Body condition, hormonal correlates and consequences for survival in common tern chicks

Although sibling competition in avian species has been extensively studied, the proximate mechanisms remain largely unknown. Recent research proposed that steroid hormones, in particular testosterone (T) and corticosterone (CORT), might play a role either in promoting competitive behavioral displays or in response to chronic nutritional stress accompanied by a sustained competitive situation. Here, we examine body condition, endogenous T levels and fecal glucocorticoid metabolites (FGM) as non-invasive measures of CORT in sibling broods of wild common tern chicks (Sterna hirundo) during three post-natal developmental stages. In this species, distinct within-brood size asymmetries are imposed by an asynchronous hatching interval, and sexes show slightly different growth patterns. First-hatched (a-)chicks were in better condition than their later-hatched sibling (b-chick). FGM levels inversely covaried with condition and were elevated at the end of pre-fledging development. T levels of a- and b-chicks changed with age, although the direction of the changes differed, with b-chicks eventually having higher levels than their older siblings. Survival to fledging was not associated with FGM but with T levels, which tended to be higher in surviving chicks. Our results are discussed with regard to how plasticity in steroid hormones could be involved in mediating sibling competition in common terns.