Semipalmated sandpiper Calidris pusilla arrives on the coasts of Venezuela between the middle of August and early September, staying there until May when birds prepare for vernal migration. To investigate possible relationships between corticosterone levels and body mass changes during preparation for migration, we sampled birds during wintering (January) and vernal premigration (May) over 2 years. Body mass and fat increased significantly from wintering to vernal premigration in both years. Capture stress levels of corticosterone varied between 30 and 150 ng mL(-1), the highest being during wintering. These results strongly suggest that increase in body mass of C. pusilla during preparation for vernal migration in a tropical environment was promoted by a decreased corticosterone secretion response to stress.