Following the rules? Different thermal and temporal life‐stage responses of biometrics across four decades in a songbird population

Global warming is widely associated with changes in body size and shape, with these variations typically framed within the context of Bergmann, Allen and the Temperature‐Size rules. However, evaluating phenotypic change only against these rules can limit our understanding of how immediate plastic reactions to temperature interact with long‐term trends. Also, phenotypic responses may differ among life stages, due to their different thermal sensitivity and life‐history role. We examined body size and shape variations across life stages in a Mediterranean great tit ( Parus major ) population monitored over four decades, during which mean annual temperatures increased by approximately 1.7°C.

Adults were the life stage most responsive to temperature and their mass declined by 0.84% for every 1°C of warming, while wing length shortened. Instead, tarsus length, a measure of structural size, increased with temperature in both nestlings and adults at a comparable magnitude, consistent with Allen's rule. Nestling mass near fledging showed no thermal response. Among reproductive traits, clutch size tended to decrease with temperature without reaching statistical significance and egg mass showed no thermal response, yet total clutch mass, an integrated measure of reproductive output, declined significantly with warmer laying temperatures.

Long‐term temporal trends did not consistently mirror these thermal effects. Adult mass declined across the study period, matching the direction of its thermal response, whereas adult tarsus length shortened over time despite lengthening with temperature. Wing length and nestling mass both remained stable. Egg mass declined and clutch size increased across the study period, despite their lack of thermal response.

These results demonstrate that while warming can proximately affect body size, long‐term temporal trends can be decoupled from immediate thermal effects, and that mass and structural size do not necessarily vary together. This creates an underappreciated mosaic of phenotypic responses across life stages and generations, with potential consequences for fecundity, morphology and fitness. Our findings highlight the importance of integrating ontogenetic perspectives into evolutionary ecogeographic theory and raise questions about which ‘rules’ will govern the trajectories of avian populations under climate change.