Noninvasive methods for measuring and manipulating corticosterone in hummingbirds

The adrenocortical response to stress has been shown to be important in ene rgy management of vertebrates. Although hummingbirds (Trochilidae) are usef ul models for studying energy balance, they are not amenable to traditional methods of studying hormones. In this study we report noninvasive methods for measuring and manipulating corticosterone (CORT), the principal stress glucocorticoid in birds. CORT was measured in cloacal fluid (CF) collected from unrestrained rufous hummingbirds (Selasphorus rufus). We demonstrate t hat CF CORT can be measured by radioimmunoassay without extraction. CF crea tinine, when used as a reference measure for CF CORT, corrects for changes in hydration state. As in other birds, CORT in both plasma and CF rose in r esponse to capture and handling stress and decreased after the termination of that stress, except that changes in CF concentration were delayed with r espect to changes in plasma. When CORT, complexed with cyclodextrin to impr ove solubility, was added to artificial nectar, CF CORT concentrations chan ged in a predictable, dose-dependent fashion. Measuring CORT in CF is advan tageous because it allows frequent and repeated sampling without itself pro voking a detectable stress response and because baseline samples need not b e obtained within the very short time between the onset of a stressor and t he appearance of CORT in the plasma, as is true for blood sampling. Adminis tration of exogenous CORT in the food offers a noninvasive, nonstressful, t emporally sensitive method for experimentally manipulating hormone levels i n an avian model that has already been used extensively for studies of ener getics. (C) 2000 Academic Press.