water rail, (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.