In birds, siblings typically compete over food, which frequently escalates into the mortality of the weakest sibling(s). Parents may promote such competition, by increasing hatching asynchrony, to allow an efficient adjustment of brood size to food availability. Common terns (Sterna hirundo) lay up to three eggs per clutch, which hatch asynchronously such that earlier siblings have a considerable developmental advantage in competition with their later hatching siblings. This project investigates how this early-life disparity among siblings translates to individual success in competition over food, age-specific chick mortality, and the long-term consequences for their reproductive value in adulthood. By also estimating how earlier chick mortality saves parental resources wasted on chicks that do not fledge, under varying levels of food availability, we aim to explain how sibling competition and early-life mortality can be adaptive in a long-lived species with unpredictable annual variation in food availability, and shape the species’ life-history strategy.