Reducing nest box entrance diameter impacts mammal occupancy

Tree hollows are an important resource for cavity‐nesting birdlife. When large, old trees are removed from a landscape, nest boxes are often installed as part of restoration efforts to replace lost natural cavities. If nest boxes are to be successful conservation tools, non‐target species, particularly competitive and predatory species, need to be prevented from entering nest boxes. Several different modifications to nest boxes aimed at excluding non‐target species have been trialed in previous studies. We tested the effectiveness of reducing the entrance diameter of nest boxes to exclude non‐target species in Southeast Queensland, Australia. We used restrictor plates to reduce the entrance diameter of nest boxes from 90 to 60 and 50 mm and compared the wildlife occupancy of nest boxes with these three entrance diameters. We found that Common brushtail possums, a predator of cavity‐nesting birds and eggs, were significantly less likely to occupy nest boxes with reduced entrance diameters and were excluded from nest boxes with a 50‐mm entrance diameter. Squirrel gliders occupied nest boxes with all three entrance diameters. Introduced species, namely the Common myna and the European honeybee, also occupied nest boxes with restrictor plates. Installation of nest boxes with reduced entrance diameters is a simple and effective strategy to exclude brushtail possums from nest boxes. However, a reduced entrance diameter cannot exclude other non‐target species, and we suggest that other exclusion strategies should be used in conjunction with reduced entrance diameters to increase the occupation of nest boxes by native birds.