Daily and Seasonal Variation in Response to Stress in Captive Starlings (Sturnus vulgaris): Corticosterone

Abstract

Captive starlings were used to examine daily and seasonal changes in basal and stress-induced corticosterone levels. Birds were bled at 4 times during the daily cycle and during three different simulated seasons: under a short-day photoperiod (mimicking winter), under a long-day photoperiod (mimicking summer), and while undergoing a prebasic molt. Basal corticosterone samples were assayed from blood collected within 3 min of disturbance and corticosterone increases in response to handling and restraint were monitored in blood taken at 15, 30, and 45 min postdisturbance. Handling and restraint elicited robust increases in corticosterone at all times of the day and during all three seasons. Both basal and stress-induced levels varied with the time of day (with the exception of basal samples during molt). Levels were higher at night, during the bird's inactive period, and decreased during the day. These data indicate that starlings have daily rhythms in both basal corticosterone levels and in their response to stress, with more corticosterone released during the night in response to identical stimuli. Starlings also show pronounced seasonal variation in both basal and stress-induced corticosterone levels. Although birds held on short and long days had equivalent corticosterone levels, both basal and stress-induced levels were lower during molt. This parallels data from free-living birds and provides a laboratory model for studying seasonal corticosterone regulation.