Facing the heat: nestlings of a cavity-nesting raptor trade safety for food when exposed to high nest temperatures

Increasing temperatures due to climate warming may expose many species to thermally dangerous conditions, especially during less mobile phases of their life cycle when the chances of relocating to cooler sites are limited. We investigated how altricial nestlings of cavity-nesting lesser kestrels, Falco naumanni, respond to elevated nest temperatures. In the study system, nestlings develop in nestboxes placed on roof terraces. Before fledging, they often rest outside the nestbox, near the entrance hole, where they can be fed by parents and rapidly re-enter if threatened. This behaviour is similar to pre- mature fledging observed in some species during heatwaves, which causes extensive nestling mortality. We experimentally tested whether high nest temperatures increased the likelihood of nestlings temporarily leaving their nest, thereby exposing themselves to higher predation risk. This may occur either because nestlings attempt to escape unbearable temperatures inside the nest or because resting outside provides them with priority access to parentally delivered food. After the eggs hatched, we reduced the maximum nest temperatures by ca. 4 C by shading the nestboxes. We found that nestlings from control (unshaded) nestboxes were twice as likely to be detected outside than those from shaded ones. Regardless of shading, nestlings left the nestboxes more frequently in the early morning when nest temperatures were lowest. Notably, hourly nest temperature did not significantly affect the probability of nestlings being detected outside. Nestlings resting outside monopolized parentally delivered food; however, parental provisioning did not differ between shaded and control nests, implying a costly, zero- sum competition game among nestmates. Our results suggest that exposure to high nest temperatures associated with climate warming can increase mortality due to dehydration and impaired growth and may also induce nestlings to trade their safety for enhanced access to food, thereby exacerbating the negative effects of climate warming on birds' reproduction.