Differential heat tolerance in nestlings suggests sympatric species may face different climate change risks

For endotherms, a major threat of climate change will be the increasing frequency of extreme climate events, including heat waves. Thus, the ability of different species to tolerate high environmental temperatures is likely to have important consequences for ecology and population dynamics. We investigated the impacts of exposure to high temperatures on survival, mass gain and physiological stress of nestlings of 2 sympatric bird species subjected to the same climatic conditions in the Mediterranean basin. Results showed species-specific responses to high temperatures. Whilst hyperthermia or acute dehydration caused 36% mortality among lesser kestrels Falco naumanni, none occurred among European rollers Coracias garrulus. Within survivors, high maximum daily temperatures significantly reduced mass gain, especially among kestrels. Moreover, mass loss during heat events was shown to result in carry-over fitness costs only for lesser kestrels, by decreasing fledging condition and likely impacting post-fledging survival. High nest temperatures strongly elevated physiological stress levels of kestrels. Overall, rollers exhibited greater resilience to heat than kestrels, surviving nest temperatures up to 50 degrees C and recovering from mass losses, suggesting that nestling development is flexible enough to cope with the constraints imposed by occasional heat waves. Although predicted increases in the frequency of extreme temperatures can accentuate lethal and sublethal fitness costs for both species, the higher thermal tolerance of rollers makes them more resilient. Our results highlight that sympatric species are not equally at risk when facing climate change and suggest that successfully predicting species response to global warming will require a better understanding of species-specific thermal tolerance.