Stress response in the rufous hummingbird (Selasphorus rufus): mechanisms of personality and social dominance

Abstract

The glucocorticoid stress response has recently been linked to characteristic, stable suites of behavior (“personality”) that hold fixed through time and environmental context. To lay groundwork for future study of this personality-CORT association, the stress responses of wild-caught rufous hummingbirds (Selasphorus rufus) were characterized by determining the corticosterone (CORT) concentration in cloacal fluid (CF) collected noninvasively over 60 min of restraint. On the basis of previous studies of sparrows and tits, we hypothesized that social dominance would be inversely correlated both with baseline CF CORT concentration and with the response of CF CORT to restraint. After capture, restrained birds were held in the hand and fed for 45 min, during which a separate CF sample was collected over each of three 15-min periods. For the final 15 min, birds were moved to a flight cage and a fourth CF sample was collected without handling the bird; all samples were analyzed by direct RIA. In contrast to our predictions, baseline CF CORT (first 15-min sample) of all three age-sex classes did not differ. Although the predicted relation between previously published dominance status in this species (adult females > first-year males > first-year females) and CORT levels was not supported, young males tended to develop higher CF CORT concentration in response to restraint than did females of any age. Of six behaviors measured in the flight cage, one was inversely correlated with CF CORT concentrations in response to restraint: birds with higher CF CORT were significantly more restricted spatially in their exploration of the flight cage. Additionally, adult and hatch-year females tended to perch more on the front and side of the cage than hatch-year males did. In contrast to a previous study of restraint stress in captive rufous hummingbirds, the wild-caught birds in our study showed significantly increased CF CORT within 30 min of capture and no significant change thereafter, suggesting that the stress response can be quantified in the field in a shorter period of time with this noninvasive method than was previously thought.