Phenotypic variation in wild populations is generally the result of many genes with small effects (additive genetic variance), that often act in combination with individual variation introduced during development (permanent individual variance), and environmental effects such as abiotic or biotic factors (environmental variance). By applying a statistical model based on a variance-partitioning approach, quantitative genetics allows to estimate the levels of additive genetic variance, its magnitude relative to the overall phenotypic variance (the so-called the trait’s heritability), as well as the correlation structure between heritable traits. The latter is important for understanding the evolutionary trajectories of traits, because only when a trait is genetically correlated with fitness can it evolve through natural selection. 

Using the information of social parents and their offspring, we can construct a social pedigree of the Banter See common terns (see figure). This pedigree is a good approximation of the genetic pedigree, because common terns exhibit very low levels of extra-pair paternity (González-Solís et al. 2001), and for the period 1992-2023, it comprises 10,581 records, covers 7 generations, and contains 912 and 798 paternities and maternities, respectively. As such, it offers the possibility of effectively studying the quantitative genetics of traits such as laying date (Moiron et al. 2020), telomere length (Vedder et al. 2022), fitness (Moiron et al. 2022), arrival date (Moiron et al. 2024), social ageing (Moiron & Bouwhuis 2024) and mercury contamination (Bertram et al. 2024).