Understanding how ecologically similar species coexist in shared environments is a central question in community ecology, particularly in systems where closely related species breed sympatrically and exploit overlapping resources. In such systems, differences in behaviour, especially in foraging decisions, may play a key role in reducing interspecific competition. Marsh terns of the genus Chlidonias provide an excellent model for examining fine-scale niche differentiation, yet comparative data on diet and foraging behaviour in sympatric populations remain scarce. We examined whether two sympatrically breeding species, the White-winged Tern Chlidonias leucopterus and the Black Tern Chlidonias niger, differ in chick diet composition and behavioural patterns of foraging habitat use by adults in the near-natural Bug River valley (eastern Poland). Chick provisioning was quantified from video recordings at nests and adult foraging behaviour was assessed using transect scans near mixed colonies. Diet composition differed between species and varied with brood size. White-winged Terns delivered proportionally more terrestrial prey, particularly Orthoptera, whereas Black Terns provided more fish and showed stronger associations with aquatic prey. Foraging patterns exhibited a pronounced species by habitat interaction, with White-winged Terns foraging mainly over meadows and Black Terns over oxbows and the river channel. The close correspondence between foraging habitats and chick diet composition indicates that species-specific foraging decisions by adults directly shape offspring provisioning. These results demonstrate multidimensional niche partitioning across dietary and spatial axes, which likely reduces interspecific competition and facilitates stable coexistence in mixed colonies, highlighting behaviour as a key mechanism underlying coexistence between sympatric marsh terns, where the White-winged Tern occurs near the western margin of its range and the Black Tern occupies the range core.