Surface reflectance drives nest box temperature profiles and thermal suitability for target wildlife

Thermal properties of tree hollows play a major role in survival and reproduction of hollowdependent
fauna. Artificial hollows (nest boxes) are increasingly being used to supplement
the loss of natural hollows; however, the factors that drive nest box thermal profiles have
received surprisingly little attention. We investigated how differences in surface reflectance
influenced temperature profiles of nest boxes painted three different colors (dark-green,
light-green, and white: total solar reflectance 5.9%, 64.4%, and 90.3% respectively) using
boxes designed for three groups of mammals: insectivorous bats, marsupial gliders and
brushtail possums. Across the three different box designs, dark-green (low reflectance)
boxes experienced the highest average and maximum daytime temperatures, had the
greatest magnitude of variation in daytime temperatures within the box, and were consistently
substantially warmer than light-green boxes (medium reflectance), white boxes (high
reflectance), and ambient air temperatures. Results from biophysical model simulations
demonstrated that variation in diurnal temperature profiles generated by painting boxes
either high or low reflectance colors could have significant ecophysiological consequences
for animals occupying boxes, with animals in dark-green boxes at high risk of acute heatstress
and dehydration during extreme heat events. Conversely in cold weather, our modelling
indicated that there are higher cumulative energy costs for mammals, particularly
smaller animals, occupying light-green boxes. Given their widespread use as a conservation
tool, we suggest that before boxes are installed, consideration should be given to the effect
of color on nest box temperature profiles, and the resultant thermal suitability of boxes for
wildlife, particularly during extremes in weather. Managers of nest box programs should
consider using several different colors and installing boxes across a range of both orientations
and shade profiles (i.e., levels of canopy cover), to ensure target animals have access
to artificial hollows with a broad range of thermal profiles, and can therefore choose boxes
with optimal thermal conditions across different seasons.