Co-occurring species with similar resource requirements often partition ecological niches. In the Northwest Atlantic (NWA), endangered roseate terns (Sterna dougallii) nest almost exclusively in coastal island colonies alongside common terns (S. hirundo). Roseate terns are prey specialists compared to common terns, which are opportunistic generalists; however, the two species forage on similar resources during the breeding season. The degree to which these species overlap in their adult foraging ecologies is not well understood. We compared the isotopic niches of nesting adult roseate and common terns by analyzing stable carbon (δ13C) and nitrogen (δ15N) isotopes in eggshell membrane tissues collected in 2018 and 2019 from ten colonies that span their NWA breeding range. Our aim was to characterize multi-scale patterns in isotope values, isotopic niche breadth, and isotope niche overlap between the tern species. We additionally examine subregional differences between “cold-water” colonies in the Gulf of Maine and “warm-water” colonies in Southern New England and Long Island Sound, and interannual differences between 2018 and 2019. Our results indicate similarity in isotope values and a high degree of isotopic niche overlap among adult roseate and common terns at the range-wide scale, but variable overlap at the individual colony scale. This suggests these species generally forage in isotopically similar habitats, but local resource availability influences partitioning within colonies. The isotopic niches of roseate terns were generally narrower than those of common terns, consistent with their respective specialist/generalist tendencies. Observed subregional and interannual differences were inconclusive and may reflect isotopic baseline shifts.