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

Birds respond to short-term deterioration in foraging conditions by increasing their plasma level of corticosterone but the physiological effects of long-term deterioration in food supplies are not well known. In resident passerine birds that winter in temperate climates, such as the mountain chickadee (Poecile gambeli), the food supply may be limited and unpredictable over long periods of time. Whether the long-term limited and unpredictable food 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 supply. The food-restricted birds had significantly higher baseline levels of corticosterone than those maintained on ad libitum food. All birds responded to the acute stressor by an increasing secretion of corticosterone but there were no differences between the treatment groups in their stress response. There was a significant effect of sex on the stress response, with females reaching higher levels of corticosterone and responding at a faster rate than males. These results suggest that permanent resident birds wintering in harsh environments may have elevated levels of corticosterone on a long-term basis. Whereas other factors, such as day length and ambient temperature, may contribute to energetic hardship during the winter, the results showed that limited and unpredictable food alone can trigger significant changes in baseline levels of plasma corticosterone. The potential costs and benefits of long-term increased corticosterone levels in resident food-caching birds are discussed.