The term BITTERN RELATIVE is related to three crossword solutions: EGRET (5 letters), HERON (5 letters), and MARSH (5 letters). All three of these terms refer to a type of bird that is closely related to the bittern. Egrets are a type of HERON, which are long-legged wading birds with long necks and pointed bills. MARSH birds are typically found in wetlands and shallow water, and herons are one of the most common types of MARSH birds.