Food-supplementation studies on birds during egg laying have generally manipulated macronutrients and energy. Such studies have shown variable effects on egg size in American Coots (Fulica americana). We determined the relative influences of local environment, food quantity, and food quality on egg size in American Coots by supplementing food and carotenoids at three sites in Saskatchewan. Eggs were collected and analyzed for carotenoid content and stable isotopes (δ¹⁵N and δ¹³C) in the yolk to determine whether variation in the type of food eaten contributes to egg size. We also report on the isotopic analysis of American Coot tissues and their eggs to assess evidence of endogenous versus exogenous protein and lipid allocations to reproduction. We provide isotopic evidence that American Coots used endogenous lipid, but not protein, reserves for egg formation and that egg size is more dependent on exogenous sources of nutrients. Laying sequence was the only variable across nesting locales that had a significant influence on the size of eggs. There was an effect of provisioning food at only one site; however, this response was driven by a component of diet quality other than carotenoids. Yolk δ¹³C values were positively correlated with egg volume, which further suggests the importance of diet quality in determining egg size in American Coots.