Body size as a mediator of climatic effects: Insights from a long-term study of social Iberian magpies

The importance of considering body size in assessing evolutionary responses to climatic change is increasingly recognized, as body size correlates with morphological, physiological, and ecological traits that are climate-sensitive and influence fitness. However, the role of body size in mediating climatic effects in vertebrates remains poorly understood. Here, we examined structural body-size responses to natal and adult-experienced climate in a social bird and assessed whether body size mediates the direction of climatic effects. Using 26 years of data on cooperatively breeding Iberian magpies Cyanopica cooki, we found that adult males reared in late spring nests were smaller than those hatched earlier in the breeding season. This pattern was driven by smaller females, which reproduced later and produced smaller sons. Larger males born in hottest years had lower lifetime fledgling production, while larger males experiencing hotter springs as adults produced more fledglings over their lifetime. Additionally, larger males born in driest years or raised in nests with many brood mates had shorter lifespans. Despite a significant increase in temperature in the study area over 26 years, the average tarsus length of males has not changed, likely due to opposing effects of natal and adult climate on body size. Our findings illustrate how inherited body size mediates climatic effects at different life stages, with these effects acting in contrasting directions on fecundity, resulting in apparent trait stasis despite ongoing climate warming.