Extra food buffers thermoregulatory costs during early development but does not reduce heatwave-induced mortality in nestlings of a cavity-nesting raptor

Global climate change is increasing both average temperatures and the occurrence of extreme weather events, such as heatwaves. These changes pose major challenges for wildlife, especially during vulnerable life stages. In altricial bird species, nest-bound offspring are particularly susceptible to thermal stress due to limited behavioural thermoregulation, where high nest temperatures can impair development and induce elevated mortality, particularly when exceeding critical thresholds. We experimentally manipulated nest temperature and food availability in lesser kestrels ( Falco naumanni ) rearing their nestlings in nest boxes to investigate whether compensatory feeding offsets the negative effects of high nest temperatures on offspring size and pre-fledging survival. Nestlings were reared under three conditions: control (high nest temperatures), shaded (reduced temperature), and food-supplemented (high temperature with extra food). While parental provisioning rates did not differ among experimental groups, nestlings in food-supplemented boxes showed greater body size than control nestlings, despite similar thermal exposure, and matched the size of nestlings in shaded boxes. This indicates that increased food can buffer the energetic costs of thermoregulation under elevated, but non-lethal, temperatures. Mortality during development did not differ significantly among groups, as temperatures remained below critical thresholds. However, during a naturally occurring heatwave, mortality in control and food-supplemented nests was similarly high (∼40 %), whereas it was very limited in shaded nest boxes (∼4 %). These results suggest that food supplementation cannot prevent heatwave-induced mortality, likely caused by hyperthermia rather than starvation/dehydration. Nest microclimate may thus play a key role in mitigating the negative impacts of intense heatwaves in avian species.