Temperature-related developmental plasticity, not selection, affects offspring body size and shape in a bird of prey

Recent climate warming has led to a reduction in bird body size and a relative elongation of their appendages, consistent with Bergmann’s and Allen’s ecogeographical rules. These changes are generally interpreted as thermoregulatory adaptations for more efficient passive heat dissipation; however, direct evidence supporting this assumption is currently missing, and laboratory studies failed to find significant thermoregulatory benefits associated with body size or appendage length. To test whether body shrinking and shape-shifting provide a fitness advantage under climate warming, we experimentally altered nest temperatures in a lesser kestrel (Falco naumanni) population exposed to high temperatures during the nestling stage. We found that nest temperature was associated with nestling mortality. Among nestlings that survived to fledging, temperature was negatively correlated with their near-fledging body size (mass and tarsus length) and positively correlated with their relative bill length. Contrary to the thermoregulation hypothesis, we found that nestlings that were larger at hatching had higher survival, irrespective of the nest temperature, whereas relative bill length did not confer any significant survival advantage. Collectively, our findings suggest that temperature-related developmental plasticity, rather than selection, is a key driver of observed morphological changes in natural bird populations, and that these changes are not adaptive.