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Understanding the trophic interactions and coexistence processes among waterbirds in floodplain ecosystems is of fundamental ecological importance. In the upper Paraná River floodplain, we assessed the diet of five sympatric waterbird species (Ardea alba, Ardea cocoi, Egretta thula, Nannopterum brasilianum, and Nycticorax nycticorax) and tested their differences in the diet, in addition to food overlap, differences in trophic niche breadths and stratum in the water column where they forage. The waterbirds’ stomachs were collected quarterly on the upper Paraná River floodplain, analyzed under a stereomicroscope, and the food items were identified. Only Egretta thula was classified as omnivorous, consuming various food resources, including insects, decapods, and fish. The other species were piscivorous, but differed in the types of prey. Food overlap was higher in the piscivorous species, but they exhibited high trophic niche breadth. The piscivorous waterbirds employ different strategies for exploiting food resources that allow species coexistence in the floodplain, avoiding direct competition. Our results highlight the importance of waterbirds as connectors across diverse environments, especially aquatic-terrestrial ecosystems. Since these waterbirds’ diet relies on fish, effective conservation and management strategies targeting fish communities are fundamental to maintaining biodiversity and functionality within the upper Paraná River floodplain.