The impact of a mega-fire on tree hollow loss and nest box occupancy by secondary cavity-nesting cockatoos

Tree hollows are important structural habitat features for secondary cavity-nesting birds, however, there has been limited research on how fire severity and frequency affect hollow abundance. Following a mega-fire on Kangaroo Island during the 2019–2020 Australian bushfire season, we conducted an opportunistic post-fire assessment of the fate of 144 monitored hollows in Eucalyptus cladocalyx and Eucalyptus leucoxylon trees used by endangered glossy black-cockatoos (Calyptorhynchus lathami halmaturinus) and yellow-tailed black-cockatoos (Zanda funerea). We investigated the relationship between burn severity, inter-fire interval, tree species and landscape position on the likelihood of tree hollow loss. We also compared the occupancy of nest boxes by both cockatoo species in fire-affected catchments pre-fire (2015–2017; 59 nest boxes) and post-fire (2020–2022; 52 nest boxes). Surveys revealed that 64.4% of monitored tree hollows within the burnt area were destroyed, as were the trees that contained them. Most monitored tree hollows (76.9%) were in areas that had not experienced fire in >50 years prior to the 2019–2020 fires. Tree hollow loss in 2020 was 29 times greater than the mean annual loss of 1.6% recorded in the 22 years prior. Burn severity, inter-fire interval, landscape matrix and tree species were poor predictors of hollow loss. However, increasing distance from forest edge slightly increased the probability of hollow loss. Post-fire, the proportion of nest boxes used for nesting by glossy black-cockatoos per annum increased to 0.70 ± 0.09, compared to 0.50 ± 0.03 pre-fire. However, the proportion of nest boxes used for nesting by yellow-tailed black-cockatoos per annum remained similar (post-fire 0.32 ± 0.12; pre-fire 0.28 ± 0.05). The mega-fires on Kangaroo Island in 2019–2020 resulted in tree hollow loss on a much larger scale than recorded in the 22 years prior, and subsequently, nest boxes have become an important resource for the endangered glossy black-cockatoo.