The Collaborative International Dictionary of English v.0.48:

Bittern \Bit"tern\, n. [OE. bitoure, betore, bitter, fr. F.
   butor; of unknown origin.] (Zool.)
   A wading bird of the genus Botaurus, allied to the herons,
   of various species.
   [1913 Webster]

   Note: The common European bittern is Botaurus stellaris. It
         makes, during the brooding season, a noise called by
         Dryden bumping, and by Goldsmith booming. The American
         bittern is Botaurus lentiginosus, and is also called
         stake-driver and meadow hen. See Stake-driver.
         [1913 Webster]

   Note: The name is applied to other related birds, as the
         least bittern (Ardetta exilis), and the sun
         bittern.
         [1913 Webster]

WordNet (r) 3.0 (2006):

least bittern
    n 1: small American bittern [syn: least bittern, Ixobrychus
         exilis]