Long-term unpredictable foraging conditions and physiological stress response in mountain chickadees (Poecile gambeli)

Birds respond to short-term deterioration in foraging conditions by increas ing their plasma level of corticosterone but the physiological effects of l ong-term deterioration in food supplies are not well known. In resident pas serine birds that winter in temperate climates, such as the mountain chicka dee (Poecile gambeli), the food supply may be limited and unpredictable ove r long periods of time. Whether the long-term limited and unpredictable foo d supply has an effect on (a) baseline levels of corticosterone and (b) the adrenocortical stress response to a standardized acute stress of handling and restraint in mountain chickadees was assessed. For a period of 94 days, one group of chickadees was maintained on limited and unpredictable food ( food-restricted) and the other group was maintained on an ad libitum food s upply. The food-restricted birds had significantly higher baseline levels o f corticosterone than those maintained on ad libitum food. All birds respon ded to the acute stressor by an increasing secretion of corticosterone but there were no differences between the treatment groups in their stress resp onse. There was a significant effect of sex on the stress response, with fe males reaching higher levels of corticosterone and responding at a faster r ate than males. These results suggest that permanent resident birds winteri ng in harsh environments may have elevated levels of corticosterone on a lo ng-term basis. Whereas other factors, such as day length and ambient temper ature, may contribute to energetic hardship during the winter, the results showed that limited and unpredictable food alone can trigger significant ch anges in baseline levels of plasma corticosterone. The potential costs and benefits of long-term increased corticosterone levels in resident food-cach ing birds are discussed. (C) 2001 Academic Press.