INTERACTIONS OF CORTICOSTERONE WITH FEEDING, ACTIVITY AND METABOLISM IN PASSERINE BIRDS

Corticosterone (B) may play a direct role in the promotion of feeding behavior under conditions of nutritional stress. However, effects of exogenous B and nutritional stress in passerines indicate a complex relationship of fed state, perceived or anticipated nutritional stress, and previous history. In a series of investigations on caged White-crowned Zonotrichia leucophrys and Song Sparrows Z. melodia, foraging behaviors and feeding rates were unaffected by exogenous B in fed birds. When food was returned, birds implanted with B refed for longer and with greater intensity following the 24 hour fast. Additionally, B-implanted birds showed lower activity (perch hopping) than controls when fed ad libitum, but when fasted this trend reversed with B-implanted birds showing increased activity and apparent escape behavior. In a second study, small flocks of Pine Siskins Carduelis pinus and Dark-eyed Juncos Junco hyemalis held in aviaries and fasted for 24 hours had higher plasma levels of B than similar flocks that had been allowed to refeed for 1 hour after fasting. However, the highest levels of B in fasted birds were still lower than in free-living conspecifics exposed to capture stress. This low amplitude modulation of plasma B levels in relation to fed state was corroborated by results from free-living Pine Siskins and Lapland Longspurs Calcarius lapponicus captured during snow storms. In spite of unusually high feeding intensities during the storm, individuals of only some Pine Siskins and no Lapland Longspurs had elevated circulating levels of B. In the latter species, serial samples revealed very high levels of B within 15 minutes of capture suggesting enhanced adrenal secretory activity as a result of the storm. Finally, in White-crowned Sparrows, exogenous B substantially reduced overnight metabolic expenditure by reducing the frequency and amplitude of arousal bouts during the night. This suggested a possible energy savings effect of B during the hours when foraging was impossible and, possibly, insurance that the individual will retain sufficient resources to seek food the following day. Taken together, our data suggest that B may be important for initiation of food-searching during periods of food deprivation. Only small increases in B secretion appear to be required thus facilitating a rapid return to homeostasis after food is located. Although under prolonged nutritional stress the basal levels of B remained low, adrenal potential to secrete B in response to further stress increased dramatically.