Heart Rate and Heart-Rate Variability Responses to Acute and Chronic Stress in a Wild-Caught Passerine Bird

The cardiovascular-stress response has been studied extensively in laboratory animals but has been poorly studied in naturally selected species. We determined the relative roles of the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS) in regulating stress-induced changes in heart rate (HR) in wild-caught European starlings (Sturnus vulgaris). In both heart-rate variability (HRV) analysis and receptor blockade (atropine and propranolol) experiments, baseline HR was controlled predominantly by the PNS, whereas the increase in HR resulting from acute restraint stress was controlled predominantly by the SNS. These results indicate similar cardiac control of baseline and acute-stress-induced HR in wild-caught starlings and mammals. We further investigated HR responses during chronic stress. Driven primarily by changes in PNS regulation, baseline HR increased during the day but decreased at night. In addition, elevated HRs during acute restraint stress were attenuated throughout chronic stress and were accompanied by decreased HRV. This suggested that increased SNS drive elevated HR, but the attenuated HR response combined with resistance to the SNS blocker propranolol suggested that the sympathetic signal was less effective during chronic stress. Overall, chronic stress in wild-caught starlings elicited profound changes in cardiac function that were primarily regulated by changes in the PNS.