Spout hollow nest boxes provide a drier and less stable microclimate than natural hollows

Artificial tree hollows (e.g., nest-boxes) are commonly deployed to mitigate the loss of mature trees within human-disturbed landscapes. Their effectiveness as a habitat resource, and thus conservation management tool, is strongly influenced by the suitability of internal microclimate conditions. In south-eastern Australia, spout hollows are a nesting resource used by a diverse community of vertebrate species. We tested the suitability of a novel nest box design (spout boxes) that mimicked the physical characteristics of spout hollows. We monitored the occupancy (n = 193) and internal microclimate (n = 131) of natural hollows and spout boxes within a woodland where natural tree hollows were once abundant. Both natural hollows and spout boxes were occupied and used for breeding by birds and mammals. Natural hollows had consistently higher humidity, and thermal maxima and minima were buffered, when compared with spout boxes. These differences were largely explained by wall thickness. Spout boxes displayed even more extreme temperature variation and lower humidity when not shaded. While more extreme microclimate conditions did not prevent usage, tolerable thresholds for hollow-dependent species may soon be exceeded under current climate change projections. Managers need to carefully consider nest box design and positioning to ensure the suitability of these supplementary resources for conservation purposes.