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The acquisition of body reserves by migrating shorebirds at specific staging sites is critical for reaching their breeding or wintering grounds. As many North American shorebird populations are declining, major staging sites need to be identified for effective conservation plans. Changes in body mass of Semipalmated Plover ( Charadrius semipalmatus Bonaparte, 1825) and Semipalmated Sandpiper ( Calidris pusilla (L., 1766)) juveniles, as well as length of stay and daily movements by Semipalmated Plover juveniles during fall migration, were studied in the St. Lawrence River Estuary in southern Quebec, Canada. Most juveniles of both species captured early in the migration period weighed less than the estimated mean fat-free mass. Body mass increased steadily in juveniles of both species during staging. Mean body masses of birds captured in our study area were similar to or higher than those of juveniles of both species captured along the North Atlantic coast. Length of stay of radio-tracked Semipalmated Plover juveniles ranged from 2 to 28 days. Body mass at capture and length of stay were negatively associated, suggesting that individuals refuelled before resuming their migration. Overall, these results provide evidence that the St. Lawrence River Estuary is an important staging site for Semipalmated Plover and Semipalmated Sandpiper juveniles. [ABSTRACT FROM AUTHOR]