Combined effects of molt and chronic stress on heart rate, heart rate variability, and glucocorticoid physiology in European starlings.

Molt is an important life-history stage in avian species, but little is known about the effects of chronic stress during this period. Three weeks after the onset of molt, captive European starlings (Sturnus vulgaris) were exposed to 18 days of chronic stress, induced with four 30-minute randomized stressors presented daily. Birds showed no chronic-stress-induced changes in heart rate or heart rate variability when measured either during the middle of the day or at night. These data suggest that chronic stress did not alter the balance between sympathetic and parasympathetic nervous system regulation of cardiovascular function, which contrasts with data from an earlier study indicating that chronic stress profoundly alters cardiovascular function in non-molting starlings. Additionally, there was a significant increase in restraint-induced corticosterone secretion the first week of chronic stress that subsequently returned to pre-chronic-stress levels by the second week of exposure. The attenuated corticosterone response again contrasts with data from non-molting starlings that showed significant decreases in corticosterone responses. Consequently, the resistance to cardiovascular and corticosterone changes indicates that the physiological changes induced by chronic stress are greatly attenuated in molting birds. Overall, the data suggest that molt requires a degree of physiological stability that must be protected, so that if a bird is exposed to chronic stress during this life-history stage, molt takes priority.