The reproductive success of ducks nesting in association with gulls on islands in Miquelon Lake, Alberta, was studied over a period of 3 years. Gulls protected nesting ducks by preventing other egg-eating birds from foraging on the islands. Pilfering of duck eggs by gulls was correlated with the point in incubation when ducks were trapped and handled. Without trapping, we estimated hatching success would exceed 90%. However, duckling mortality, concentrated in the 1st week posthatching, was positively correlated with the numbers of associated gulls. The survival of ducklings declined to zero when the number of nesting gulls approached 500 pairs. The origin and maintenance of this ecologically unstable relationship is discussed.