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Nest Box Design I don't think the nesting habits of suburban brushtails are known in great detail. We've seen, or heard about, them sleeping in various unlikely and uncomfortable places such as rolled-up garage doors, or the forks of trees. On the other hand, our comparatively safe and comfortable possum boxes have often remained unused for months at a stretch. This might be a result of the possums deliberately giving one spot a rest to put predators off the scent, but I suspect that our nestbox design is far from perfect. There are a number of nestbox designs available on the web. Most of them appear to be little more than modified bird nesting boxes. I think that comparatively little science has gone into nestbox design and we're a long way from having the ideal nestbox. The two boxes we have constructed so far have been based on plans provided on the Ipswich City Council web site (click on "+Habitat/Nest Boxes "; the PDFs are at the bottom of the page) with a few modifications. However, we've found that there's much more to nest boxes than simply following a plan like the above one and if you're contemplating putting up your own nest boxes, you might find the comments below helpful. Although these notes are primarily about nestboxes for brushtail possums, they will also be relevant to other species as well. Materials Plywood is probably the best material to use. It's very strong for its weight, it stands up to weather very well without warping or splitting and it's easy to work with. You must use exterior grade plywood, which incorporates a waterproof adhesive - if the plywood doesn't say what type it is it probably isn't suitable for exterior use (although thick "structural" grade plywood might be OK if painted). You can also use marine grade plywood if you want, but this is more expensive and harder to get and doesn't have any special advantages for this application. [Apparently the difference between marine and exterior grade ply is in the choice of timber, with the wood used in marine grade being free of holes and splits so it won't leak when immersed in water. The adhesive used is the same for both.] I used 15mm exterior grade plywood for both boxes. Different plans I've seen specify thicknesses from 12mm up to 20mm, but I believe 15mm ply is a good choice. Thicker types are more expensive, harder to find, and make the box heavier (which can be an important point when you're balancing on top of a ladder trying to attach it to a tree), whereas anything thinner is difficult join without splitting the wood. It's possible to make a possum box out of solid wood, and this isn't a bad option if you have some scrap wood lying about and want to save money, but you'll find it's more difficult to do and you'll end up with a heavier, clunkier box. The Victorian DPI site describing nestboxes for wildlife warns that solid wood may warp and suggests that you "Arrange timber so that growth rings radiate away from the centre of the box. Subsequent 'warping' will force the box together rather than apart." A picture is worth a thousand words here, and the illustrations on their site make it clear what they mean. Don't use treated pine. Some possums are fussy housekeepers who lick the walls of their box clean, and we've also occasionally seen them biting at their boxes apparently to remove splinters and sharp edges, and therefore treated pine, which contains toxic chemicals, is not a good material to use. Don't use craftwood, particle board, laminex or similar composition timbers either; even with painting these will disintegrate rapidly when it rains. This type of timer product would be OK as the internal layer of a double-walled box, but should never be exposed to the weather. I would advise against using plastic or metal, even just for the roof of the box (although a metal or plastic covering over a wooden roof would be OK). Possums sleep during the day time when it's hot and something without the insulating properties of wood can get stifling. In fact, I have my suspicions that even a standard plywood possum box would benefit from extra insulation [this is discussed below under "Insulation/Temperature Control"]]. Consider that possums naturally sleep in tree hollows, which are insulated by many centimeters thickness of tree trunk. Fasteners The box should be fastened with screws rather than nails as these are stronger and less likely to pop out under stress. You must use screws suitable for exterior use, such as galvanised ones. I chose ones intended for use with treated pine; these have a Climacote™ coating which appears to be a very tough form of enamel rather than conventional galvanising. I used size 8-10 x 57mm countersunk screws for assembling the sides and bottom and 8-10 x 25mm for attaching other items such as climbing rails. Possum boxes can apparently be assembled using glue in addition to screws, but I don't think this is necessary. A properly constructed box will hold together perfectly well without glue (in fact, the first possum box we made survived a fall of over 4 meters from a tree without damage), and I don't see the benefit of running the risk (albeit a minor one) of using an adhesive which may be toxic or give off a smell that could deter possums. If you really want to use glue, I would recommend PVA woodworking adhesive (e.g. Aquadhere), as this is relatively non-toxic and odour-free. I wouldn't use building adhesive (e.g. Liquid Nails).
Left: Screws for assembling box Right: Screws for attaching box to tree Painting We initially didn't paint our boxes and they stood up to all sorts of weather for nearly four years without any problems. The wood faded to a greyish colour which to some extent made the boxes blend in to the natural colours of the garden. I think possum boxes constructed from exterior grade plywood could safely be left unpainted if necessary. When we refurbished the first possum box, we decided to paint it. This was partly done to pretty it up, partly in the hope this would make it cooler in hot weather, and partly because extra protection from the elements wouldn't hurt. If you've got a neat orderly garden, painting your box(es) to match your colour scheme is highly recommended - why not make them a nice feature of the garden, rather than something that needs to be hidden away ? I've seen possum boxes painted a dark green, and I'm not too much in favour of this. The dark shade chosen often contrasts with the natural colour of the surrounding foliage and, rather than camouflaging it, makes the thing stand out even more. Also dark colours will make the box hotter inside if it's exposed to sunlight. We painted our box a colour called Poncho, which is an off-white. This matches the house and also goes with the off-white colour of the tree to which it is attached. Only paint the outside of the box. The inside doesn't need it, and, as with adhesive, it might be toxic or offputting to the residents. Box Size The plans we used recommended that a brushtail box should be 350mm wide x 350mm deep x 500mm high (interior dimensions). Our second box was intended for ringtails and was 200mm x 200mm x 450mm. I believe these dimensions are far from ideal. Our experience is that the brushtails actually prefer the considerably smaller "ringtail" box. Even our biggest brushtail, Olaf, would always prefer the smaller box, despite the fact that his body filled almost half of it. Ringtails, on the other hand, steadfastly refused to visit either box. We've read that possums prefer to squeeze into the tightest space they can manage, presumably in order to exclude larger predators, and this seems to be born out by our observations. It's possible that brushtail box dimensions quoted were in fact intended for southern brushtails. It seems that the further south you go, the bigger the possums get, with Tasmanian brushtails being gigantic fearsome things almost twice the size of our Queensland ones. If I had to recommend the dimensions for a brushtail nestbox, I'd probably go with those specified on a leaflet from the University of Ballarat, which suggests: 300mm wide x 250mm deep x 510mm high (interior dimensions). It's also much easier to position a camera in a deeper, narrower box like this. Box Shape Our boxes, in common with most designs, are of the "vertical" type (i.e. with the longest dimension vertical), however the Australian Nestbox Company claims various advantages for a horizontal orientation. This is one area of box design that needs more investigation. I can't really comment on the suitability of different box shapes except to note that the classic vertical type has the advantage that a box camera mounted at the top of the box can see the whole interior. Entrance Hole The diameter of the entrance hole is said to be a very important parameter in nestbox design. For the first box we built, the plans recommend a 120mm diameter entrance hole. Elsewhere, hole sizes of up to 150mm were suggested. 120mm looked a little small to me, so I used 150mm instead. This was probably a mistake since the second (ringtail) box we made with a 100mm hole was preferred by the brushtails. Early in 2008 I fitted a collar around the hole in the larger box, reducing the size to 110mm, and since that time the box has been used more than formerly, although it still remains less popular than the other. I think a hole 100-120mm diameter would be a safe size for brushtails. If you were putting up a number of boxes, it wouldn't hurt to use a number of different hole sizes. On one hand, a mother plumped up with pouch young might need a larger hole (and possibly also larger box), on the other, a recently independent baby possum might prefer a smaller one. The original design had an entry hole in the front of the box, i.e. on the side opposite to the tree. I think this is a hangover from the bird-box history of the design, and isn't ideal for possums. We built our first box this way, but in this case, branches from a nearby tree came alongside the box, making it easier for possums to enter and exit. Our second box had a hole in the side adjacent to the tree trunk, which is a better option. If you're making a box for smaller species like sugar gliders, it might even be preferable to have the hole on the side of the box against the tree. [Obviously the box would need to be mounted so that it stands a few inches away from the trunk to allow access to the hole.] One site mentioned that sugar gliders prefer hollows where the entrance is concealed from view. Roof of Box The top of the box should be hinged so that you can open it up when you need to. Some designs call for the roof of the box to be fixed in place permanently, but I believe this is a bad idea. You might need to clean out ant or wasp nests if they decide to take over the box (a common problem), and it might be necessary to remove a sick or injured possum. We used a strip of standard piano hinge along the back. This is cheap and easy to work with, but has the disadvantage that it rusts after a couple of years out in the weather. The ideal material would be stainless steel piano hinge but this is more expensive. Probably ordinary brass or galvanised hinges would work just as well. Some designs recommend using a strip of inner-tube rubber for a hinge on the basis that it's cheap and waterproof. I'm not sure how long this type would last when exposed to sun and rain; it's claimed to hold up very well. On the opposite side to the hinge, you should put some sort of catch or fastener to prevent the lid from flying open in strong wind. Some designs recommend a sloping roof to allow rain water to run off more easily. This is probably not a bad idea, although it makes construction a little more difficult. For what it's worth, both of our boxes have flat roofs, and we haven't had any problems with rainwater accumulating or running back into the box. Baffle It's not essential, but I'd recommend that the box incorporate a baffle. This is basically a fascia board (i.e. flap of wood hanging down vertically from the roof) positioned like a visor in front of the entrance hole. Its function is to screen the hole so that it's not possible to look directly into the hole from horizontally in front of the box. The use of a baffle has been suggested to prevent Indian Mynahs nesting in the box (apparently they won't nest inside if they can't fly directly in). The baffle also makes the interior of the box darker and more appealing to possums, as well as keeping out wind and rain. Our second box was fitted with a baffle - this can be clearly seen in the photos - and this is one possible reason why the possums prefer this box. Possible disadvantages of a baffle are that it might make the box hotter in warm weather and that it prevents possums from entering the box from above. For this reason it's essential to fix one or more rungs under the hole to allow them to climb up from below. Although only a single rung is shown in the photo of box #2, I ended up adding another two rungs underneath it to form a little ladder A compromise is to use a partial baffle (i.e. one that only partly hides the hole). If you don't have an Indian Mynah problem, this is probably the best solution. Floor of Box The floor of the box should fit inside the walls and be fastened with screws going horizontally (as opposed to the walls sitting on top of the floor with screws vertical). This is a stronger method of construction and also looks neater. It is normal to drill a drain-hole of about 1/4 inch diameter (5 mm or so, it isn't critical) in each corner of the box floor to allow any water that enters the box to drain out. We've never had a problem with rain entering a box (they seem to stay dry in even the fiercest of storms), however a few times we've had to pour water into a box to shift ant nests. Inside the Box Most designs suggest you should fix wire mesh inside of the box to allow young possums to climb in and out easily. Something like this is required, because the concave plywood surface of the interior of the box is too smooth for possums to climb. Wire mesh, however, may not be the ideal material. Sometimes it appeared as if baby possums were getting their claws caught behind the mesh while playing (although we never witnessed a possum having serious problems with this). Also, all the sharp ends of the mesh have to be bent back to prevent them from projecting into the box, which is a bit tedious to do. If you use galvanised wire mash (as we did), it will corrode over time and this can't be healthy for possums who decide to lick the wire (and we've seen them do this). When we rebuilt the first box, we removed the wire and replaced it by a series of wooden rungs fixed to the side of the box like the rungs of a ladder. I think this is a better solution than wire. You could also use wooden dowels, or a piece of board with horizontal grooves cut in it with a router. Take care to avoid splinters and the projecting ends of screws on the inside of the box. Baby possums fling themselves around inside the box with great abandon (see the Cocoa Leaping movie to see what they get up to) and could easily be injured. It's true possums are hardy creatures and often have to sleep in much more hostile places, but then you often see possums with various injuries. If you manage to put a screw in crooked and the end projects, probably the best thing to do is to replace it with a shorter one, and sand off any splintered wood. With brushtails, I don't believe it's necessary to place wood shavings, leaves, or other nesting material in the box. Brushtails are generally quite happy to sleep in a bare box. Occasionally we've seen them take in leaves, but the reason for this is unknown because they don't ever sleep on them. One theory we have is that mother possums might take leaves into the box for their babies to play with; a few times we say baby Cocoa playing with leaves brought in by Ginger. Ringtails and sugar gliders, on the other hand, do build nests, and there seems to be some evidence that sugar gliders appreciate a bed of shredded paper. Even so, I don't think it hurts to provide unfurnished accommodation, since there's nothing to stop possums from bringing in their own bedding. Insulation/Temperature Control I'm not sure whether anyone has studied the thermal characteristics of tree hollows, but they are likely to be very different to those of a relatively thin-walled box in the open air. In the Bywong Community Sugar Glider Project they built sugar slider boxes with 60 mm thick walls consisting of inner and outer wood layers, with foam insulation in between. The intention was to emulate the insulation provided by a natural tree hollow. The gliders apparently like these boxes, but the site mentions gliders occasionally sleeping outside of the box on very hot days, so the thermal characteristics of this type of box might sill not be perfect. It's possible brushtail possums may be more hardy than sugar gliders, but they are by no means insensitive to temperature. We've observed possums in our boxes and below 25° C they curl up into balls to conserve heat. Much above this and they tend to lie on their backs with their legs outstretched (often in quite amusing poses). Once, on a very hot day, a possum (shown below) spent the afternoon with the upper part of his body outside of his box. He would periodically lick his forearms and hold them out in an attempt to cool them in the breeze. It looked like he was very uncomfortable.
Queek finding it a bit too hot in his box. We don't know whether this behaviour is normal in the wild or whether our box was too hot. The baffle probably blocks the wind and keeps the box hotter than it would otherwise be, and there's also a camera inside which puts out about half a Watt of heat as well. On the other hand, our boxes are well below the canopy and shaded for the entire day. I'd hate to think how hot one of those dark green boxes up the top of a gum tree gets when exposed to the direct sun all day. In the absence of any hard data I can only make some vague waffly recommendations regarding box temperature control:
Attaching the Box to a Tree It is recommended to attach possum boxes at a minimum height of 4 metres above ground level (measuring from ground level to the bottom of the box). When you're up a ladder, you'll find that this seems very high indeed and you'll be tempted to fix the box much lower. Certainly you shouldn't risk life and limb for the sake of a possum box, but on the other hand, a box that's too low is either not going to be used, or will expose its inhabitants to unnecessary risks. Take care and use your common sense here. Use a ladder that's the right length, place it on level ground (use a spade to level the soil if necessary), and tie the top securely to the tree trunk with rope. You'll be up the ladder for quite a while, so it's worth taking the time to make everything secure. Having a second person hold the ladder while you're working is not recommended because of the danger of accidentally dropping stuff onto them. For what it's worth, boxes 1 and 2 were initially 4.2 and 4.1 m respectively above ground level. After it was replaced, the new box 1 was positioned 5 m above ground level. You probably don't need to go this high, but I'd think that anything under 3 metres would be unacceptable. The box should be positioned as far as possible in the shade and with the entrance hole sheltered from prevailing winds (this is less of a problem if you fit a baffle over the hole). Most importantly, the hole should face roughly south-east (one place said that 100° true was the optimal direction), and at all costs avoid facing it west. I believe the logic is that possums like to sneak out of the box as soon as possible after it gets dark, and when the sun is setting the west side of the box will be illuminated while the east is in shadow. Actually fixing the box to the tree is a more difficult task than you might think and something that requires careful consideration. Trees are rarely completely even and vertical, and when you add the constraints of box height and orientation, you'll often end up having to fix the box in a very awkward spot. Also, it's very much preferable to have the box mounted level - the possums probably won't mind if it's leaning at an odd angle, but you'll find that it looks very slovenly to the human eye, and if the box is in a visible location you'll regret it every time you look at it. Wooden shims located behind the box can be used to even it up and are worth the effort to use. It might also be necessary to shim the box so that it stands out from the tree in order to allow for the top of the box to swing open. It's best to attach the box to the tree using screws. Nails can be pushed out of the tree when it grows, and any sort of rope or chain around the tree runs the risk of ring-barking it. According to our local tree-surgeon, screws damage the tree less than any other method and hold very securely because the tree callouses around the screw, locking it in place. According to our tree-surgeon, the weakest point of a screw attachment is the portion of the screw immediately outside the trunk of the tree, which is in danger of rusting. You should therefore always use coated or galvanised screws. The original design showed two screws being used to mount the box. Considering the size and weight of our boxes, this seemed grossly insufficient. In the end, we used a total of six 14 gauge x 100mm long Bugle Head screws (shown above) for each box. It is necessary to pre-drill holes for screws of this size. We initially attached the first box by simply screwing through the back of the box into the tree. This was difficult to do at the time and was also the main cause of the box eventually falling out of the tree. Owing to the difficulties we had with the first box, we attached the second box via a separate back plate (described below), and when we reattached the first box after the fall, we also used a separate mounting frame. I think that attaching a box like this (i.e. in such a way as to make it easily removable) is a very useful feature, but unfortunately something that rarely seems to be taken into account with nest box designs,. We have found that this approach has the following advantages:
What is needed is a simple way to solidly attach a possum box to a tree, but also to have it quickly and easily removable. Neither of my two designs quite achieved this (they both fell down when it came to the "simple" part), but they're worth considering as a starting point. The design used for box 2 is probably preferable because it's simpler and requires fewer special parts, however it's only useful for attaching a relatively narrow box onto a straight tree. This design consists of a back-board permanently attached to the tree, with the box attached to the back-board by use of four lift-off hinges. To prevent the possibility of the box lifting off the hinges during high winds, and also for added rigidity, an angle bracket was placed on the back-board underneath the box and attached to the box by two cup-head bolts. Again, a picture is worth a thousand words:
To remove the box, we undo the two nuts on the cup-head bolts and lift the box off the back-plate. This method works well and is very secure. On the other hand, it was a tedious process to align all of the hinges well enough so that the box could be easily slid on and off, and a simpler method of attachment would be preferable. The refurbished box 1 used a more sophisticated mounting frame in place of the simple back plate. The method used is documented in the Box 1 Replacement section below. Cameras Installing a camera in your possum box adds significantly to the cost and difficulty of the project, but we found it to be well worth the effort. Checking a box from the top of a ladder is something you're only going to want to do occasionally, and if the possums aren't visiting regularly, you might not ever know if the box is being used. And if your visit does happen to coincide with that of a possum you'll discover that brushtails really don't appreciate being woken up. We've found that the ability to look into the box at any time, and also to record what's happening in there, without disturbing anything, has been really helpful. We placed a Swann "Day/Night Camera" in each possum box. This was one of the lowest cost cameras available at the time, and is much more suitable for the application than many more expensive models. It is about the size of a box of matches and has infra-red LEDs which allow it to be used in complete darkness. These cameras have proved to be very reliable, with both having been running continuously for nearly three years at the time of writing. These cameras have the (relatively slight) disadvantage of a narrower than ideal field of view and also the image quality could be better. This is the camera placed in our second possum box. The strip of wood above the camera was added to prevent possums from bumping the camera when entering and leaving the box.
Nest Box Camera Given that new technology is appearing every day, it might be possible that you'll find a better camera out there if you look. You should consider the following points when choosing a camera: 1) Check the minimum light level it will work at. You need one that says 0 Lux (i.e. total darkness) and this means it will incorporate infra-red LEDs to provide (invisible) illumination. If the camera claims it's suitable for nighttime use, but the fine print mentions a minimum light level of 1 or 2 Lux, it's probably not suitable. 1 Lux is about the intensity of moonlight, and your box will often be darker than this. [Although a fair amount of light leaks into a box in the day time, a lot of the action happens when possums enter or leave the box, which happens during the hours of darkness.] If possible, check the type of infrared LEDs used, since some types emit a dull red glow in addition to the infrared, and this is undesirable. 2) CCD cameras are said to give better quality images at low light than CMOS cameras, but are more expensive and use more power (which means they run hotter, which could raise temperatures in your possum box). Our camera was a CMOS one, but it's possible that CCD ones might now be available for a reasonable price. Don't expect to be able to find a camera (even a CCD one) which will operate in colour mode at low light. Even if the camera says it's a colour one, it'll still switch to black-and-white in night mode. 3) Consider the angle of view of the camera lens. This is 52° for our cameras, which is a bit narrower than we'd like. Some cameras have 90° angle lenses, which would be much preferable. Be a little wary of fitting replacement lenses; I've heard that in some cases a replacement lens will prevent a camera from being used in night-mode (I assume it blocks the infra-red in some way), despite being suitable in all other respects. 4) Some cameras don't support audio. Ours had a built-in microphone and we found this to be useful on occasions; for instance, it alerted us to the problem with Queek's eye when we heard him emitting little shrieks of pain/anger when he was in the box. 5) Our cameras have analogue audio and video outputs, which are connected into the A/V input of a VCR. This is a simple and low-cost arrangement which requires no special technical knowledge to set up. Other cameras such as webcams are available which use USB or ethernet instead. These types currently seem to be less suitable. USB cameras are limited by the length of USB you can use, which I think is around 5 m, and would be inadequate in almost all cases. Ethernet cameras usually incorporate their own video server are theoretically very good (especially if the camera uses Power Over Ethernet, because then you only need a single Cat 5 cable to hook it up) but the only cameras of this type I've seen so far have been very expensive. 6) It might be possible to get a PCB camera (i.e. the electronics are on a naked Printed Circuit Board, not enclosed in any sort of housing) cheaply, but without modifications, these aren't suitable for possum cameras. Possums have been seen to sniff, lick and even box the cameras and geckos will sometimes be attracted to the heat given off by a camera. Trying to save a few dollars by using an inadequately protected camera is not recommended. The camera doesn't need to be waterproof or anything special, it just needs to be enclosed. Cabling Most of the difficulty and annoyance related to installing a possum box camera is caused by the need to bring a cable back from the camera. The cable supplied with the Day/Night Camera didn't look suitable for use outdoors. Cables for exterior use need to have UV stabilised insulation, or else they'll go brittle in the sunlight and they also need to be physically strong. We ran our cable inside a mixture of 20mm solid and 25mm flexible PVC electrical conduit. When I replaced the first possum box, I replaced some of the 20mm conduit with 25mm because of the difficult of coaxing the relatively bulky connectors on the end of the cable through the conduit. I think that using 25mm conduit from the start would pay for itself in terms of reducing the frustration involved, and I'd also recommend limiting the amount of flexible conduit you use as far as possible; the ridges on this stuff make it really difficult to get cables through. You'll find that in addition to conduit, you'll also require a surprising quantity of miscellaneous fittings (elbows, saddles, junction boxes etc.) to make a good installation, but it's worth the time to do it properly. It's usually best to buy this sort of stuff from your local electrical supplier (e.g. Ideal Electrical or Haymans) rather than a normal hardware store because they've generally got lower prices and a better range. And, just to note; conduit saddles sometimes come with nails for fixing them in place. When using saddles to attach conduit to a tree, do yourself a favour and replace the nails with screws. I initially used nails and the tree pushed them out after a short period. Although our cameras came with 20m of cable supplied, we needed to add another 18m extension cable to reach the camera in the back yard. It seems the cable supplied with these cameras is selected for smallest size and lowest cost rather than the best performance because this length of cable degraded the quality of the picture noticeably. I'd suggest that if you need a long cable run (i.e. longer than the cable supplied with the camera) and want the best possible pictures, you should make your own cable using proper coaxial cable (Single-shielded RG-59U is said to be the best choice here, apparently it works even better than the more expensive quad-shielded RG-6U recommended for digital TV, because it provides better shielding at lower frequencies. Most types of RG-59U coax are tough enough to be used outdoors with a protective conduit.) for the video signal. Probably shielded audio cable would be OK for the power and audio. Our first possum box was installed on 19/2/2005. On 11/12/2008, after nearly 4 years of use, it fell out of its tree. I've documented the process of repairing, improving and replacing the box below. I've found it easier to do it this way than to try to incorporate this information into the possum box notes above.
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