LYNN, S.E.*; BREUNER, C.W.; WINGFIELD, J.C.: Short-term fasting in a songbird leads to elevated free corticosterone levels due to decreased corticosterone binding globulin

Studies have shown that unpredictable events such as severe storms lead to an increase in circulating corticosterone (CORT) in breeding birds. This increase in CORT is often accompanied by an increase in foraging and irruptive behavior. We were interested in determining if acute food restriction (such as might occur during inclement weather) is a sufficient cue to elicit irruptive activity, increased CORT secretion, and/or decreased circulating levels of corticosterone binding globulin (CBG) in white-crowned sparrows (Zonotrichia leucophrys gambelii). Male Z.l. gambelii were housed individually in environmental chambers on long days (LD 20:4) to simulate daylength during the breeding season. Birds were fed ad libitum, and on select days, food was removed 2 h after lights on (fasted treatment), or was removed and replaced (control). The duration of fasting was randomized, and we analyzed CORT and CBG levels after 1, 2, 6, 22 (lights on), and 23 h under fasted and control conditions. We also measured activity during the 23 h experiment. Fasting as little as 2 and 6 h significantly increased males� activity levels and total CORT levels compared to controls, although after 22 h, fasted total CORT levels matched control levels. Plasma CBG decreased after the 22 h fast, and remained low after the 23 h fast. This change was sufficient to significantly elevate free CORT levels in 22 h fasted birds as compared to controls, despite the lack of difference in total CORT levels. Thus, short-term fasting quickly elevated activity and circulating CORT levels, and more slowly increased free CORT levels by altering plasma CBG.