(Rallus aquaticus), slender marsh bird of the family Rallidae (order Gruiformes), native to most of Europe and Asia. Its length is about 28 cm (11 inches), and it has a moderately long beak. The sides of the bird have black and white bands. The name water rail also is used as a general term for the larger group, or tribe, to which R. aquaticus belongs. Rallus aquaticus can be distinguished from the tribe Rallini by its relatively long beak. See also rail.