The stress response of European stonechats depends on the type of stressor

The acute increase in plasma levels of corticosteroid hormones in response to a stressful situation is essential for adequate physiological and behavioural responses to unpredictable events. The pattern and amplitude of the corticosteroid stress response has been extensively studied to compare endocrine adaptations between different life history stages, but also to determine environmental and physiological disturbances. Most studies on birds have used a simple and repeatable standardized stress-protocol. After capture, blood samples are taken at fixed time intervals from the bird kept in a cloth bag. Capture and handling during this procedure are stressful events which typically result in enhanced secretion of corticosterone. However, it is not known whether the stress response obtained with the cloth bag protocol is comparable with a life-threatening situation such as the confrontation with a predator. In this study, we investigated the plasticity of the stress-response towards different stimuli. We compared the stress responses of European stonechats (Saxicola torquata rubicola) exposed to different stressors: (1) a standard cloth bag; (2) space restraint by caging; and (3) caging and additional confrontation with a live predator, a tawny owl (Strix aluco). Our results show that the stress response induced by the predator was greater and the stress response induced by the cage tended to be greater than that induced by the cloth bag protocol. Thus, the present study showed that the stress response may vary depending on the type of stressor.