Effects of entrance size, tree size and landscape context on nest box occupancy: Considerations for management and biodiversity offsets

Abstract

The effectiveness of nest boxes as a management and biodiversity offset tool remains equivocal and controversial. Improving nest box programs requires urgent empirical research to identify the spatial factors that affect occupancy outcomes. Understanding which fine, local and landscape-level attributes influence nest box selection by wildlife can assist practitioners in refining nest box designs and placement in the field. We asked: Does entrance size, tree size and landscape context affect nest box occupancy? We monitored 144 nest boxes with six different entrance sizes (20, 35, 55, 75, 95 and 115mm diameter), secured to individual trees of three sizes (small 20–50cm DBH, medium 51–80cm and large >80cm) situated in four different landscape contexts with varying degrees of modification (reserves, pasture, urban parklands and urban built-up areas). We found that six common native and exotic species accounted for 89% of nest box occupancies. Entrance size had a significant effect on overall occupancy. Nest boxes with larger entrance sizes (115, 95, 75 and 55mm) were occupied more (⩾77% of nest boxes occupied) than nest boxes with smaller entrance sizes (35 and 20mm; ⩽45% of nest boxes occupied). Tree size and landscape context had no significant effect on overall occupancy. However, multinomial analysis revealed that entrance size and landscape context affected occupancy by common fauna (i.e. species that occupied ⩾5% of nest boxes). Nest boxes with small (20 and 35mm), intermediate (55 and 75mm) and large (95 and 115mm) entrance sizes were predominately occupied by the European honey bee Apis mellifera, common exotic (e.g. common myna Acridotheres tristis) and native birds (e.g. eastern rosella Platycercus eximius), and the common brushtail possum Trichosurus vulpecula, respectively. Nest boxes in reserves and pasture had near equal occupancy by common fauna while nest boxes in urban parklands and urban built-up areas were predominately occupied by the common brushtail possum and the European honey bee. Establishing nest boxes with different entrance sizes could maximise occupancy by a variety of common hollow-nesting species. Targeting occupancy by some species requires consideration of landscape context but not tree size. Nest boxes were predominately occupied by a few common native and exotic species, suggesting that nest boxes may not be highly effective management and biodiversity offset tools for rare and threatened taxa in modified landscapes. Management policies and practices aimed at avoiding the loss of large, hollow-bearing trees must be prioritised.