Oviparous mothers transfer to the eggs components that have both independent and combined effects on offspring phenotype. Functional interactions between egg components, such as antioxidant and hormones, lead to expect that a change in the concentration of one component has effects on offspring traits that depend on the concentration of other interacting components. However, the combined effects of variation in different egg components are virtually unknown. Bird eggs contain vitamin E, a major antioxidant, and also maternal corticosterone. The independent consequences of variation in the egg concentrations of these compounds for offspring phenotype are largely unknown and no study has investigated their combined effects. We manipulated the concentration of vitamin E and corticosterone in the eggs of the yellow-legged gull (Larus michahellis) by administering a physiological (2 standard deviations) dose both independently and in combination. We tested for an effect on chick post-natal growth, plasma antioxidant capacity (TAC) and oxidative compounds (TOS). Separate administration of vitamin E or corticosterone caused a reduction in body mass relative to controls, whereas the combined administration of the two compounds reversed their negative effects. These results suggest thatmaternal egg components, such as antioxidants and steroid hormones, interact and mothers must balance their concentrations in order to achieve optimal offspring phenotype.The functional relationship between vitamin E and corticosterone is corroborated by the observation of positive covariation between these compounds.