Experimental nest cooling reveals dramatic effects of heatwaves on reproduction in a Mediterranean bird of prey

Future climatic scenarios forecast increases in average temperatures as well as in the frequency, duration, and intensity of extreme events, such as heatwaves. Whereas behavioral adjustments can buffer direct physiological and fitness costs of exposure to excessive temperature in wild animals, these may prove more difficult during specificlife stages when vagility is reduced (e.g., early developmental stages). By means of anest cooling experiment, we tested the effects of extreme temperatures on differentstages of reproduction in a cavity-nestingMediterranean bird of prey, the lesser kestrel(Falco naumanni), facing a recent increase in the frequency of heatwaves duringits breeding season. Nest temperature in a group of nest boxes placed on roof terraceswas reduced by shading them from direct sunlight in 2 consecutive years (2021and 2022). We then compared hatching failure, mortality, and nestling morphologybetween shaded and non-shaded(control) nest boxes. Nest temperature in controlnest boxes was on average 3.9°C higher than in shaded ones during heatwaves, thatis, spells of extreme air temperature (>37°C for >=2 consecutive days) which hit thestudy area during the nestling-rearingphase in both years. Hatching failure markedlyincreased with increasing nest temperature, rising above 50% when maximum nesttemperatures exceeded 44°C. Nestlings from control nest boxes showed higher mortalityduring heatwaves (55% vs. 10% in shaded nest boxes) and those that survivedfurther showed impaired morphological growth (body mass and skeletal size). Hence,heatwaves occurring during the breeding period can have both strong lethal and sublethalimpacts on different components of avian reproduction, from egg hatchingto nestling growth. More broadly, these findings suggest that the projected futureincreases of summer temperatures and heatwave frequency in the Mediterraneanbasin and elsewhere in temperate areas may threaten the local persistence of evenrelatively warm-adaptedspecies