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

In avian young that are dependent on parental food pr visioning, brood size and corresponding sibling competition are key environmental factors a ffecting present performance, future developmental trajectories, and ultimately fitness. Within-brood competition for nutritional resources as well as the energetic costs of social competition may have a general impact on the sibship, but often also differently affect chicks w ithin brood hierarchies based on hatching rank and sex. Testosterone (T) and corticosterone (CORT) , steroid hormones associated with aggression and stress response, respectively, are f ocal candidates for proximately mediating sibling conflicts and the physiological consequence s. I studied the effect of sibling competition on chick condition and possibly underlying behaviou ral 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 behaviou r, and T and CORT levels. I hypothesized T and CORT levels to vary with hatching order and sex of the chicks. Condition was better in senior than junior chicks and as a tendency also be tter in male than female chicks. In contrast, neither competitive success nor hormone levels vari ed with hatching rank or sex. However, T levels were affected by the interaction of the sex of the senior chick in a brood with hatching order: Male seniors had lower and female seniors ha d higher T levels than their junior siblings, respectively. Correspondingly, with respect to cond ition male seniors tended to be more superior to their younger brood mates than did female senior s. Second, to investigate the effect of sibling compet ition 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 brood s. I tested the hypotheses that the junior chick remaining in the colony will in response show increased feeding rates, ameliorated condition, and decreased T and CORT levels. In the absence of their siblings, juniors' condition indeed improved. Their CORT levels covaried with co ndition and tended to drop, and significantly rose again upon the senior's return. In contrast, the experiment did neither affect juniors' feeding rates nor T levels. Overall, the results show that sibling competition mposes limits on chick condition, at least in junior chicks, and underline the role of elevated C ORT 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 endoge n us T does not hold in common terns. Hatching rankand sex-related condition patterns d o not appear to be governed by baseline endogenous steroid hormones, but could instead be i nfluenced by short-term level elevations or maternal yolk steroids.