ABSTRACT Niche construction can lead to coevolutionary episodes and greater specialization, and can have implications for the evolution of biological diversity, so understanding how individuals adjust their behavior to conspecifics and heterospecifics over changing social and environmental gradients presents an interesting research problem. The objective of our study was to test the hypothesis that mixed flocks of gulls and diving ducks congregate as a result of interacting over food. Ducks are vectors that carry zebra mussels (Dreissena polymorpha) up from the bottom of a water body, and gulls consume this type of food. We investigated the interactions of 2 gull species with 3 benthivorous duck species—Common Pochard (Aythya ferina), Tufted Duck (A. fuligula), and Greater Scaup (A. marila)—in the Szczecin Lagoon, Poland. We found that the presence of gulls in duck flocks was closely correlated with foraging by ducks. Eighty-four percent of foraging duck flocks (>5% of the flock members were foraging) and...