Tolerance to high temperature by arboreal mammals using nest boxes in southern Australia

Nest boxes are used to manage populations of tree-cavity dependent birds and mammals. Concerns have been raised that due to their poor insulative properties nest boxes may cause heat stress and occasionally death during summers of extreme maximum temperatures. Our study investigated whether this nest box heat stress hypothesis applies to two small cavity-dependent mammals (brush-tailed phascogales and sugar gliders). Focusing on days when ambient temperature reached ≥40 °C, we recorded: i) temperatures within occupied nest boxes, ii) temperatures within nearby unoccupied tree cavities, iii) the duration of temperatures of ≥40 °C within nest boxes, iv) whether direct mortality was observed, and v) the relative abundance of these species in nest boxes before and after a very hot summer. When ambient temperature reached ≥40 °C, nest boxes were equivalent to ambient or 1-2 °C cooler, whereas tree cavities were 3-7 °C cooler than ambient. Exposure in nest boxes to temperatures of ≥40 °C lasted an average of 2-8 h. We observed no mortality over 65 records of phascogales and 31 records of gliders in nest boxes on days when ambient reached ≥40 °C. No decline in abundance was recorded after a summer with 11 days of temperatures ≥40 °C, with each species subsequently occupying >40 nest boxes. Our observations suggest these species are tolerant of the high temperatures that occurred. Nonetheless, provision of nest boxes designed to minimise summer heating is recommended.

Does temperature variation influence nest box use by the eastern pygmy-possum?

Cavity-using birds and mammals reliant on nest boxes may be negatively affected by the poor thermal buffering of nest boxes. I investigated whether nest box use by the eastern pygmy-possum (Cercartetus nanus) over a 4-year period was influenced by maximum ambient temperature, which ranged from 15.6 to 34.9°C during survey occasions. Occupancy modelling of 144 site detections over 30 survey occasions suggested that a model that included maximum temperature had little support whereas a model involving time-varying detection (i.e. detection differed across sample occasions) was the most plausible. I also investigated how temperatures in nest boxes and tree hollows varied over the four hottest days of summer, including one day when the temperature reached 40.6°C. Maximum temperatures were 3–4°C cooler in plywood nest boxes and 5–8°C cooler in tree hollows compared with ambient temperatures. Together, these results suggest that eastern pygmy-possums using nest boxes in coastal areas are unlikely to experience heat stress. Cavity-using species are a heterogeneous group such that empirical studies are required to identify those that may be vulnerable to heat stress if nest boxes are used to provide population support.