Maternal effects can increase offspring phenotypic variation. In birds, mothers can influence the developmental environment of chicks by secreting various substances into the egg yolk. Recent studies have demonstrated that yolk substances can interactively affect offspring phenotype, but the implications of such effects for chick fitness and phenotype in natural populations have remained unclear. We measured 31 yolk components including steroid hormones, antioxidants and fatty acids in wild great tit eggs (Parus major) during two breeding seasons. We tested for associations among yolk substances, and for relationships between component groupings and offspring fitness and phenotype. Hatching and fledging success were explained by yolk fatty acids (including saturated, mono- and polyunsaturated fatty acids) – but not by androgen hormones and carotenoids. Fatty acids also accounted for more variation in nestling oxidative status and size than androgens and carotenoids. Our results indicate that fatty acids are important yolk substances that shape offspring fitness and phenotype in wild populations. Therefore, future work on maternal effects needs to incorporate the multivariate composition of the egg and avoid focusing on single yolk components. Since several fatty acids are of dietary origin, our study also highlights a possible role of the environment in shaping maternal effects and consequences for offspring.