A comprehensive study of factors controlling the erodibility of fine-grained intertidal sediments found that sediment strength increased with the arrival of large numbers of migratory shorebirds. Before the birds came, sediment cohesion resulted in part from secretion of polysaccharides by benthic diatoms whose production was controlled mainly by a grazing amphipod, Corophium volutator. When the birds arrived, Corophium behavior and abundance changed, bioturbation and grazing pressure on the diatoms decreased, and the production of cohesion-inducing carbohydrates rose. The results emphasise the importance of biological processes in affecting sediment stability, and the limitations of laboratory-based measurements of sediment properties used in models of cohesive sediment behavior.