Tawny Owl, Strix aluco, nestbox, nest box, design

THIS PAGE takes a critical look at the designs of nestboxes for Tawny Owls. I show why the two main designs sold in the UK are unsatisfactory, and suggest that a simple box design (like the one on the left) is a better deal both for the owls and for you if you want to see them.

The main difference between them is the inspection hatch in the chimney type, which is a big advantage for cleaning out. Note too that neither has a ledge, although it wouldn't be difficult to add one to the letterbox.

A major feature of sold nestboxes is that although manufacturers follow the designs advocated by the major bird bodies, almost all ignore the recommended dimensions and make them too small.

I found these examples, bought from Jamie Wood Products of Framfield, East Sussex, to be sturdy, with no looseness or play when moderate force was applied. Sold boxes are reviewed on page 9.

The American letterbox design is distinguished in having larger side dimensions and being less tall, which makes it an altogether owl-friendlier box. Many of the criticisms expressed below do not therefore apply to it.

So what's the problem?

It seems to me that the problems with this "tall box" or "tube" design arise from the fact that it's based on a concept (owls nest in tree holes, don't they) that doesn't translate well to machined wood.

The justification that's always given for this design is that it mimics the holes or cavities in a tree trunk (the letterbox) or a branch (the chimney) that Tawny and other cavity-breeding owls favour.

So, let's run through the problems. The illustrative pics show the chimney box

1. No grip in smooth interior Tawnies nest in tree cavities -- if they can find one. That's fine. But the problem here is that, unlike the interior of a tree hole, machined wood is smooth, so the owl has nothing to grip on inside the box walls when it enters or exits, and these boxes are too narrow for a wing-assisted jump or drop. The dangers posed to the delicate chicks, and especially their eyes, as a parent tries to control its descent, or kicks off to reach the exit far above, are not pleasant to imagine. Videos of tawnies in such boxes show that they brace their wings against the walls when climbing up. No commercial boxes are provided with climbing aids like interior rungs. I haven't seen a study of the number of young that emerge from these boxes with eye lesions, but such lesions are common in tawnies, and one wouldn't want to contribute in any way to early damage to their eyesight.

2. Boxes are too tall The first problem is compounded by the unnecessary height (two and a half feet) of these boxes. They would be more acceptable with a foot or more lopped off their top ends. In this respect the chimney version is less hazardous as the box slopes and a lip at the lower end of the hatch door provides some grip.

3. And too narrow The floor area is much too small. Around 10" is the side length recommended by the RSPB and BTO, but manufacturers go right down to 8 in. This makes a mean space for a mother and two or more chicks, let alone when Dad drops in. Ok, so owls may use smaller tree holes, but it seems perverse to make things so cramped for what are quite large birds.

4. Damage to feathers The surface of plywood is covered with numerous small splinters or wood "hairs". Owl feathers, which are soft and delicate, are damaged or pulled out when they catch on these splinters.


Three problems relate to the chimney type only

5. Inconveniently shaped space at bottom The fact that this type of box is fitted at an angle can make an uncomfortable situation even worse, as mother and chicks have to arrange themselves to fit in the hard V formed by the base and the front panel. If there's litter in the box this problem may not be so bad. The problem is examined further on Page 11: Approved designs as it varies with the angle of the box.

6. The open top In some circumstances -- especially if the fixture is steeper than about 40 and the branch above doesn't protect the entrance, the open top is going to let in rain. This should run out of the drainage slot, but probably not before it's made someone, or the litter, wet. Unlike those of other birds, owl feathers are very wettable, and for a chick over an open drainage vent wet feathers can mean hypothermia. Mother isn't with her chicks from about 10 days after the first one hatches. This problem is also examined further on Page 11.

7. Fixture itself is hazardous It is often recommended that the chimney box be suspended under a branch using wire, but if it is screwed the box will be hanging from its attachment, with its weight acting to pull fixing screws down and out of the branch. The curvature of the branch may also mean that only the centre screws will go fully into the wood. In unskilled hands or with hard (i.e. impenetrable) or rotten wood this could prove disastrous. Arriving owls land with quite a jolt, and the extra weight of a father owl on or in the box adds to the risk that a box of this type will be pulled from its attachments.


Two more problems relate to both types of box

8. No ledge With the letterbox this problem is easily remediable, but no owl nestbox should be without a ledge. First, it makes landing easier for the parents, and second, it gives the chicks a safe place to take stock of their surroundings in the week before they fledge. The problem here is that a second chick pushing up from behind one already perched in the door may force the first into an ill-considered flight that can end up on the ground. This could be bad news for a fledgling that can barely fly.

9. Mother is cut off from the outside world My impression is that open nests have two big advantages that the mothers appreciate. First, they can see what's going on around them, providing interest and entertainment during the six long weeks that they're stuck with their brood. Second, they can -- and do -- communicate easily with their mate while he is out hunting to feed the family. Neither of these is possible if you're holed up at the bottom of one of these nestboxes.


One last problem with the chimney box

10. Chicks may not be able to get out I have no evidence for this, but one does wonder whether some chicks never make the two-foot jump up a smooth-sided box to the door.

SO SMALL The small floor area of the boxes makes a very mean space for an owl -- and Sophie is small for a female tawny! Pic shows the splinters on the plywood, and how in the cramped space the ends of her flight feathers are jammed against the wall. View is through the cleaning hatch.

SO DEEP At the bottom of a lift shaft! The boxes are excessively tall, which adds to the risk of feather damage, egg breakage or injury to chicks when mother comes in and out.

BAD FOR FEATHERS Detail from previous photo showing how the owl's tail is bent right round (lower right), with obvious risk to feathers. Also note the pulled feathers on the floor -- and all she's been doing is standing patiently.

GET ME OUTTA HERE I'm holding the box at an angle as if it were slung under a branch. Although she's got a grip on the back of the hatch door, the box is more like a trap than a safe nesting site. Look at what's happening to those all important flight and tail feathers. The gap beyond her foot is the drainage slot.

Potential attachment problem (hazardous fixture)

On this chimney box the wide spacing of the screw holes for attaching the box to the under side of a sloping banch means that only the centre screw is going to go fully into the branch. (There's a similar set of holes at the lower end.) This is easily remediable, but it never should have been allowed into the design in the first place.

Do these boxes have any advantages?

With a ready-made box you don't have to make one yourself. A second undoubted advantage is that for all the problems a chick is safer in one than on an open nest.

As may be evident, my opinion of these boxes is that they're pretty awful and shouldn't be used. Interestingly this view was shared by a conservation officer I spoke to recently. His apt word for them was "diabolical". The main problem is that they're just too darned small. So in the rest of these pages I'm going to consider spacier, more box-like designs. These are illustrated in detail on the next page, Making your own.

There's a review of commercially available nestboxes on page 9, where dimensions and vendors are also given.

An alternative to the "tubes": The box design

This is my own nestbox. It's totally over the top, but I made it for Sophie's mother, an owl I'm very fond of and I had some spare time on my hands. If you make a nestbox from scratch, give a simple box design a go. The roof can be a flat panel. The key point is that the entrance must be some way above the floor -- 5-6 inches minimum. If you put it lower the chicks may fall out. The inner dimensions of this box are 15 in wide by 10 in deep by 12 in high to the inner ceiling panel (not shown here). Quite compact but, as shown below, still roomy, giving mother room to stretch out and the chicks space to exercise their wings.

On the next page there are construction plans for a basic version of this box.

A box of this type and size will make your local mother tawny very happy. She can while away long hours on the eggs by looking out. She can hear and call to her mate. She will be comfortable. Even my quite small box gives an owl parent plenty of room, and the youngsters are able to play and, later on, exercise their wings. There is little risk of them falling out, but getting out in the week before they fledge is easy.

The box has been outside for two years. Yacht varnish may not be a usual finish, but it certainly keeps everything in good condition. Construction is screw and glue. Always use screws in preference to nails.


Two important caveats

Predators A design like this has one disadvantage you should be aware of, and that is that it doesn't give the eggs or chicks as much protection against predators as those with an entrance that's higher up. Crows may be egg-raiders, and a crow would be capable of taking an unguarded very young chick. Grey Squirrels compete with Tawny Owls for nestbox sites and may harass a brooding female, sometimes resulting in breakage of her eggs. Where they are present Goshawks are the only bird of prey that takes tawnies in the UK. If you have them in your area you might think twice about using a low-door design. Never place this type of nestbox within climbing reach of cats. In the US and Canada fishers (a marten) are able climbers and can pose a dangerous threat to owls nesting in any type of site. The equivalent in Europe and some parts of the UK is the Pine Marten, and these are a well-documented threat to owl nestlings. Predator-resistant box designs are given on the next page.

So far, touch wood, I haven't had problems. The mother owl is usually on the nest at all times during the day and only leaves for periods of varying length at night. She is there almost continuously until the first chick to hatch is about 10 days old, when she takes up a guard post nearby where she can watch the nest. We don't have martens or Goshawks, no crow that I know of has come anywhere near, and the local squirrel leaves the nest well alone when the owls are in occupation.

And NOT for use by Barn Owls The Barn Owl Trust, Devon, advise that low-profile boxes should NOT be used for Barn Owls. Their carefully collected data show that "Out of 238 recorded cases of [Barn Owl] owlets falling from nestboxes 75% were from the old-style all-on-one-level boxes [i.e. low-profile boxes] and only 4% were from deep boxes." Barn Owl chicks just seem to be more given to doing foolish things when they're too young than tawny chicks. As a result, the BOT say, "In 2008 we decided to stop using (and stop providing information on) Barn Owl nestboxes with low entrance holes. They are just not safe enough." See this BOT page — Getting the best Barn Owl nestbox for your site — for their excellent advice if you are thinking of putting up a Barn Owl box. They do say, however, that a low-profile box is ok for Barn Owls as long as there are plenty of branches around so errant chicks can get back in.


So why is a low-profile box all right for tawnies?

I find this Barn Owl stuff fascinating as with all five tawny chicks in the two broods we've had in our low-profile box the evidence has been quite surprising: the chicks stayed inside the box right up to fledging. I would have expected them at least to come out on to the ledge in the days immediately before fledging — but they haven't even done that.

The only explanation I can come up with is based on observation of their behaviour. First, in our rather open box the chicks can see out easily and so feel no need to come out. Second, and even more important, by just being near the wide door they can all be in an equally good place in the queue for food. In fact I find this combined explanation pretty convincing and it's backed up by all the behaviour I've seen. Chicks I've been able to watch closely via a nestbox camera appear to be quite content to look at the world from the safety of the box—and they spend quite a lot of time doing this, especially at night. Then, when a food delivery is expected they can all take up an advantageous position to intercept it as food is delivered through the door, not on the ledge.

Compare this with the situation in a deep, narrow box like the chimney. Towards the end conditions inside are foul (and tawnies do have enough of a sense of smell to know that!) and cramped, and up at the top there's this brightly lit exit to a world they can't see. Not only that, but that hole at the top is where you want to be to make sure of getting fed. It's a no-brainer, and all the chicks struggle to make it up to the exit—often all at the same time, and usually some time before they're able to fly. If there's no ledge, or no nearby branches, or the door isn't big enough, the situation can become very risky for an inexperienced pre-fledgling.

It's possible, of course, that the chicks produced by our owl pair are unusually timid, or sensible! But for all that it's another reason why I think the tube boxes are actually hazardous nesting places for Tawny Owls.

Index page

Page 4: Making your own

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Tawny Owl Nestboxes:

The UK Designs

Definitely not a UK design, this one! More below . . .

The commercially available "tube" boxes

The two examples shown below are almost the only types of nestbox for Tawny Owls on the market in this country. The "letterbox" is designed for vertical fixing to a trunk, while the open-topped "chimney" is fixed or slung at 45 or so, usually beneath a sloping branch. They're both about the same size, with dimensions of 8-10" (sides) by 30-36" (height). Holes for fixing with nails or screws are provided in the back panel. In both cases drainage for chick poo is through a narrow slot at one side in the base. Construction of the two shown here is from 9-mm exterior (weatherproofed) plywood, pins and glue. The thinness of the ply means that they're quite light.