Field experiences with nest boxes

Three research projects included some investigations of box occupancy by western ringtail possums at field sites (conservation estate, not urban) – see Clarke, 2011, Moore, 2007, Harring-Harris, 2014. If they report a frequency of use by ringtail possums at all, it is always very low.

In the main study, that focussed on the issue, animals frequently checked the boxes out but they did not stay in them.
When occupancy of a box was confirmed for several hours (iButton and hair sample) the user was most often a common brushtail possum.
The method of using iButtons either only seemed to work in (wooden) rectangular and hexagonal boxes or the option of ‘artificial dreys’ were not at all used by possums in the field. (Moore, 2007)

During another study 2 radio-collared translocated ringtails were recorded in nest boxes for 2 nights each – the rectangular and the hexagonal style.
(Clarke, 2011)
I would assume that these animals came from care and were used to sleeping in a box and that the sightings happened shortly after release.

In captivity occupancy of boxes was largely determined by the individual’s preference for a nest box design. (Moore, 2007)
However, as boxes were the only available shelter (no tree hollows or building materials for a drey), the research would not give any insight into whether  animals would choose to use a box at all in a more natural environment. 
The study tried to work out which type of box would be the most popular, however as this was tested in a cage situation, it might not apply to animals in the wild.
In rehab situations usually only one box option is provided and they are in the vast majority taken up by the animal, no matter what design or style.
Some adults coming into care prefer to sleep on a bunch of leaves (the food provided) at least for the first few days.

In the study nest-box occupancy was also affected by the ambient temperature and rainfall. Box usage dropped off when temperatures were higher than 17°C and increased on rainy days. (Moore et al, 2010)
These findings are in contrast to our experiences. In general, box use increased on hot days and was unaffected by rain. 
I would argue that it is the style of box and the positioning/orientation that matters most in summer. Coir mat dreys are for instance extremely popular in summer – but mostly in suburban backyards - much less so in winter. They seem to provide a degree of protection without warming up above ambient temperature. Quite often tails can be seen hanging out (see photo).

Also, boxes in release cages that are placed fairly low (cooler), in a shady, protected environment and with a water source close by, were significantly more often occupied in the heat of summer than under any other weather conditions.
Even in heavy rain there was rarely any occupancy whenever checked (all seasons). However we mainly camera-monitored and they tend to function badly in rain – the rain seems to mask the higher temperature of the animal.
Those results were obtained in a bush area similar to that in which the study was conducted.

The need or the usefulness of nest boxes of whatever style is highly dependent on the habitat and its limitations. 
Urban backyards and urban reserves with inadequate vegetation profit most from the supply of boxes.

We were alerted to ringtails seen on the ground in a small urban reserve with quite thick understorey but youngish, inadequate trees. As the reserve was heavily used by dog owners who let their pets run free, we installed 7 boxes (coir mat style) in spring. A night count in the reserve revealed 10 animals (3 of them juveniles). It took close to 1 month for the animals to check out the boxes and start using them. 3 weeks after the first recorded occupancies the boxes were vandalised and 3 destroyed. 4 were partly damaged but still in the trees - 3 of those were still occupied by ringtail possums!
Newly installed boxes were placed so high up in the trees that they would be fairly safe from vandalism but unfortunately it was then impossible to check the boxes without a several metre long ladder.

A revegetated semi-urban site (approx. 1 hectare) from which a small number of ringtails were successfully released, also proved to greatly benefit from boxes as the mainly young vegetation lacked hollows. A heavy jarrah box proved particularly popular as it seemed to provide better insulative properties and probably was more similar to tree hollows.
Where ever there is a common brushtail population they seem to be the main beneficiaries of boxes and try to squeeze in even small entry holes.
Amazingly the nest boxes in the stationary cages (in well shaded positions and with ‘bridges’ leading into the vegetation) are also always heavily used – not only by the ringtails released from there but also years later by their offspring.

We had a similar experience on another small semi-urban bush lot (2-3 acres) but with some good mature trees. Ringtails still took to the nest boxes as the hollows might have been occupied by brushtail possums.

Brushtail possums were the main occupants in the study (Moore, 2007) which confirms our own observations on most properties where ringtails and brushtails cohabit. Sometimes brushtails quickly moved into boxes (probably due to scarcity of natural hollows) provided for ringtails and they evicted ringtails or stopped them from entering a box.

A 50 acres bush property where a number of ringtail possums were released however contradicts the research finding that nest box occupancy is low in bush areas.
Released animals again used the boxes in their cages for a long period but in this case some seemed to have no interest in ever moving out. However, after boxes were installed in trees nearby, they quickly vacated the cage box for the nest box in the tree.
Only wooden boxes were occupied but often altered by possums – e.g. a drey built inside. In summer coir mat dreys were added and they were taken up too.
My hypothesis regarding this unusually high box use in a mixed bush area is that the brushtail possum population limited ringtails’ access to natural hollows but it was not substantial enough to also take over the boxes.

Relocation success could potentially be improved by the provision of additional rest sites such as nest boxes which however require regular maintenance and replacement.
After relocations of ringtails to Leschenault Peninsula, the first drey was recorded less than 6 weeks after release. (De Tores et al, 1998)
Mainly to provide shelter for this early establishment phase, we always provide a few nest boxes in nearby trees when releasing ringtails on one of our 3 main sites, however long-term monitoring confirms Moore’s result that they are rarely used and do not provide major benefits.

Box use in the early days after release is high but particularly in boxes in release cages. Some animals – in particular males - tend to come back to the cage for weeks, in rare cases for many months (half a year!). Usage of outside boxes is more sporadic and drops off after a few weeks after release.

Animals that take to nest boxes as such are more likely to use an outside nest box if their cage is removed. As would be expected, ringtails that were reared in captivity and are used to living in nest boxes are more likely to use them after release while wild adults that spent some time in rehab (and in a nest box) would only rarely come back to boxes in a release cage and also not make much use of boxes in trees.

Another observation hints at the association between nest boxes – particularly in cages – and safety. Some females that were not seen for weeks or even months came back to a box in a cage when they had offspring in their pouch.
A mother with twins once deposited one young in a box on its own and it can only be assumed that she was unable to feed both and left one behind in perceived safety.

We also once discovered a wild juvenile with a serious tail injury in a box in a release cage. 

A wild mother with a small daughter moved into a box in a cage in a stormy, wet period in June but left when the weather improved.
Even though I would have assume it more likely that wild animals use the outside boxes, when we had wild visitors it was mostly in the boxes in cages, while whenever outside boxes were checked and they were occupied, it was one of the released animals. This could however have been purely coincidental as the frequency that boxes were checked was low.

In fact, all the above observations are based on a low checking frequency and without a strategic plan in place and have therefore no statistical weight.

Only the number of nest boxes on site A is high enough for any investigation. We have used 3 designs – large wooden boxes as usually recommended, some made from 19 mm thick new untreated pine, some made from marine plywood – and small (approx. 25 cm high) boxes made from recycled, untreated medium-weight duty pallets. The latter have always been the most popular even though the low height might reduce safety and can heat up more quickly than a larger box (with ventilation slits).
Unless we want to specifically look at the role boxes can play for newly released animals, I think that general box use can only be investigated after at least one year in the wild.
In the heat of summer the  occupancy rate of small boxes was up to a maximum of 45%, while up to 33% of the pine boxes were used and only up to 20% of the marine plywood boxes. However, this could well be an attempt of quantifying coincidence.
Box use in cold weather was very low in all styles – only 10-15% of all boxes were occupied.
The reason for the popularity of the small boxes could well be completely unrelated to the design. They could just be better placed as they are lighter and easier to handle.

Monitoring ringtails that do not wear radio collars is difficult and recognition of individuals in a tree is impossible. If animals occupy nest boxes and they are at least micro-chipped (or have some distinctive features) some data collection is possible. However, the disturbance caused by checking the box or trying to scan an animal might drive them away and distort perceived results.
Similarly not physically checking boxes that are under camera surveillance but show no occupancy, can distort results as there might be an unsuspected reason e.g. infestation by ants that can be a serious problem and keep ringtails away. So far one of our boxes was taken over by bees, however we constantly have to clean boxes that are infested by ants. They seem to be a far bigger and more wide-spread problem than bees.

There is no doubt that monitoring nest-box use can provide information on a species’ biology and ecology, behaviour, interactions and reproduction, but whatever we do, we potentially influence/distort the results through our own action or inaction.

In summary, whether provision of nest boxes is beneficial depends on the habitat and its features/values. They can clearly enhance sub-optimal habitats but are probably superfluous in adequate habitat with no or very few brushtail possums.
Ringtail possums are natural drey builders and we should encourage this habit and not influence it negatively through oversupply of nest boxes.

Thermal stress during extreme heat is a serious risk factor in our warming climate and nest boxes can exacerbate this danger. Some research into thermal properties (design features such as paint and wall thickness) and placement/orientation should always be carried out before boxes are provided in a new area. 

However the tree canopy cover available at the various installation sites is the main feature that should guide placement and box style.

Boxes in isolated trees without canopy connection to other trees, will increase predation risk as the animal would have to come down to the ground before moving away from their rest site. This is a major problem on many release properties that lack dense vegetation. Trees are either too isolated or nest boxes are placed so far away from release spots that they might not be found or not attract interest.
There is also the question whether a light colour of a box could make it conspicuous to predators and therefore render their use counterproductive. However boxes that blend in well (brown or dark green) increase the risk of heat stress significantly.