's the easiest to glue and screw in, and when screwed at top and bottom won't dislodge when the doorstep is fitted. Then measure and cut the doorstep carefully. Fix in place with glue and by screwing through into a small backpiece made to run fully across the back of both added pieces inside the box.

Lower right: The inside of my front panel showing the lath used to secure the doorstep. Make your lath long enough to stretch across both added door pieces (ignore what I've done to avoid the window!). Place it just under the door to add width for an owl to grip. If you haven't sawn off the front panel like this, screw from outside but ensure that there are no protruding screw points inside.

IMPORTANT! In fitting the inside lath, make sure to leave enough space below it to take another internal fixing lath later into which the ledge support screws will go. Check with Step 9 (the ledge) to see why and how much space before proceeding further.

Step 5: Internal corner battens

Adding battens in the corners will strengthen the box.

Camera installers: These battens must be put in as they're what the front panel is screwed back on to.

The photo for Step 6 (below right) shows the camber on the battens to reduce the risk of harm to chicks. I use a small plane to do the camber, but a sanding disk or even sand paper can be used to take the sharp edge off.

At the open right-hand end of the box set the battens far enough back so that a panel can be fitted snugly into the recess to close this end off in Step 7. Camera installers: This door is the main access to the camera and light compartment.

It's a good idea to add two vertical strips of wood between the battens at the right-hand end to improve the weather seal, especially if a camera is going in.

Step 6: Closing off the drainage slot

My box has a narrow drainage slot at the closed end (base of original box). The slot's no longer needed (and it'll create draughts), so if yours has one close it off with a piece of wood cut to fit. No need to screw, just glue into place.

How it should look (with the front of the box off!) is shown in the photo at right. If you're working on a closed box you won't see it like this of course.

Step 7: Panel to close off open end

Cut and fit the end panel (right). It goes inside the adjacent panels and should fit snugly into the recess left for it when fitting the battens at this end in Step 5. Attach to the battens with four screws. Don't glue it in as it gives useful access for cleaning and maintenance. When you've got a good fit, leave it off for the time being.

Camera installers: As mentioned above, this door is the main access to the camera and compartment. Add two vertical strips of wood between the battens and flush with them to minimise chances of rain getting in.

Step 8: Drainage holes

Drill drainage holes in the floor and over the full width and length of the box. (Camera installers: but not in the camera compartment!) These should be about 1/4 to 5/16 in diameter and on about 1 1/2 in centres. Don't worry about following these measurements exactly -- you just need quite a few, that's all! Try not to splinter the plywood by drilling through too hard. Splinters will form, though, and these are best removed by sanding after a coat of sealant has been applied. Sanding unsealed ply can just raise more splinters and make the problem worse.

It won't be possible to countersink the drainage holes like I have here if you're working on a closed box, but it's a small detail that doesn't matter. Here I've finished sealing the floor but not started the walls. The pic shows the panel for the camera compartment, which will slot in between wooden guides. Ignore the way I've done the right end batten -- a mistake!

Camera installers only: At this stage glue in slats to act as guides for the camera compartment panel. Two across the floor and two across the ceiling either side of the panel. Secure the slats with small screws if you don't quite trust exterior glue in continuous damp conditions -- I don't. The placing of the panel should leave enough room for the camera and light that are going in -- mine is a little way to the right of the owls' entrance door.

Step 9: The ledge

The ledge is a feature that adds considerably to the chances the owl chicks will survive. The reason is that for a couple of weeks before they're fully capable of flight they become very active and may start emerging from the box. With no ledge there's a risk that a chick may be pushed into the air by another trying to find a space in the door. It'll end up on the ground where it's at considerable risk from predators like foxes. A ledge can prevent accidents like this.

You don't need to follow the shape I've made -- the ledge can go the full width of the box, or be shorter and centred on the door. My ledge doesn't continue to the left because it would cut off the view of the window from the ground. The ledge should be about six inches deep.

The important point is to position it about one inch below the entrance. The owls come in at speed and use the strip under the door to bring them to an abrupt stop -- a bit as if they were landing on the deck of an aircraft-carrier.

The ledge is supported on a stout piece of planed wood (thicker and deeper than the internal battens). There's a slight construction problem here for those working on a closed box. I was able to screw from the inside, but you'll have to screw from the outside. This means that to get a secure fixing you'll have to screw through into a lath on the inside of the box. You can either hold this in place while you drill pilot holes, or it can be glued in place and drilled into after the glue has set. The details are shown in cross-section in the diagram below left.

Camera installers: In you case fix the support batten by screwing from the inside. Also, don't fit an internal lath as it will interfere with the camera panel and guide slats.

For extra fixing security use glue as well as screws for all three fittings (ledge, support and internal lath).

When screwing in the support batten from the outside, make sure that no sharp screw points break through into the inside. If they do, you'll have to file them down or use shorter screws (or a thicker internal lath). Use at least four screws along the length of the support, and the same number to screw down the ledge. There's quite a bit of leverage on this piece and you don't want it to fail in action.

The ledge is best screwed down after fitting the support batten. Avoid hitting the screws attaching the support batten!

My two photos at right below show the support before a camber was applied (shown in the diagram on left). It just looks better with a camber.

Niggling detail for those who've removed front panel, including camera installers: As the photo at right shows, the ledge support may cover the point where one of the screws that will attach the front panel to a corner batten has to go. The remedy is either to have the ledge support stop short of the end or to cut a generous notch in it (shown in the far right photo) to give access to the screw hole.

Couple of camcorder frames showing how adult owls use a ledge. Left: aircraft carrier stop against the base of the door; right: kicking off. This is a female with young. The father (fnight photo) sometimes stands on the ledge to survey the scene when he arrives with prey (thumbnail only).

Step 10 Finishing off the interior and exterior

First, fill (and stain if required) the depression left when the hatch latch was removed in Step 1.

There are two good reasons for sealing and sanding the interior, and as far as I know yacht varnish is not harmful when it and solvents have thoroughly dried out. The reasons are that (1) a smooth, non-splintered finish can be got that will not damage delicate owl feathers; and (2) plywood is highly absorbant and if not sealed will take up owl chick poo, which is very revolting stuff and could ruin the box. Cleaning becomes a whole lot simpler. A seal also protects glue and metal pins and screws from damp.

Thin some yacht varnish with about 1/5th (20%) white spirit. This makes it more penetrating and it won't sit on the surface so much. Cover the whole interior of the box, including the door edges. Brush well into the drainage holes, making sure it's taken up by the walls of the holes.

The whole of the exterior, including the ledge and end door, can be done at the same time. As this is a thinned first coat, it will be absorbed and won't give a glossy finish, but it will protect the box and increase its life. Basically plywood is not an ideal material to use outdoors. It's really up to you how many coats you put on the outside of the box. Two thinned coats will still give a matte finish that isn't at all overglossy and will protect the box well. Apply woodstain to the ledge pieces between the first and second coats as woodstain is not absorbed well and will stop varnish penetrating the wood. Don't use woodstain on any interior parts.

Seal and stain the wood you're going to use for the tree-fixing battens at the same time (for these see Step 11).

Back to the inside . . .

When the box is dry, sand the whole interior -- especially the walls -- to remove as much roughness and raised splinters as possible.

Cover the interior with a second coat of thinned varnish, and sand the walls again if necessary. The floor should be left with any roughness it may have.

That's probably enough varnish for the walls and ceiling, but continue to seal the floor (including the interior sides of the drainage holes) and about an inch up the walls until you've got a glossy finish. Don't sand the floor further -- it should be left a little rough.

Back to the outside . . .

Don't sand the top surface of the ledge as it should be left rough.

Step 11 Tree-fixing battens

There are several ways of fixing a nestbox to a tree trunk. Here I'll only show the most common of these, along with a more secure variant.

Basically, a simple plank is attached to the back of the box and this is used to nail or screw the nestbox to the tree. This form has the disadvantage that there's only a single line of contact with the trunk and the box can rock from side to side.

In the variant version two battens 4-5 inches apart are used. With two lines of contact, this gives an attachment that's less prone to rocking. Contact with the tree is improved by cambering the battens.

These points are illustrated in the diagram below.

The upper attachment point to the tree is the most important and must be the strongest as the weight of the box tends to pull it out. (Imagine that the box was only attached by a hinge at the bottom -- the top of the box would swing out and away from the tree). The lower point of attachment is less critical.

Single-batten fixing (photo at right)

Use a plank about 4 in wide and long enough to extend about 8 in above the box and 4 in below. The plank used should be stout -- e.g. 3/4 in thick. Working through the entrance door, attach it to the back panel on the nestbox using at least five screws in the pattern shown in the diagram below. Screw from the inside of the box, not from the back of the plank. This is a vital fixing -- don't take risks with your owls!

Then drill holes to take the tree-fixing nails or screws as shown below. Drill at least three holes above the box. Just two drilled horizontally across the plank below the box will be fine.

Single-batten or plank fixing in use at the Hawk Conservancy Trust. This type may be prone to side-to-side rocking. (Not clickable)

Double-batten fixing

This is the type I've used on my main nestbox (photo at right). It gives a better grip on the tree, largely eliminating any tendency of the box to rock. But it takes a little longer to make. These battens should be cambered (more on this below); use a disk sander or small plane.

You need two lengths of wood of about the same cross-sectional dimensions as the ledge support in Step 9. These battens should extend about 5-6 in above the box and 3-4 in below. They are placed 4-5 in apart (see diagram below).

Double-batten fixing on my main nestbox, here used with a cord attachment. A snug, non-rocking fixing can be obtained (right).

You can make a guess at the angle of the camber, but a good idea is to draw a scale diagram like the one on the right. From this you can measure off the angle that fits the site you have chosen for the nestbox. You'll need to measure the diameter of the trunk. Draw a circle (or part of it) with this diameter and add scale drawings of the battens in cross-section. You will probably find they're best with their centres about 4-5 in apart -- this will depend on the diameter of the trunk. The intersection of the curve of the circle with the battens gives you the camber. Measure the angle of a line that represents the average angle of the curved intersection.

If you can't do this, use an angle of 25-30°.

When that's done, make the camber and attach the battens to the back of the box using three screws down each (diagram above). As before screw from the inside of the box out into the battens. To take the nails or screws that'll attach the nestbox to the tree, drill two holes into the top of each batten and one into the bottom.

The double-batten arrangement has the advantage that it can be lashed with cord. This is not possible with a single batten as the fixture will inevitably rock.

Step 12 Last things

For those who have not removed the front panel, all that remains to do is to screw the end panel into place. Leave the box in a warm, well-ventilated place until varnish and solvents (especially white spirit) have thoroughly dried or evaporated. This may take a couple of weeks.

If you have removed the front panel to make work inside the box easier, you may need make a temporary weather seal around the join with mastic. Press this in and smooth with a filling spatula.

Then check with Page 6: Putting it up.

Camera installers: The front panel is attached to the main body of the box by screwing into the ends of the four internal corner battens. See the diagram at the bottom of the next page for how a camera and light can be fitted. Specific arrangements will depend on the equipment you have.

Index page

Next page: Adding a camera

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