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 in response to stress in captive wild starlings. Starlings were captured from 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 maintained on long days until they began a prebasic molt, the energetically costly replacement of feathers. Throughout the daily cycle we took a basal blood 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. Experiments 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 glucose levels, however, showed a significant effect of season, with birds held on long days exhibiting the highest levels, molting birds showing intermediate levels, and birds held on short days exhibiting the lowest levels. Basal glucose levels also showed a circadiel rhythm in all three seasons. Regardless of season, however, the daily peak in basal levels occurred at midday with nadir in the middle of the scotophase. This trend was paralleled in the overall weights of the birds. Although stress-induced glucose levels showed no circadiel rhythm, the stress-induced elevation of glucose above baseline showed a significant daily rhythm, indicating that stress elevated plasma glucose levels only during the scotophase in all three seasons.