An experimental test of the capture-restraint protocol for estimating the acute stress response

Stress-induced increases in glucocorticoids (GCs) modulate behavior and are key in directing energy reserves. The capture-restraint protocol was developed to experimentally stimulate and quantify the magnitude of the acute stress response by comparing baseline GC levels with those collected after restraining a subject for a period of time, typically 30 min. This protocol has been used extensively in the field and lab, yet few studies have investigated whether it parallels hypothalamic-pituitary-adrenal (HPA) activation under natural acute stressors. We examined the hypothesis that acute stress from the capture-restraint protocol accurately mimics the adrenocortical response induced by a natural acute stressor. Using wild-caught rock pigeons Columba livia in a repeated-measures design, we compared plasma corticosterone (CORT) concentrations at baseline, after exposure to acute capture-restraint (30 min in a cloth bag), after tethering in a harness (30 min), and after a real but nonlethal attack by a predatory raptor. As found in previous studies, the capture-restraint treatment significantly increased CORT levels of pigeons compared with baseline. However, we also found that when pigeons were exposed to an attack by a raptor, their CORT levels were more than twice as high compared with the capture-restraint treatment. Our results provide evidence that an authentic acute stressor can activate the HPA axis to a greater extent than the capture-restraint protocol and also suggest that real predation attempts can have a significant effect on acute stress levels of wild birds.