Shrinking body size under climate warming is not associated with selection for smaller individuals in a migratory bird.

How species are responding to climate change is a key topic in evolutionary ecology. Increasing temperatures are expected to affect phenotypic traits involved in thermoregulation, thus decreasing body size and/or increasing body appendages associated with heat exchange, as predicted by Bergmann's and Allen's rules. Results from long-term studies of variation in morphology over time have generally provided results supporting these predictions. However, two outstanding questions are frequently raised in studies relating changes in phenotypes to increasing temperatures: (1) whether such changes involve a shift in animal shape through the non-proportional variation of different body parts; and (2) whether they result from adaptive evolutionary responses. Relying on capture-recapture histories of almost 9000 breeding individuals from a declining Italian population of an Afro-Palearctic migratory bird, the barn swallow (Hirundo rustica), we documented a decrease in some body size traits (body mass, keel and wing length) over a 31-year period (1993-2023), with body mass declining the most (up to 4.0% in males). However, this was not the case for bill and partly tarsus length. Intra-individual lifelong changes in morphological traits of sexually mature birds showed only a limited contribution to trends over time in phenotypically plastic morphological traits. Viability and fecundity selection analyses revealed that smaller individuals did not enjoy greater success compared to larger ones. For some traits, the opposite was actually the case. The shifts in body size and, partly, shape over time we observed were coherent with predictions deriving from Bergmann's and Allen's rules. Yet, natural selection did not consistently favour smaller individuals. We thus call for caution in interpreting recent decreases in body size as adaptive evolutionary responses to climate warming, as they may rather reflect phenotypically plastic responses to changing climatic/environmental conditions occurring during early ontogenetic stages.