Authors: Corregidor-Castro, Alejandro; Morinay, Jennifer;Morganti, Michelangelo;Assandri, Giacomo;Ramellini, Samuele;Morbiato, Elisa;De Capua, Enrico;Cecere, Jacopo G.;Pilastro, Andrea;Rubolini, Diego;

The increased frequency and intensity of heatwaves associated with climate change can have major impacts on bird conservation. The lesser kestrel Falco naumanii dependency on old buildings for nesting, along with agriculture transformation, caused a decline of the Mediterranean population in the last decades, but recent heatwave events represent a cause of further conservation concern. We report the effects of the 2021 heatwave on the reproductive success of a large lesser kestrel nestbox colony in southern Italy (Matera). To tease apart the heatwave effects on nest microclimate from those on the external environment, we either shaded or left unmanipulated (control) nestboxes at hatching. As expected, shaded nestboxes were on average 4.2°C cooler inside than controls. We observed an increased survival (shaded 85%, control 20%), body mass (+12%) and tarsus length (+3%) in nestlings from shaded nestboxes. These results indicate that heatwaves have direct, strong effects on nestling (and presumably post-fledging) survival. These results suggest that nest microclimate will be a major factor affecting population dynamics and conservation of this and possibly other Mediterranean birds breeding in nestboxes, underlining the need of designing nestboxes that can minimize the effect of future heatwaves.