Niche partitioning in diet and foraging habitat between two sympatric marsh terns

Understanding how ecologically similar species coexist in shared environments is a central question in community ecology, especially where closely related species breed sympatrically and exploit overlapping resources. Marsh terns of the genus Chlidonias provide an excellent model for examining fine-scale niche differentiation, yet comparative data on diet and foraging habitat use in sympatric populations remain scarce. We examined whether White-winged Tern Chlidonias leucopterus and Black Tern Chlidonias niger breeding sympatrically in the near-natural Bug River valley (eastern Poland) differ in chick provisioning diet and adult foraging habitat use. Chick provisioning was quantified from video recordings at nests, and adult foraging behaviour was assessed using transect scans near mixed colonies. Overall chick provisioning diet did not differ significantly between species, indicating broad dietary overlap at the multivariate level. However, taxon-specific analyses suggested directional tendencies, with White-winged Terns tending to deliver more terrestrial prey, particularly Orthoptera, and Black Terns tending to provide more fish. Brood size also influenced diet composition, suggesting that provisioning patterns varied with brood demand. In contrast, adult foraging habitat use differed strongly between species: White-winged Terns foraged mainly over meadows, whereas Black Terns used river channels more frequently. Thus, niche differentiation was most evident in foraging habitat use, whereas trophic differentiation in chick provisioning was weaker and taxon-specific. Such partial, multidimensional niche partitioning may reduce interspecific competition and facilitate coexistence in mixed colonies. Our results highlight the importance of behavioural differentiation in sympatric consumers in heterogeneous wetland landscapes.