Foraging in stopover areas influences nutritional condition of birds during spring migration. Our purpose was to determine if body mass, percent carcass water, and serum biochemistry would predict energy reserves (carcass fat and protein) in northern pintails (Anas acuta) at a spring staging area, Lake St. Pierre in Quebec, Canada (46 degrees 11 'N, 73 degrees 08 'W). Northern pintails were collected during spring 1997 (14 April-9 May). In this staging area, body mass and percent body water successfully estimated carcass protein and fat in male northern pintails, but only carcass protein in females. None of the seven blood parameters we used accurately estimated nutritional reserves in staging northern pintails. These findings suggest that investigators must use direct estimates of carcass reserves to examine nutrient reserve requirements for egg production, migration, or body maintenance during spring migration.