Why are avicularia sometimes compared with tiny bird heads?

Avicularia are specialized zooids found in certain bryozoan colonies that are often compared to tiny bird heads due to their physical structure and movement.12

In these specialized zooids, the operculum—a lid-like structure typically used to close the orifice—has evolved to become proportionally larger and is controlled by strong adductor muscles. This modification effectively turns the operculum into a functional jaw. In genera such as Bugula, these structures are mounted on short stalks and possess a movable mandible, creating a striking resemblance to the head of a bird. The name itself is derived from the Latin avicula, meaning "small bird".213

While their exact purpose remains a subject of study, these structures are believed to help keep the colony clean by removing debris and preventing other organisms from settling on the colony's surface.2