A group of bitterns is commonly called a Pretence. Other collective nouns you may see include Sedge and Siege.
A collective noun for bitterns is a "sedge" of bitterns. Bitterns are a family of wading birds known for their elusive behavior and skillful camouflage. Their collective noun, sedge, hints at their preferred habitat: the thick, marshy sedges and reeds where they reside. This term captures their tendency to congregate and nest in these environments, often hidden from view. Sedges of bitterns can be spotted with patience near wetlands and dense vegetation, tantalizing observers with their distinctive calls and occasional brief glimpses. These collective nouns not only highlight their habitat preference but also highlight their communal nature, presenting an image of a group of bitterns hidden among the rushes, unified in their surroundings and demeanor.
Known collective nouns for Bitterns include: