Daily and seasonal variation in response to stress in captive starlings (Sturnus vulgaris): Glucose

We investigated the seasonal and daily variation in plasma glucose levels i n response to stress in captive wild starlings. Starlings were captured fro m the wild during the winter, held on short days (11L:13D, mimicking winter ), and then shifted to long days (19L:5D, mimicking summer). Birds were mai ntained on long days until they began a prebasic molt, the energetically co stly replacement of feathers. Throughout the daily cycle we took a basal bl ood sample within 3 min of disturbance and took subsequent blood samples at 15 and 30 min. Birds were kept in cloth bags (restraint) between bleeds. E xperiments were repeated during all three seasons (short day, long day, and molt). Starlings showed no sexual difference in circulating glucose levels at any time of the day or in any season. Both basal and stress-induced glu cose levels, however, showed a significant effect of season, with birds hel d on long days exhibiting the highest levels, molting birds showing interme diate levels, and birds held on short days exhibiting the lowest levels. Ba sal glucose levels also showed a circadiel rhythm in all three seasons. Reg ardless of season, however, the daily peak in basal levels occurred at midd ay with nadir in the middle of the scotophase. This trend was paralleled in the overall weights of the birds. Although stress-induced glucose levels s howed no circadiel rhythm, the stress-induced elevation of glucose above ba seline showed a significant daily rhythm, indicating that stress elevated p lasma glucose levels only during the scotophase in all three seasons. (C) 2 000 Academic Press.