Chainsaw hollows carved into live trees provide well insulated supplementary shelters for wildlife during extreme heat

Context: Supplementary shelters for hollow-dependent fauna, such as timber or plywood nest boxes, have much drier and less thermally insulated cavity microclimates than do natural tree hollows. Hollow-dependent endotherms can experience hyperthermia and dehydration when occupying poorly insulated nest boxes during extreme heat. Aims: We investigated the effectiveness of three different types of artificial hollows in buffering hollow-dependent birds and mammals against hyperthermia and dehydration during extremely hot summer weather (ambient air temperatures >40°C). Methods: We recorded microclimate (temperature and relative humidity) data inside (1) chainsaw hollows carved into live trees, (2) log hollows, and (3) plywood nest boxes, during extremely hot weather events in Australia in December 2019-January 2020 (austral summer). We quantified temporal variation in microclimates inside the different supplementary shelters relative to ambient conditions and used statistical models to evaluate the effects of different factors (wall thickness and solar exposure) on internal microclimates. Key results: Microclimates inside chainsaw hollows were significantly different from those in log hollows and nest boxes, remaining >16°C cooler and 50 percentage points more humid than ambient conditions when daytime air temperatures reached 45°C. In comparison, nest boxes closely tracked ambient conditions throughout the day. Log hollows had an intermediate microclimate profile, getting warmer and drier than chainsaw hollows during the day, but remaining cooler and more humid than nest boxes. Conclusions: Our results showed that artificial hollows more effectively mimic the stable microclimates inside naturally occurring hollows if placed inside the tree (e.g. carved into the tree trunk of live trees), rather than attached to the outside. Implications: The chainsaw hollow design we tested could provide microclimate refugia that reduce the risks of hollow-dependent wildlife experiencing either hyperthermia in regions with hot summer climates, or hypothermia in areas with cold winters. We encourage managers to consider incorporating chainsaw hollows into existing nest box programs to provide fauna with well insulated microclimate refugia.