Adaptive management of artificial hollows: a case study evaluating carved tree hollows as an emerging restoration tool for hollow-dependent fauna

The global decline of hollow-bearing trees threatens hollow-dependent fauna and highlights the need for effective artificial refuges in habitat restoration. Nest boxes are widely used but often exhibit poor thermal performance, limited durability, and narrow species suitability. Carved hollows in living trees are an emerging alternative that may better replicate natural cavities, although their ecological performance and implementation remain poorly understood. This case study evaluated carved hollows using a purpose-designed tool (HollowHog) within a long-running nest box program in urban bushland remnants in Sydney, Australia. The study assessed ecological outcomes and implementation challenges, with implications for land managers and future research. Monitoring included 535 nest boxes and 53 carved hollows. Occupancy, attrition, and internal temperature were recorded, with observations of factors affecting hollow persistence. Nest boxes recorded 21% native vertebrate use, and 12% of carved hollows were occupied by target species. Carved hollow availability declined due to termite occupation (n = 19), water ingress (n = 5), and native bee occupation (n = 3), collectively rendering 51% hollows unusable for target fauna. Many cavities were also created smaller than design specifications, further reducing suitability. Carved hollows exhibited lower and more stable daytime temperatures than nest boxes. Despite some identified challenges, carved hollows have potential as durable and thermally stable artificial refuges if these challenges are addressed. Successful application requires attention to cavity dimensions, entrance design, tree selection, and management of woundwood, termites, and moisture. Artificial hollows cannot replicate natural cavity diversity; therefore, protecting and recruiting hollow-bearing trees remains essential for conserving hollow-dependent wildlife.