BITTERN (Botarus Stellaris):A rather large brown heron, blotched, mottled and streaked with black,gape blue-green.
Very skulking, so less often seen than heard uttering its permeating low booming or “mooing” song,
like a distant cow or foghorn.
Stands hunched up like a large domestic fowl, but looks very different in
alert
or alarm
positions and striking crouched threat posture with wings outstretched. Flight call a
harsh “kwow” or “kwah.”
Extensive reed-beds, fens and swamps, not colonial, only feeding in the open in hard weather. (Collins Pocket Birds
of Britain and Europe.)
"And the loud Bittern from the bull-rush home Gave from the Salt-ditch
side the bellowing boom"