In avian young that are dependent on parental food provisioning, brood size and corresponding sibling competition are key environmental factors affecting present performance, future developmental trajectories, and ultimately fitness. I studied the effect of sibling competition on chick condition and possibly underlying behavioural and hormonal patterns and mechanisms in the common tern Sterna hirundo, a semiprecocial bird species exhibiting hatching asynchrony, slight sexual size dimorphism, and profound effects of chicks' mass growth on their fitness. First, I conducted an observational study on sex-, hatching rank-, and brood size-specific patterns in chick condition, success in competitive behaviour, and testosterone (T) and corticosterone (CORT) levels; second, to investigate the effect of sibling competition on chick condition, feeding rates, and T and CORT levels, I experimentally varied the extent of within-brood competition by a one-day removal of the senior siblings from two-chick broods. Overall, the results show that sibling competition imposes limits on chick condition, at least in junior chicks, and underline the role of elevated CORT levels as a response to connected energetic stress. T seems to especially play a role for female senior chicks as a means to impose their dominance in the brood hierarchy in spite of a hardly superior physique, but the proposed general link between sibling competition and endogenous T does not hold in common terns. Hatching rank- and sex-related condition patterns do not appear to be governed by baseline endogenous steroid hormones, but could instead be influenced by short-term level elevations or maternal yolk steroids.