Roasting in nest boxes: physiological, behavioural and fitness consequences of extreme heat in cavity-nesting birds and mitigation strategies

Simone Messina , Marcel Eens

Artificial nest boxes are widely used to mitigate the loss of natural cavities caused by deforestation and habitat degradation, and they have become an important tool in both avian conservation and ecological research. However, the capacity of nest boxes to provide suitable microclimatic conditions for breeding birds is increasingly challenged by climate change. In particular, more frequent and severe episodes of extreme heat may expose developing embryos and nestlings to potentially harmful thermal conditions and may also reduce parental food provisioning. In this review, we synthesised current knowledge on the impacts of extreme heat on cavity-nesting birds breeding in nest boxes. Following a mechanistic framework, we first examined thermoregulatory responses to high temperatures, including evaporative cooling and facultative hyperthermia, together with dehydration risk and cellular stress responses. We placed particular emphasis on the sublethal effects of early-life exposure to extreme heat and their potential long-term fitness consequences. We further synthesised evidence for behavioural responses of parents and nestlings, including changes in parental care, food provisioning, and nestling behaviour, and considered their potential consequences for offspring development, survival, and subsequent fitness. We also examined evidence for heat-related mortality and potential population-level consequences, identifying ecological factors associated with variation in vulnerability. Finally, we evaluated strategies for reducing harmful thermal exposure, including changes in nest-box materials, wall thickness, surface reflectance, shading and placement. Integrating species-specific thermal physiology with nest-box design and placement will be important for maintaining the conservation value of artificial nesting structures under ongoing climate change.