Ecological harshness mediates reproductive trade-offs in a great tit population

Lack’s seminal work on bird clutch sizes has spurred expansive research on reproductive trade-offs, especially focusing on offspring quantity–quality trade-offs and the potential fitness consequences for the parents. The environment is a critical driver of the expression of individual reproductive traits, influencing them through plastic responses. However, the plasticity of reproductive trade-offs themselves across environments has seldom been studied, and these studies were often limited to experimental approaches and dichotomous environments. Using 58 years of detailed data from a great tit population, we employ the recently developed ‘covariance reaction norm’ (CRN) model to explore how continuous environmental variation influences the shape of reproductive trade-offs among individuals. Our analysis reveals that the offspring quantity–quality trade-off is predominantly stable across years, with minimal variation linked to ecological harshness during the breeding season. However, the CRN also demonstrated that the among-mother correlation between offspring mass and future offspring recruitment was positive, but only under harsh environmental conditions, suggesting that producing larger offspring provides fitness benefits when breeding conditions are suboptimal, which may reflect the importance of size for early-life competition. Altogether, this work highlights that there is temporal variation in some of the phenotypic correlations, mostly driven by environmental conditions, which shape the expression of offspring investment across breeding seasons. Our study shows the benefits of exploring old ecological questions in the light of new statistical methods, highlighting the importance of understanding how environmental variation shapes the expression of life history trade-offs and the evolution of plasticity in reproductive strategies.