We studied the basic biology of the Red Fox (Vulpes vulpes) and its interactions with the waterbird colonies in the Ebro Delta natural park, Spain. Dens (earths) were found in areas characterized by sandy dune vegetation, a few buildings, areas where human entry was prohibited, and areas where hunting of waterfowl was banned. Regression associated the presence of dens to the abundance of dunes, halophytic vegetation and prohibited entry. Birds constituted 96% of the prey found at dens, mainly waterfowl (68%), gulls (16%) and rails (10%), but this differed markedly between areas. During the breeding season, foxes obtained their food from a reduced number of sources in the area surrounding their dens, and impacted the nearest breeding birds. We propose changes in the management of foxes in the natural park to safeguard internationally threatened bird species and to prevent them from decreasing, or even disappearing, as a result of fox predation.