(noun) The lower half of a bird's bill. Both the upper and lower halves of the bill may be commonly referred to as mandibles, but with "upper" or "lower" used to distinguish between them, or else the upper mandible is called the maxilla. The lower mandible, when referred to by itself, is just called the mandible.
While many birds have identical colors and markings on both halves of the bill, some species do have distinct mandibles. Look for markings such as spots, colored tips or subterminal bands to help identify a bird based on its mandible, such as the different colored mandibles of some toucans or the spots on the mandibles of some gulls. Mandible shape can also be important for identification, such as the upward curved mandibles of crossbills. Similarly, mandible size can be a good identification clue, such as the long mandible of black skimmers.
Photo – Fiery-billed Araçari © Frank Vassen