Understanding daily lipid-reserve changes in migrating wild birds is important for habitat assessment and species conservation. Plasma-lipid metabolites have been used to estimate rates of lipid accumulation or catabolism in small-bodied wild birds (<75 g), but this has not been validated for larger-bodied wild birds such as waterfowl. We developed an index for detecting whether individual birds accumulate or catabolize lipid reserves by regressing plasma-lipid metabolite levels (triglyceride and β-hydroxybutyrate) and known one-day mass changes (daily mass change) of 22 free-living Lesser Scaup (Aythya affinis; a larger-bodied species with typical mass range 600–1,000 g). Triglyceride and β-hydroxybutyrate predicted 75% of the variation in daily mass change (F = 28.85, df = 2 and 19, P < 0.001). Triglyceride was positively correlated (P = 0.029) with mass change, and β-hydroxybutyrate was negatively correlated (P < 0.001) with mass change (daily mass change = −54.49 11.82 [triglyceride] − 28.65 [β-hydroxybutyrate_log]). Our results indicate that triglyceride and β-hydroxybutyrate can estimate one-day changes in mass of free-living wild Lesser Scaup, which provides an index to daily changes in lipid reserves and should be useful for assessing quality of migration habitat.