4.3 | The effect of capture event on fat score change in winter
Interestingly, we found that robins that had previously been captured tended to have more fat reserve in all three models. Two possible hypotheses were constructed from this result: (1) birds perceive capture as a heightened predation risk and respond by reducing body mass or (2) birds perceive capture as an interruption to foraging and respond by increasing body mass. Our results supported the second hypothesis and were consistent with a previous study on great tit (Parus major ), in which the body size of the bird showed a significant increase on the day after capture and persisted for one week (Duarte, 2013; Macleod and Gosler, 2006). Meanwhile, from a physiological perspective, a capture event could cause acute stress and additional status change. During an acute stress response, the adrenal gland of the bird secretes corticosterone via the hypothalamic–pituitary–adrenal axis (HPA). The initiation of HPA would then result in increased synthesis of glucocorticosteroids that mediate behavioral and physiological changes (such as an increase in body mass) to help the organism cope with the stressor (Dickens et al., 2009). This outcome can be a reference case for further study using classical ornithological field study methods, helping to analyze disturbance caused by human capture.