Variation in isotopic niche partitioning between adult roseate and common terns in the Northwest Atlantic

Co-occurring species with similar resource requirements often partition ecological niches at different spatial and temporal scales. In the Northwest Atlantic (NWA), federally 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 2 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 10 colonies that span their NWA breeding range. Our aim was to characterize interspecific patterns in δ¹³C and δ¹⁵N values, isotopic niche breadth, and isotope niche overlap. We additionally examined interannual and subregional differences between ‘cold-water’ colonies in the Gulf of Maine and ‘warm-water’ colonies in Southern New England and Long Island Sound. At the range-wide scale, there was a high degree of overlap in the overall isotopic niches of the 2 species; however, more variable patterns were observed at the colony scale, ranging from nearly complete overlap to complete separation. The isotopic niches of roseate terns were generally narrower than those of common terns, consistent with their respective specialist/generalist tendencies. While the influence of isotopic baselines limits our interpretation of interannual and subregional differences, isotopic niche breadths and overlap suggest consistency of relative foraging ecologies across these scales.