Nest boxes – an overview

The use of nest boxes for possums is probably as old as the notion of caring for them. Whenever the general public wants to give our wildlife a helping hand, the two main suggestions are feeding and putting up a nest box.

While feeding should be strongly discouraged (unless containers with fresh leaves are put out), a recommendation for or against the use of a nest box is trickier.
We need permission to put nest boxes in public spaces - rightly so as for instance damage to trees can occur if not done professionally and/or a box can turn into rubbish long before its ‘use-by-date’.
Unfortunately our experience is also that boxes in public spaces attract not only the attention of possums – the worst case was an act of (juvenile) vandalism that put the possums into harm’s way.

Whether we put up boxes in our backyards is a personal choice, however it then also turns into a personal responsibility as boxes need to be maintained and monitored if they should be of any use and not potentially harmful.

Unfortunately possum boxes are not the magic solution to the problem of ‘possums in roofs’ or other unpopular (with humans) spaces. If possums have made a roof space their home, they will not voluntarily abandon it no matter how nice the box provided looks (to us).

We have worked with several different box designs and none of them is perfect. They all have their particular advantages and disadvantages and sometimes the environment (including the humans) where they are to be used limits the choice of design.
As a best case scenario nest boxes should provide similar protection against environmental extremes as natural shelters.

There is no scientific evidence that ringtails prefer any particular design. A research project (however with only very few individuals) showed that it comes down to the individual’s ‘taste’.
In my personal experience, ringtails sometimes move readily into the most unlikely structures and sometimes ignore the most skilful dwellings – or even show what they ‘think’ about it by placing a drey next to it or on top.

Ringtails are extreme food specialists but generalists when it comes to shelter – the use of boxes seems to be mainly dependent on the habitat. If needed, anything is taken up, if not needed, the most elaborate design will be ignored. Still, mimicking the characteristics of natural hollows would provide the highest chance of them being used and being beneficial for the species. 

Some birds need hollows to be able to breed and if there are none, boxes are vitally important. The same applies to for instance greater gliders in the eastern states. However, boxes then have to be species-specific. The height in which the box is placed, its size and the size and shape of the entrance hole are all important and often not fully researched.

Ringtails in heavily modified environments such as our backyards seem to be the most likely candidates to benefit from the provision of nest boxes. The removal of large trees has resulted in scarcity of mature hollow-bearing trees.
Research in natural bush environments in WA and the eastern states points at low ringtail occupancy rates. Common brushtail possums are far more likely to move into boxes than the target species.

Types of boxes

Artificial dreys
Two wire hanging baskets put together with some tape or wire to form a round structure and with a cut out entry/exit hole can then be cushioned and camouflaged with leaf material. Peppermint is best for this purpose as it keeps pliable for years.

Wrapping some insulation tape around the entrance hole will reduce the risk of injury through sharp edges but as it is unclear whether possums nibble our materials, this protection can also turn out to be a danger.
The more foliage, the denser the drey, the better it will insulate from the elements like rain and wind.
In Moore’s research (Moore, 2007) and in our own projects this type of drey was not used at all in bush areas.

 

Advantages
light-weight
suitable even in flimsy vegetation
easy to make
easy to attach in a tree with a few cable ties
retains almost no heat in summer
very cheap

Disadvantages
not sturdy and can be destroyed quickly by e.g. a raven/crow
the entry hole will always be fairly big (approx. 15 cm diameter) due to the structure of the baskets and provide little protection against intruders
little insulation against the elements and in winter

Coir-mat dreys
A cheap coir mat of approximately 70 x 45 cm without plastic/rubber backing or edges is rolled up to form a tube and fixed (wire, small cable ties, strong twine) in that shape. Saucers for flower pots or any other plastic disks in the right size are attached (drill small holes) to the sides of the drey. An entrance hole of 9 cm is cut into one of them.

Advantages
very easy and fun to make – great for making them with kids
light-weight
suitable even in flimsy vegetation
easy to attach in a tree with a few cable ties
retain almost no heat in summer
give adequate protection from rain and wind

Disadvantages
should be well-watered prior to use as the chemicals in the mat can be toxic
(however, we don’t even know whether the chemicals are water soluble and can be ‘watered out’)
the material is easily damaged (fibres pulled out) as it is popular with birds as nesting material -
ravens were seen ripping holes into the mat and attacking the animal inside
not durable
very popular with rats (roughed up behind the entrance is an indication)
provide little insulation in winter

Timber boxes

Various materials can be used for the more traditional box.
Pine of up to 19 mm thickness, plywood, MDF and marine plywood are the most popular.
Untreated pine is available; boxes are very durable but also very heavy.
Light-weight plywood consists of wood veneer glued together.  It is not durable and potentially toxic due to formaldehyde in the adhesive. Phenol-formaldehyde glue is water resistant and therefore more suitable for outdoor structures.

MDF (medium density fibreboard) and chipboard are the least durable light-weight materials and boxes therefore turn into environmental waste quickly.  They are based on bonded wood chips and the same warning applies as to plywood.
Marine plywood is not too heavy but durable even in humid and wet conditions. However the treatment for durability most likely increases the chemical burden and therefore toxicity. It is also very expensive.  

Whether toxicity is an issue or not depends on whether chewing at boxes is common in the wild and not just in captive (bored) possums. I could not find any research into this topic.
My personal experience is that common brushtail possums would chew big enough holes in a wooden cage to escape while ringtails only very rarely chew the entrance hole of their nest boxes.

Whenever boxes are painted to increase their lifespan in wet conditions, the paint could also be quite toxic. Acrylic paints give of volatile organic compounds (toxic fumes) and should never be nibbled. The costs for non-toxic paint and high quality wood will be economical in the long run as the box will not decay prematurely.

Everything that makes a box more durable (protection against termites, borers, fungi, wet weather etc.) increases the toxic chemical load  - not only for insects and fungi.

Untreated wood is probably the safest option, but those boxes are often invaded by bees, wasps or ants. 
By lining the ceiling with carpet the wax comb of European honeybees cannot be attached which might lower the attraction. Any use of insecticides can be hazardous. (Gleeson and Gleeson, 2012)

There are suggestions to use talcum powder to deter ants from invading; however studies with rats, mice and hamsters in the US have had incidents of tumour formation. (https://www.cancer.org/cancer/cancer-causes/talcum-powder-and-cancer.html)
Possums that come into contact with any sort of powder would always lick themselves clean.
Diatomaceous Earth is used as an organic pesticide for the control of ants, fleas, mites, ticks and other insect invasion and should be safe as long as it is ‘food grade’. (http://www.stopskinmites.com/products/diatomaceous-earth) However, my experience was that it kept not only the insects away from the box but also the possums.

Wooden boxes also heat up significantly in summer and keep the heat even when the worst of the day is over – a place to rest can quickly turn into a place to roast.

Boxes in various shapes are available.
A hexagonal box constructed from 10-mm MDF with one or two entry/exit holes (each 90mm diameter) and a hinged lid for monitoring seems to have lost popularity in the last years.
An advantage of this shape is the added escape hole which however also lets more daylight in.

Rectangular nest box constructed from up to 19-mm-thick pine, with one entry/exit hole (90mm diameter), a sloping, hinged lid for monitoring access and drainage holes is probably the most common box for ringtails. Size matters and 32 cm deep boxes (20X24 cm base size) give better protection from predators (apart from snakes), a darker environment and prevent overheating better than a small box – the animal will give up heat too.  Ventilation in the sides of the box or a sun shield could also give relief.
All wooden boxes maintain a higher temperature for longer than other designs which could be favourable in winter but lethal in summer.

A large box can quickly turn into a death trap for a small animal that cannot climb out. Steps sawn into the front panel might reduce the risk but not for all species. The advantage of a smaller box is also that it reduces the incident of hive building and in general ringtails seem to prefer smaller volumes.

The entrance hole should not exceed 90 mm as then most adult brushtails would not be able to enter and that size might also exclude predatory species better.

Hollow logs
In many people’s experience these logs are the most popular dwellings as they are the closest to the real thing – a nice tree hollow.

Main obstacles are that you need to find suitably big hollows that can be closed off on one side – unless they are very long – and that your trees are big with thick branches that can hold an extremely heavy structure. A weight-lifting class would be a good preparation if you want to install a few of them.
Cable ties won’t hold a hollow log (at least not for long), so you have to use wire which then needs to be well insulated (e.g. threaded through a garden hose) so that you don’t damage your trees. It’s a big job to fix a hollow log securely to a tree.

Plastic/poly pipe shelters
We never used any of those as we feared that they would get too hot in our summers. They probably can only be used in a very shady, protected spot or with a sun shield on top.
The plastic can also give of potentially toxic fumes and the slippery material allows no traction for clawed feet.

All boxes should be removed when they are dilapidated!

Factors influencing usefulness and occupancy
Apart from box design features, placement – how high up in the tree, how easily accessible – and orientation will influence occupancy and the microclimate in the box.
The opening should be facing away from prevailing winds and at best the whole box should be shaded from direct sunlight by a dense canopy. Protection from inclement weather is a major reason for putting out boxes after all.

While natural tree hollows usually give good protection from severe heat, next boxes are in summer up to 8 degrees hotter than a good hollow. (Rowland et al, 2017) Taking the ringtails’ tendency to overheat at an ambient temperature from 35°C upwards (Yin, 2006, Yokochi et al, 2015b) into account the importance of retention of tree hollows as refuges is paramount and the lack of them cannot be mitigated by boxes.

Only in winter boxes might provide a slight advantage as they were found to be 3°C warmer than natural hollows (Rowland et al, 2017).
New research points out that painting boxes can be a major manipulator of the inside temperature of a box.

Most often various shades of green or brown are used as they blend well into the environment. Less visibility might even reduce the risk of attracting predators or human vandals. (Griffiths et al, 2017)

Boxes in the popular dark green colour however carried the highest risk of overheating in summer. They performed almost as badly as black boxes. Ringtails prone to heat stress and dehydration in extreme temperatures could either perish or have to leave the box and increase their risk of being noticed by predators.
Those darkly painted boxes were only advantageous on cold sunny days in winter as they kept warmer than light coloured boxes.

Light green or white boxes which absorb less radiation are more suitable in summer but less so in winter. (Griffiths et al, 2017)