Early-life environment drives long-term decrease in adult body mass in a wild bird population

Body mass is a key organismal characteristic that impacts many physiological and ecological processes and is often a strong determinant of fitness. Many recent studies have documented temporal phenotypic changes in this trait in wild animal populations, but identifying the mechanisms underpinning these changes can be difficult. While most research conducted to date has focused on temperature changes as a driver of these trends, the relevance of other environmental variables remains to be analysed, limiting our understanding of the factors driving these changes. Here, we use 47 years of data to decompose mechanisms behind temporal changes in adult and nestling body mass in a great tit Parus major population in Wytham Woods (UK). Further, we link those changes to temperature, the main driver of temporal trends in mass according to the literature, and to two other environmental variables previously recognised as drivers of body mass: intra- and inter-specific competition and temporal mismatch with a key prey during breeding, winter moth Operophtera brumata caterpillars. At the population level, we report a marked decrease in body mass in adults between 1978 and 2024 (-0.042 Haldanes; -0.020 g year^-1) and show that this results from phenotypic plasticity, driven by a negative between-cohort trend likely reflecting carry-over effects of the early environment. Within cohorts, however, trends were consistently positive, reflecting an age-dependent mass increase. The temporal change in adults was paralleled by a change in nestling body mass (-0.036 Haldanes). Nestling mass was negatively associated with estimated intensity of intraspecific competition, as well as inter-specific competition from blue tits Cyanistes caeruleus, as quantified by local population density. These effects carried over to adulthood, as shown by a negative association between adult mass and the population density of great tits and blue tits experienced in early life. Seasonal and developmental temperature, and mismatch with the caterpillar food supply, despite being associated with adult and nestling mass, did not explain the observed declines in mass. Our results illustrate the potential for effects mediated early in development to carry over into long-term phenotypic change at later life-history stages, and emphasise the importance of considering different environmental variables as drivers of phenotypic change in natural populations.