Bird's Mouth is a term used to describe the BEAK of a bird. The BEAK is the part of the bird's anatomy that is used for eating, drinking, and preening. It is made up of two parts, the upper mandible and the lower mandible, which are connected at the tip. The term Bird's Mouth is derived from the shape of the BEAK, which resembles an open mouth. The crossword solution for the term Bird's Mouth is BEAK, which is a four-letter word.