Sibling competition in common tern Sterna hirundo chicks : underlying hormonal and behavioural patterns and mechanisms

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.