Stress, corticosterone responses and avian personalities
Birds are constantly responding to stimuli from their environment. When these stimuli are perceived as threatening, stress responses are initiated, with activation of the hypothalamo-pituitary-adrenal axis and the release of corticosterone from the adrenal gland. The basic emotion of fear is also experienced during a stress response. Corticosterone stress responses and behavioural responses to stimuli vary markedly between individual birds, raising questions about the significance of these individual differences and about the relationship between corticosterone responses and fearfulness in birds. Although fearfulness can be challenging to measure, data from several species indicate that corticosterone responses and fear behaviour responses are linked in individual birds. Consistent profiles of behavioural responses of birds to a wide range of stimuli can be identified and are called personalities. Personalities vary along a continuum, but are usually classified as proactive or reactive. Individual corticosterone and behaviour responses depend on each bird’s personality. Birds with proactive personalities have relatively active behavioural responses and relatively low corticosterone stress responses, whilst birds with reactive personalities have relatively passive behavioural responses and large corticosterone responses. Relationships between the physiological and behavioural characteristics of avian personalities can now be explored in detail to determine the significance of individual differences in stress responses and personalities in birds.