It has become increasingly apparent to me from my own experiences, and from reading the published research, that there is no one best nest box orientation strategy that will suit all habitats. Rather the best outcomes will be achieved from adopting a strategy that best suits all the different target species and the attributes of that particular ecosystem, including climactic factors.
Installation orientation (which side of the tree?) and climate are important considerations when installing a nest box, as are the intended, and likely species that will use the nest box. In your location, how hot are Spring and Summer, and how cold does it get in Winter? Which orientation recieves the hot Summer sun for the longest during the day, or recieves sun during the hottest part of the day? The amount of available shade in the habitat where nest boxes are to be installed, also affects how hot nest boxes may become during scorching Summer months. If you can find a spot to install a nest box that is completely shaded from the afternoon hot sun, then the orientation of the nest box doesn’t matter so much for overheating potential.
Also important to consider, is not just the amount of sunshine that a nest box is exposed to all day, but rather the timing of this. Being warmed by sun during the hottest time of the day when heat may be deadly, is much worse than being exposed to sunshine during the morning when the temperatures are survivable. For winter, early morning sunshine provides greater warming benefits than late afternoon sunshine, when the temperatures is already much more comfortable than that of an icy cold dawn.
Orientations that receive more sunshine (all day): East, North-East, North-West, West, South-West, and all the variations in between. These orientations are great for Winter warming nest boxes, for occupants such as Sugar Gliders.
Orientations that receive more sunshine (hot summer afternoons): North-East to North-West, and all the variations in between. Avoid these orientations for nest boxes with Summertime occupants, such as nesting birds, or year-round nest box users including Ringtail Possums.
Orientations that receive less sunshine (all day): South, South-East, East and all the variations in between. Great nest box orientations for year-round nest box occupants, early season nesters etc.
Orientations that receive less sunshine (hot summer afternoons): East to South-East. That is, the opposite side of the tree to North-West. For late season nesting species where temperatures can be 35-40+ degrees, this is the best orientation to reduce risk of death-by-heat.
Not sure which direction North is? Just grab your mobile phone. Most phones have a compass app, or download a free one. This is a quick and easy way to figure out what side of a tree to install a nest box.
Some regions of Australia, like where I live near Canberra, have the extremes of scorching hot summers, and subzero winters. I have the challenge of considering cold season and hot season needs of our various nest box occupants.
Mammals: Installing nest boxes particularly for mammalian occupants, at a range of orientations would probably be much appreciated, because of there different seasonal needs. Such as nice, warming sunshine during winter vs sunshine that will overheat and kill nest box occupants during a summer heatwave. A nest box oriented to recieve sunshine for most of the day during a frosty winter, I have found is more likely to be occupied than one that recieves very little warming sunshine. They prefer to use tree hollows instead if available, as thickly wooded tree hollows probably retain warmth better. However nest boxes installed in sunny orientations may be abandoned by mammalian occupants over summer, as they reach temperatures even higher inside, than the already unbearably hot outside temperatures. A leafy, shady located nest box is much preferred over the warmer months.
Birds: For avian nest box occupants, nesting may be triggered by changes in temperature, such as after the first post-winter warm week, or after significant rain events that triggers an increase in food availability. The earlier nesting species are much less susceptible to late Spring and Summer heatwaves. However the species that wait for rain to occur may nest during the crazy hot months, and so are much more susceptible to extremene heat. A shortage of nesting options may result in queues for nesting spots even for the species that prefer to nest earlier, resulting in the later occupants being more affected by warm season extreme heat events.
There are numerous North American studies that recommend nest box installation orientation to be warmed by daytime sunshine, for species that nest in cooler temperatures, where a sunny spot on a tree will provide benefical warmth for nestlings. The opposite may be required for the warmer parts of Australia, where our temperatures can soar above 40 degrees during nesting season, and the canopy of many ecosystems such as Eucalyptus woodlands is quite open. Nest boxes exposed to sunshine will be even hotter than the ambient outside temperatures, in the more open canopied ecosystems there may be no all-day shady option for nest box installation. Hot temperatures can be deadly for nest box occupants, as we discovered during a nesting season heatwave in 2019.
Dense, tall forests: For dense-canopied forests, the installation orientation of a nest box is probably less of an issue. A dense canopy will limit the amount of sunlight able to reach an installed nest box, and forest temperatures don’t vary between morning, afternoon, and night time as much as temperatures in an open canopy environment.
Cool Climates: For cool climates with spring and summer maximums below 30 degrees, installing nest boxes in orientations where they will be warmed by morning and midday sunshine may benefit nesting birds and snoozing mammals, particularly during the cooler months. Extreme heat isn’t something to be concerned with for these climates, so winter warming nest box orientations can be the key priority.
Climates with Extreme Heatwaves: For climates where spring and summer maximums are in the range of 35-40 + degrees, nest box orientations that are exposed to the least amount of midday to late afternoon sunshine is the preference for both nesting birds and snoozing mammals. A nest box exposed to the hot afternoon sun to the West will be significantly hotter than a nest box on the opposite side of a tree, shaded by the tree trunk. A hot nest box can be easily fatal for chicks of any age, as they cannot escape the deadly heat. Nocturnal mammals simply won’t use nest boxes that become too hot during summer days. We had a Ringtail using a North-facing parrot box, until after a really hot summer day where the poor thing spent the entire day with it’s head hanging out the entrance hole trying to get a bit cooler.
On our property last year, Crimson and Eastern Rosella chicks survived a four-day heat wave of 36-37 degree temperatures, while a family of three Crimson Rosella chicks perished during a four-day heat wave of 40-42 degree temperatures.
Studies significantly differ in findings of internal nest box temperatures compared to outside ambient temperatures over the hot summer months, with being considerably cooler than outside temperatures (Goldingay 2019), or being significantly hotter than outside ambient temperatures (Rowland, Briscoe, Handasyde 2017).
I have found our nest boxes to be a little bit warmer than the outside air temperatures, within our Scribbly Gum regenerating woodland. The tree canopy is very open, so it is virtually impossible for the nest boxes not to warmed by sunshine if not completely shaded by the trunk of the installation tree.
Last year I purchased some Thermochrons to measure temperatures inside the exact same type of empty nest boxes, installed at different orientations during the second heatwave. Results over two very hot spells indicated that there was between a one to three degree temperature difference between the North-East and South-East nest box orientations during the hottest times of the day. This could make the difference between nestlings surviving a heat wave, or perishing. I’ll be doing the same again this Summer, to have a better look at the impact of nest box orientation and the peak heat period of the day, and the length of the peak heat. The graph below may not be representative of the timing of peak heat during the day for your location, which will affect which orientations will be the absolute worst for overheating any occupants where a nest box recieves direct or dappled sunshine during this period.
References:
Does temperature variation influence nest box use by the eastern pygmy possum? (R. L. Goldingay; 2019): http://www.publish.csiro.au/am/AM19023
Comparing the thermal suitability of nest-boxes and tree-hollows for the conservation-management of arboreal marsupials (J. A. Rowland, N. J. Briscoe, K. A. Handasyde 2017): https://www.sciencedirect.com/science/article/abs/pii/S0006320717301970?via%3Dihub