The term RELATIVE BITTERN is a combination of two crossword solutions, EGRET and BITTERN. An EGRET is a type of heron, a long-legged wading bird with a long neck and bill. Egrets are found in many parts of the world and come in a variety of sizes and colors. Bitterns are a type of marsh bird that are related to herons. They have a distinctive booming call and are usually found in wetlands. The term RELATIVE BITTERN is used to describe a group of birds that includes both egrets and bitterns.