Plasma corticosterone concentrations increase when birds experience a stressor, and in this study we quantified variation in corticosterone responses for the first time in a species of free-living bird. Adelie penguins (Pygoscelisadeliae) nesting at Cape Bird on Ross Island in Antarctica were sampled on three occasions. Penguins with relatively low or high corticosterone responses on the first occasion had consistently low or high responses, as previously found for great tits and chickens. A model for birds is proposed in which birds with low corticosterone responses and proactive personalities are likely to be more successful (have greater fitness) in constant or predictable conditions, whilst birds with reactive personalities and high corticosterone responses will be more successful in changing or unpredictable conditions. There is thus no linear relationship between the size of a corticosterone response and fitness. Whilst the absolute magnitude of corticosterone responses varies markedly across species of birds, coefficients of variation are similar. Individual corticosterone responses are generally repeatable, with significant statistical repeatabilities for 30 min corticosterone concentrations and integrated corticosterone concentrations in the Adelie penguin, great tit and chicken. Coefficients of variation in corticosterone responses between birds and power analyses were used to provide a rule of thumb for determining differences between groups of birds in mean corticosterone concentrations to enable statistical analyses to have acceptable levels of statistical power for given sample sizes. It is suggested that power analyses and this rule of thumb be adopted in future investigations of corticosterone responses in birds.