Evaluating the effectiveness of heat shields on the thermal microclimates of nest boxes for cavity nesting birds

Rising global temperatures and frequent heatwaves pose threats to biodiversity, with cavity nesting birds especially at risk due to declines of naturally occurring cavities and the enclosed nature of their nests. Nest boxes are commonly utilized to offset habitat loss, but exhibit poor thermal insulation and internal temperatures that exceed those of natural cavities, potentially influencing fitness of nestlings. Various methods to cool nest boxes have been proposed. The heat shield (a second layer of material installed above the roof to deflect solar radiation) is poorly studied despite availability on the commercial market. We tested the effectiveness of heat shields made of aluminum, plywood, and PVC plastic in buffering internal temperature extremes of 31 standard wooden nest boxes for secondary cavity nesting birds during the breeding season in open grassland in southern Michigan, USA. We monitored internal and ambient temperatures of occupied and unoccupied nest boxes using thermal loggers. Heat shields did not lower daytime maxima or reduce daily temperature fluctuations relative to ambient, and only plywood, the thickest material, significantly increased nighttime minima relative to ambient (0.48 °C above controls). Occupancy increased temperatures relative to ambient regardless of treatment. Our findings suggest that roof-mounted heat shields alone are insufficient as a strategy to mitigate elevated thermal microclimates in nest boxes. Given the marketing of heat shields to the public and the urgent need for effective thermal mitigation strategies, we recommend further research into alternative materials, designs, and implementations before heat shields are promoted as a conservation tool.