HIEBERT, S.M.*; SALVANTE, K.; RAMENOFSKY, M.; WINGFIELD, J.C.: Seasonal differences in the response of corticosterone and nocturnal torpor to energy challenge in rufous hummingbirds

The energy balance of captive rufous hummingbirds (Selasphorus rufus) was challenged during the autumn migratory season and during the late winter prebasic molt by diluting the artificial nectar on which the birds feed. Body mass, cloacal fluid corticosterone, and duration of nocturnal torpor were then compared between food dilution and normal food treatments, and between migratory and molting birds. Differences between migratory and molting birds in the response patterns of all three variables to the same food dilution treatments demonstrate that corticosterone concentrations alone are not sufficient to explain the differences in torpor use in migratory and molting birds, and argue against an earlier hypothesis that corticosterone could serve as the ultimate integrator of information about physiological condition and environmental energy availability to the torpor regulation system. Additional experiments demonstrated that the sensitivity of torpor to exogenous corticosterone during late summer differs between the sexes in parallel with differences in the timing of southward migration by adult males and females. Although duration of nocturnal torpor was influenced by administration of exogenous corticosterone in several experiments, the results do not distinguish between a direct influence of corticosterone on torpor, and an indirect influence of corticosterone on torpor through other variables such as food consumption and body mass.