What is "frontal shield"

A frontal shield, also known as a facial shield or frontal plate, is a feature of the anatomy of several bird species. It consists of a hard or fleshy plate of specialised skin extending from the base of the upper bill over the forehead. The size, shape and colour may exhibit testosterone-dependent variation in either sex during the year. Functionality appears to relate to protection of the face while feeding in, or moving through, dense vegetation, as well as to courtship display and territorial defence. It is a characteristic of some water birds in the rail family, especially the gallinules and moorhens, swamphens and coots, as well as in the Jacana family. The watercock's frontal shield is extended above the head into a horn-like protuberance. A bird from a different order, the extinct Choiseul pigeon, had a blue frontal shield.