Noninvasive methods for measuring and manipulating corticosterone in hummingbirds

The adrenocortical response to stress has been shown to be important in energy management of vertebrates. Although hummingbirds (Trochilidae) are useful 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 that CF CORT can be measured by radioimmunoassay without extraction. CF creatinine, 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 response to capture and handling stress and decreased after the termination of that stress, except that changes in CF concentration were delayed with respect to changes in plasma. When CORT, complexed with cyclodextrin to improve solubility, was added to artificial nectar, CF CORT concentrations changed in a predictable, dose-dependent fashion. Measuring CORT in CF is advantageous because it allows frequent and repeated sampling without itself provoking a detectable stress response and because baseline samples need not be obtained within the very short time between the onset of a stressor and the appearance of CORT in the plasma, as is true for blood sampling. Administration of exogenous CORT in the food offers a noninvasive, nonstressful, temporally sensitive method for experimentally manipulating hormone levels in an avian model that has already been used extensively for studies of energetics.