Fig. 5. Effects of RU486 treatment on plasma corticosterone (ng ml^-1) in spring migrants and wintering birds. Corticosterone concentrations were determined from blood samples collected in under 3 min of disturbance. Administration of RU486 caused a decrease in plasma corticosterone during both life-history stages. Black bars, controls; white bars, RU486-treated birds. Sample sizes are shown. Asterisks indicate significant differences between treatment groups. Error bars indicate S.E.M.