What does "sedge" mean?

Sedge: (Zoöl.) A flock of herons. Sedge ken (Zoöl.), the clapper rail. See under 5th Rail. -- Sedge warbler (Zoöl.), a small European singing bird (Acrocephalus phragmitis). It often builds its nest among reeds; -- called also sedge bird, sedge wren, night warbler, and Scotch nightingale.