Beginning in the 1960s, Ring-billed Gulls' (Larus delawarensis) historic breeding range expanded from inland habitats and freshwater wetlands to marine sites in Atlantic North America. Adults winter on salt water but salt tolerance may entail costs (e.g. maintenance of salt glands) and favor reduced parental use of marine resources for feeding chicks. Diets of Ring-billed Gull chicks nesting on a marine island off Prince Edward Island, Canada, were investigated using soft regurgitant collection and stable isotope (δ¹³C and δ¹⁵N) analysis of both regurgitants and blood samples. Isotopic signatures in blood of chicks of Ring-billed and sympatrically-nesting Herring (L. argentatus) and Great Black-backed Gulls (L. marinus) were also compared. Regurgitants of Ring-billed Gull chicks contained freshwater/ terrestrial and marine invertebrates and fish. A stable isotope mixing model incorporating both δ¹³C and δ¹⁵N in chick regurgitants and blood estimated that 42.2% of Ring-billed Gull chick diet came from the marine environment, whereas estimates from blood for Herring and Great Black-backed Gulls were 43.6%, and 73.5%, respectively. Further, differences existed among gull species in estimates for dietary contribution from various food sources from marine and terrestrial/freshwater environments. Although Ring-billed Gulls can use marine food sources to feed their young, whether salt in marine food imposes a physiological cost on these young in comparison to young reared exclusively on non-marine foods is unclear.