Evaluating variation in acute stress response in captive Wood Ducks (Aix sponsa)

Acute stress response has been linked to body condition and associated with the allocation of finite energy resources in wild, free-living birds. However, the relationship between the body condition of individuals and the acute stress response is less clear for birds kept in captive settings, where energy resources are abundant and readily available. We evaluated how individual variation in body condition, reflected as body mass, relates to the acute stress response in adult captive Wood Ducks (Aix sponsa) hatched from eggs collected in the wild and reared in captivity while birds were fed ad libitum, in and out of the breeding season. We determined plasma corticosterone (CORT) levels from blood collected within 3 min of capture and at 10, 30, and 60 min post-capture from 28 ducks during spring and fall, in and out of breeding season. Body condition measurements were recorded once for each bird in early spring. The effects of body mass, age, and time of year (in vs. out of breeding season) on total and maximum CORT secreted were analyzed in Generalized Linear Mixed Models (GLMM). No relationships between age, body mass, and total or maximum CORT were found. There was a significant difference in response in and out of the breeding season for total CORT (p < 0.01) and maximum CORT (p < 0.001), with a higher response during the breeding season. Measures of individual responses for total (p = 0.001) and maximum CORT (p < 0.01) were consistent across seasons. Baseline CORT levels were consistently low, with 53.6 % and 78.6 % of the samples below the detection limit during and outside of breeding season, respectively. These results highlight differences in baseline CORT maintenance compared to studies on wild Wood Ducks, potentially relating to environmental differences such as unlimited food supply and protection from predators in captivity. Additionally, we found that captive birds maintain seasonal variation in acute stress response, which reflects some trends found in wild birds, although more research across multiple seasons is warranted. We also found that some captive Wood Ducks continue to increase CORT secretion at the end of the 60 min handling period, indicating that longer handling times may reveal differences in total CORT secreted. Further investigation should be done to assess the costs and benefits of variation in body condition in terms of coping ability across life stages.