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Two flocks of Red Knots Calidris canutus, belonging to the temperate-wintering subspecies islandica and to the tropical-wintering subspecies canutus, were kept in outdoor cages at north-temperate latitudes over two annual cycles during which their plasma concentrations of corticosterone were measured at 4-6 week intervals. Islandica-knots maintained the schedules of moult and body mass changes of their counterparts in the wild over the two years of study, but canutus-knots showed deviations from normal annual rhythms. Even though the range of measured corticosterone concentrations was great, values remained well within those measured in free-living shorebirds. As predicted on the basis of past research, plasma corticosterone increased with respect to time required to capture and handle the birds during each sampling episode (capture-stress response). Plasma concentrations of corticosterone were also somewhat greater during spring migration periods (when birds had higher than average body masses) than at other times. In the islandica-, but not the canutus-flock, there were significant differences between individual concentrations of corticosterone. However, even after taking the foregoing factors into account, for the group showing the most natural mass and moult cycles (islandica), by far the strongest effect on corticosterone concentrations was the time since being taken into captivity. The longer the birds had been in the artificial aviary-environment, the lower the concentrations of plasma corticosterone became. Long-distance migrant bird species such as Red Knots normally encounter a wide range of environments during the course of their annual cycle. We interpret the long-term decrease in corticosterone in the light of its role in mediating appropriate physiological responses of birds faced with ecological variability in their environment and the occurrence of unpredictable events. We hypothesize that only after experiencing a full annual cycle

Long-Term Decreases of Corticosterone in Captive Migrant Shorebirds That Maintain Seasonal Mass and Moult Cycles