Individual variation in corticosterone secretion in laying hens

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Abstract
1. Plasma corticosterone responses to a handling and isolation stressor were measured in laying hens once, before two groups were mixed into a single larger group, and again after the groups were mixed. Hens were bled before the stressor was applied and within 3 min of removal from groups (0 min sample), and again 15 and 45 min later 2. Social interactions were recorded throughout the experiment using continuous, all-occurrences sampling during group observations and the social rank index of each hen was calculated. 3. Plasma corticosterone was low immediately after birds were first picked up and sampled on each occasion, and rose to a peak after repeated handling, 15 min later. It then declined and was almost back to basal levels 40 min after first handling. 4. Corticosterone responses were greater 6 d before mixing (first day of sampling) than they were 1, 7 or 21 d after mixing. Corticosterone concentrations and responses were not elevated 1 d after the two groups of hens were mixed. 5. Variation in corticosterone concentrations and responses within and between hens was quantified by calculating coefficients of variation of corticosterone concentrations and areas under corticosterone response curves. Variation tended to be less within than between hens. 6. There was a transient increase in aggressive interactions immediately after mixing. One bird was clearly dominant in each observed dyadic interaction and hierarchies tended to strong linearity in all groups. Social rank was not related to corticosterone concentrations or responses. 7. Area under the corticosterone response curve can be used to compare differences between and within laying hens in their responsiveness to a handling and isolation stressor.