Designing better nestboxes: double-walled and pale proves coolest under the sun

Context. Fauna nestboxes are used for conservation, research and mitigation against tree hollow/cavity loss. Scant attention has been given to the microclimate inside boxes until recently, with concern that nestboxes may be ineffective or a thermal trap because of high internal temperatures during summer. Aim. Our study used construction design principles to guide modifications to nestboxes to reduce maximum temperatures inside boxes. Methods. Five trials were undertaken, and modifications included addition of thermal mass, creation of a double wall system and painting the box and/or outer wall white. Nestboxes were placed in full sun. Key results. The internal temperature difference from ambient between the worst (unpainted box) and the best box design was around 7 degrees C at 30 degrees C, and 9.5 degrees C at 40 degrees C. Painting boxes white had a marked impact on internal temperatures, but the single modification giving most protection from heat gain was construction of a ventilated double wall. This created a shaded air space around the internal box. Painting the outer layer white further improved insulation, and painting both the box and outer layer gave the best result. Conclusion. Double-walled, pale nestboxes can provide significant protection from solar heat. Implications Adding an air space to insulate a nestbox has considerable advantages over alternatives - it contributes little weight (compare using denser wood/materials); avoids environmental issues associated with insulators such as polystyrene or foil batts; is inexpensive; is easy to retrofit a second layer around existing boxes and it should improve nestbox longevity as the outer layer protects the inner box from weathering.