Thermal constraints on body size depend on the population’s position within the species’ thermal range in temperate songbirds

Abstract Aim There is mounting evidence that climate warming can induce morphological changes locally, particularly size reduction. However, the direction of thermal stress may differ between climatic regions. We predicted that morphological response to temperature fluctuations should vary throughout species ranges, depending on the local climate. Hot temperature anomalies are expected to induce size reduction in hot regions where species live close to their upper thermal limit, whereas size stasis (or increase) would be expected in cold regions, where species live close to their lower thermal limit. Location France (204 sites). Time period 2000–2014 springs. Major taxa studied Songbird species (n = 9). Methods We tested whether the effect of temperature anomalies on juvenile body size varied along an 11 °C thermal gradient. Results In warmer springs, juveniles were larger overall at the coldest sites, but this effect decreased toward the hottest sites, becoming negative for two species. Main conclusions Warming should induce body size increases more frequently at the cold edge of species distribution ranges, and rather body size declines at the hot edg. The climate dependency of the effect of weather fluctuations on body size is still under‐acknowledged, and the pattern identified deserves to be investigated over broader climatic gradients and taxonomic coverage. Climate‐driven changes in body size are therefore not uniform across climatic regions and within species ranges.