There’s been lots of talk and media about ‘owl boxes’ in the City of Swan lately. It makes out that we just ‘make more owl boxes and get more owls’, which is simply not true, but typical of the simplified view humans have of our often complex ecosystems.
Importantly, there is no such THING as an ‘owl box’, only one SUITABLE for owls; of course there is never a guarantee any particular target species will use any nest-box. Whether or not it does depends on much more than using a design (dimensions + orientation) known to have been used by certain owl species in the past (such as those featured in my book ‘Hollowed Out?’).
Of greater importance is the fact that owls live in territorial pairs that occupy and defend spaces (territories) of a fairly consistent size and shape, and knowledge of existing territories and the hollows within them is vital to inform actions aimed at ‘increasing owl populations’. Territories are established on the basis of: 1) prey abundance and habitat to support a minimum threshold of prey to keep a resident pair alive throughout the year, and 2) existing hollows to be used as nest sites if and when seasonal conditions allow (i.e., not necessarily every year). This fact means there is a maximum density of owls that any landscape can possibly support (like a housing estate, but with no rezoning or subdivisions allowed). Adding more artificial hollows without using data on the density of natural tree hollows, and the occupancy of such hollows by the range of species (including owls) already using them, to inform their placement, could well lead to no boxes being used by the target species at all.
Indeed, the science supports this. In a 2015 Perth metro area study that involved placing nest-boxes at sites where Boobook Owls were not initially detected during nocturnal aural surveys, to see if the new habitat feature would result in subsequent owl detection, only 1 of 15 nest-boxes installed was used by Boobooks 3 years post-installation (Lohr 2019). It was possible in this study that boobooks already had suitable hollows in areas of placement, so had no need for surplus ones, or that the rodenticide highlighted as a conservation issue was killing owls in ‘ecological sinks’, thus reducing probability of box uptake.
Our approach at ReCycOlogy is to carry out a tree hollow assessment prior to the installation of any artificial hollow, to see if suitable natural ones exist beforehand. If they do, we simply recommend focussing on the monitoring of those natural hollows for fauna use first. If not, then we make recommendations on installing a range of different nest-box designs (none named after a single species or species group!), taking into consideration the home range size of various local fauna and the density those species are known to nest at in natural sites. The overall aim is to reintroduce a similar structural diversity that one could expect to find in an 'old growth' forest with a range of hollows spread fairly evenly across the landscape. This approach has led to a number of nest-boxes being successfully used by Boobook and Barn Owls, as shown in these photos, but it has taken quite a bit of ground work, before we take to the trees!
To help our nocturnal predators, addressing the secondary poisoning issue certainly needs to be a priority at all levels of government. But embarking on ‘owl box’ projects not informed by science or ecological professionals may lead to ‘owl-less’ boxes being destroyed by super-abundant galahs and corellas, or clogged with feral bee hives.
