Plasma metabolites, including triglycerides, β‐hydroxybutyrate, and glycerol, can be used to estimate mass change in birds. Although dietary fatty acids can be ingested and absorbed as phospholipids, they have been largely overlooked as a potential indicator of mass change. The plasma ratio of triglyceride to phospholipid could also provide insight into diet quality because a high ratio in food items indicates high relative energy content. Variability in dietary phospholipid content and triglyceride:phospholipid may also affect the relationships between metabolites and mass change. We fed Gambel’s white‐crowned sparrows (Zonotrichia leucophrys gambelii) diets differing in phospholipid content and triglyceride:phospholipid and measured metabolites during mass loss and mass gain. Plasma phospholipids were higher and triglyceride:phospholipid was lower in birds fed a diet higher in phospholipid content and lower in triglyceride:phospholipid. Contrary to our expectations, plasma phospholipids were negatively related to mass change. Plasma triglyceride levels were positively related to mass change and unaffected by diet. The relationships between mass change and both plasma β‐hydroxybutyrate and glycerol were affected by diet. Plasma triglyceride appears to be the most reliable metabolite predicting body mass changes, but inclusion of plasma phospholipids and triglyceride:phospholipid into metabolite profiles may provide additional information on diet quality.