The North American lesser scaup (Aythya affinis) population has declined since the mid-1980s. The acquisition of nutrient reserves during spring migration may play a role in explaining that trend. We studied nutrient-reserve dynamics of lesser scaup collected during spring at 3 major stopover sites and assessed whether reserves of birds on the lower Great Lakes (LGL) differed from those at other staging (or breeding) areas. At lakes Ontario and St. Clair, males had larger fat reserves than females, but no other substantial sex-related differences were observed in fat, protein, or mineral reserve levels of lesser scaup. Protein in males and mineral reserves of both sexes at lakes Erie and St. Clair did not change throughout spring. Male fat reserves remained constant at Lake Erie and increased at Lake St. Clair, whereas female fat and protein reserves increased at both stopover sites. Patterns of fat and protein dynamics in males partly may be due to energetic costs of courtship and pair-bond maintenance. However, maintenance and accumulation of fat reserves in both sexes while on the LGL is important for impending migration and subsequent reproduction. Female lesser scaup staging on the LGL had fat reserves comparable to, or slightly higher than, those reported in other studies of northern spring-staging and breeding birds. Fat reserves of LGL females, however, were much lower than those reported for birds at another midlatitude stopover site. These comparisons suggest that events occurring on Atlantic Flyway wintering areas or LGL staging areas are potential factors contributing to nutrient-reserve limitation and possibly to observed declines in scaup numbers in North America.