A look at some approved designs,
with a focus on the UK
The two organisations that publish plans for Tawny Owl nestboxes are the Royal Society for the Protection of Birds (RSPB) and the British Trust for Ornithology (BTO). These organisations do not give other designs for Tawny Owl nestboxes and they don't sell these boxes themselves. Most British retailers base their boxes on these designs. The chimney appears to be the most commonly sold box.
Royal Society for the Protection of Birds: Chimney box and letterbox
RSPB New chimney box design
Dimensions: 230 mm x 270 mm x 795 mm (9 in x 10.6 in x 31 in). Back panel overhang negligible at 1.8 in. Height to exit: 795 mm (31 in) sloping.
Internal diagonal: 12.5 in
Chimney design webpage "Attach the chimney box at 45° angle to a tree either with a batten of wood 100x710mm, or slung beneath a sloping branch." Some would say that 45° is too steep for a box with little overhang (protection from rain) at the top.
In both RSPB designs the box hangs off the back panel. This is a major design fault and could make the box hazardous as it ages. A lesser fault is that the floor panel is beneath the sides; it should be tucked inside them. Construction material indicated by panel thickness in both diagrams is 9 mm plywood; a thicker ply would be better. Means of attachment indicated in diagram.
The old RSPB chimney design. Same dimensions, but no lip at top. Thicker wood is indicated.
RSPB Letterbox design
Dimensions: 280 mm x 200 mm x 865 mm (box height without back extensions). Equivalent to 11 in x 8 in x 34 in. Height to door: 585 mm (23 in); door size: ca 7.5 in across x 8 in high (overhung by extending roof).
Internal diagonal: 12.2 in
Letterbox design webpage No instructions or guidance given.
This design has the same faults as the chimney. Plans also indicate single point of attachment at top, which, if followed, would be insecure and give little stability, especially in winds. Side pole is odd and at 60 cm extends too far. An owl landing at the far end will exert large forces on the box and its attachment to the tree.
British Trust for Ornithology: Chimney box and letterbox
BTO Chimney box design
Dimensions: 10 in x 10 in x 30 in (254 mm x 254 mm x 760 mm). Height to exit: 30 in.
Internal diagonal: 12.7 in
Design downloadable as PDF from this page.
The BTO's designs have much the same internal diagonal dimension as the RSPB boxes. The chimney shown here has no overhanging lip, so if the box is hung at 45° as suggested, the owls are going to get wet. The BTO chimney has the same risky back panel configuration as the RSPB design, but the floor panel is indicated as being inside. Instructions for buildng and attachment are limited.
BTO Letterbox design
Dimensions: 10 in x 10 in x 36 in. Height to door: 27 in. Door size: ca 9 in across x 8.5 in high.
Internal diagonal: 12.7 in
Design downloadable as PDF from this page.
Interestingly, correct construction principles have been followed here, eliminating the faults seen in the previous boxes. An extra touch is that the front panel is inside the side panels, though this isn't necessary as there is little risk that it will fall off! But the floor area of the box is still too small.
The BTO offers its Nestbox Guide from this page. The book is retailed by Jacobi Jayne, which also sells the Schwegler Tawny Owl nestbox (see previous page).
There's a set of owl box designs here, published on the Cotswold Owl Rescue Trust website. The plan for the tawny box is essentially identical to the RSPB letterbox design and dimensions, down to the side pole, and gives the same internal diagonal of just over 12 inches. The design is taken from the book Boxes, Baskets and Platforms by Sue Dewar and Colin Shaywer, £5.75 from The Hawk and Owl Trust (this page). With its cramped floor space (less than the BTO designs) and 2-foot jump to the base of the door it cannot be recommended.
How (the heck) does a Tawny Owl (and her chicks) fit into these boxes?
Before going on, let's look at what these dimensions mean for a tawny. In the diagram on the right I've assumed the mother is 15 inches from head to tail. On top of her and the chick I've superimposed the outline of a square box representing the average dimensions of all four recommended designs. With 9 mm ply this gives an internal diagonal of 13 inches, as shown by the dashed line. Click on the thumb for a larger version. (The photo has been extended by 10% vertically to compensate for the angle it was taken at.)
Hard to believe? Check with the pic of my tester in a 10-inch chimney box. At a little under 14 in from head to tail she's a small tawny! And she's pictured here at an angle.
The flawed chimney box
Suspension angle and space
The chimney box is a paradoxical creature because the space for the female and chicks varies with the angle at which it's suspended. As the scale diagrams opposite for a 10-in box and a 15-in owl show, at 60° the chimney box is little better than a similarly sized letterbox. At 45°, a commonly recommended angle, the situation hardly improves because one of the sides is becoming the roof and forcing the owl's tail back over her body. Also, she is now tending to fall into the trough formed between the base and one side. Only at angles approaching 30° does one of the sides become more like a floor and the owl can move up the box, giving her more space to stretch out.
These diagrams are for a box without litter, which is probably not uncommon in boxes that receive no attention after they have been put up. With a bed of litter the owl's situation is somewhat more comfortable simply because she has something to grip and the tendency to slide into the trough is reduced or removed altogether. Litter doesn't alter the problem of the limited space significantly.
Suspension angle and entry by rain
A remarkable feature of the chimney box that must make it unique among the world's owl boxes is that it is open-topped. Slung under an appropriate (10-inch diam) branch, of course, entry by rain will be largely prevented. But these boxes are often suspended in locations that offer no protection (e.g. on a trunk). The next diagram shows how the amount of rain that can enter is reduced as the suspension angle is made shallower. However, it's a surprising fact that even at 30° the amount is still 50% of what can enter the vertical box. At 60° (a commonly seen angle) the amount is 85% -- little reduction -- and at 45°, the angle recommended by the RSPB, it's still 70%. The BTO does not mention an angle on its webpage for the design. Some retailers show the box hung much too steeply.
It's obvious from the second diagram that a chimney box must have effective drainage if it is not to be positively hazardous to young owl chicks. Usually this type of box has an open slot along one side of the base (the side at the bottom in the diagrams) to provide this. However, this does not get round problems that will occur if litter in an unattended box becomes compacted and blocks the drain or if the box is put up incorrectly so that the drainage slot is not along the lowest side of the base. Finally, even if the box is correctly put up and the drainage slot remains open, this will not prevent litter becoming wet as water drains through it.
The British recommended designs are flawed in their combination of excessive height with limited side dimensions. The oft-given rationale for them is that the Tawny Owl is a cavity-nester. This does not amount to a convincing argument in their favour for two main reasons. The first is that few narrow holes in trees are likely to be as deep, and second, that a box made from smooth wood offers no climbing grip during entry and exit. This puts owls' eyes and other parts at risk from kicking feet and claws, and their flight and tail feathers are susceptible to damage both because of the limited space and as a result of the effort needed to exit from a tall, smooth-sided box.
Some might be surprised that designs are recommended here in the UK that might be considered cramped even for smaller owls, like the Screech Owl in the US. But even worse is that nearly all manufacturers simply ignore the already restricted side dimensions published by the RSPB and BTO and make their boxes even smaller! The internal diagonal across the floor in the "official" designs ranges from 12.2 in (RSPB letterbox) to 12.7 in (BTO both designs). No fewer than four retailers I've found sell boxes (all chimney type) with an internal diagonal of 10.4 in -- a full 2 inches less than the rough average of the recommended designs. This is for a 15-inch long owl! As we have seen, the angle at which the chimney box is positioned doesn't affect the situation much, if at all, until it approaches 30°.
The only merit I can see in these boxes is that they contain the owl chicks and prevent them from falling during the active phase before fledging. In other words, more owl chicks are likely to survive until fledging than on an open nest, and this is no mean advantage. That said, it is a misfortune for British Tawny Owls that hundreds of these boxes are sold every year when with a little thought about the designs and a decision in a boardroom they could be provided with something very much better.
Sadly the situation in France with designs that are sanctioned by a body with some authority (as opposed to a retailer or individual) is not much different to the UK.
The Ligue pour la Protection des Oiseaux itself does not advocate designs, but its Champagne-Ardenne branch publishes schematic drawings of nestbox types for a range of birds, including the tawny, on this page. The dimensions the Ligue suggests are of interest, if somewhat contradictory: "un nichoir cylindrique profond de 30 cm de diamètre au moins ou un nichoir à balcon de 600 x 250 x 250 mm". That is, it recommends either a cylindrical box with a diameter of at least 30 cm (inside or outside not stated), which gives a fairly generous circular floor 11-12 in across, or a "balcony"-type nestbox with much smaller floor dimensions of about 10 in x 10 in, which is much the same as the space available in the British boxes. The balcony nestbox is a French speciality that's supposed to give its occupants greater protection against predators. Basically it has an enclosed level, the "balcony", projecting out at the top front with the exit at the front of that.
PassionBassin has an extensive page of designs for various birds, with a letterbox type for tawnies that looks quite impressive but which turns out to be an overdeep type with cramped floor dimensions which are again similar to the British boxes (internal diagonal of about 12.5 in).
No approved designs found on web yet. Will add as and when I find any.
In the US and Canada the equivalent of the tawny in many ways is the Barred Owl. It too is nocturnal, a frequenter of woods, a cavity-nester with a widely varied diet and a territory of about one square mile. Indeed, it can almost be thought of as the North American Tawny Owl. Overall length is 17-24 in.
Frequently given nestbox dimensions are sides of 14 in by 14 in with a variable height, though the height is always less than the British "approved" boxes for the tawny. These side dimensions give an internal diagonal of 18-20 in depending on whether they are external or internal. Many webcams are posted for such boxes, and my clear impression is that the Barred Owl is provided with a more generous floor space relative to its size than the Tawny Owl is by the approved British designs. I have certainly seen nothing that suggests the cramped, uncomfortable conditions of the RSPB and BTO designs.
The Audubon Society's recommendations are fairly typical:
For Barred Owl: 14 x 14 x 22-28 (internal). This gives an internal diagonal of just under 20 inches, which is clearly sufficient for this owl.
The Society's recommended dimensions for the Screech Owl (7.75 x 9.25 x 16 (internal)) are of interest as they are pretty similar to the British recommended dimensions for the tawny, and larger than many tawny boxes sold in the UK. These gives an internal diagonal of 12 inches for an owl that's just 8-10 inches long!
Source: Audubon Educational Fact Sheet page, downloadable as pdf by clicking on "Owls".
The Department of Biology at the University of North Carolina at Charlotte has been conducting a study of Barred Owls since 2001. With 50 boxes up for a maximum of 45 breeding pairs their published design for a nestbox can be taken as authoritative. On their nestbox page they claim that "We've been working on nest boxes for a number of years and have come up with what we think is the definitive Barred Owl nest box design." Hmm, so what do these people who actually work with their owls, and presumably base their designs on how the owls react to different designs, come up with?
The recommended dimensions are 19 in x 19 in x 24 in! Wth the 3/4-in ply suggested that gives a whopping internal diagonal of almost 26 inches -- in other words more than the full length of the largest adult. A nice touch is a ledge inside the box placed 8 inches below the entrance. Also interesting is the way the height is fairly modest, giving a more "boxy" or cuboid shape. In other words a greater vertical extension to mimic some conceptual hole in a tree has been abandoned as serving little purpose and the owls are being given just a nice, roomy protected space.
Source: main Barred Owl study page and linked pages.
An older UNCC design, but it gives a good impression of the more boxy form that some designers seem to arrive at when they think about what the owl might actually like and set aside preconceived notions that it must nest at the bottom of some deep, dark hole. (Not clickable)
Nb: The UNCC drawing of the whole box is misleading as it misrepresents the relative proportions of the door and front panel, making it look like a deeper box.
The USDA Forest Service Northeastern area recommends dimensions of 13 x 15 x 16 inches high, with an entrance hole 8 inches in diameter (USDA nesting bird page 2). This is a very boxy shape with an 18.5 in diagonal.
Wildbird Magazine (USA, here, no details given on website) is also said to recommend a floor area of 14 in x 14 in with a height of 28 in (source: OwlCam "Building the Owl House; the OwlCam nestbox is based on Wildbird's recommended dimensions).
Environment Canada, a government agency, recommends floor dimensions of 14 in x 14 in for a Barred Owl nestbox. This gives an internal diagonal of nearly 20 inches (page is here, but the instructions are confusing).
powered by owls
It's not only British designs that offer female tawnies such cramped quarters for their six-week stint on the nest. But in the UK many retailers sell boxes smaller than this!
THIS PAGE TAKES A CLOSER LOOK at the two "approved" designs here in the UK. The designs can be considered as approved in that the two main bird organisations publish plans which many manufacturers follow. Some retailers cite these designs and/or state that their boxes have been approved by one or other organisation. As we saw on page 9, this may be despite the fact that they do not follow the recommended dimensions -- invariably in the interests of economy and to the detriment of the owl.
In other European countries the situation is less clearcut in that there seem to be no "officially" recommended designs and boxes range between the letterbox type and simple cuboid forms. In many cases designs published on the web tend to have similar cramped dimensions to those recommended in the UK. In the US the equivalent box is one made for the Barred Owl, a somewhat larger owl but with habits similar to the tawny's. Published dimensions as well as boxes for which photos are posted on the web indicate that they are almost all more spacious relative to the size of the owl. None I have seen are shorter than the length from the head to the tip of the wings.
To provide a means of comparing boxes I've again used the internal diagonal across the base as this is the maximum length available to a female owl to lie along. This is arrived at by lopping one inch off the external side dimensions and doing a simple Pythagorean calculation to give the internal diagonal. For boxes made from 9 mm plywood this will slightly underestimate the diagonal, but as all boxes are calculated in the same way the results are at least consistent.
As explained lower down (under "The flawed chimney box") the internal diagonal is difficult to apply to the chimney as this box is suspended at an angle. However, it's shown there that unless the box is suspended at shallow angles (around 30° or less) the internal diagonal remains a fairly good indicator of the room available at the base of the box.