(noun) A very prominent bump, swelling or projection at the base of a bird's bill, typically positioned on the upper mandible and having a bulbous, horn or plate-like shape that projects away from the bird's forehead rather than staying flat along the head like a frontal shield, and is much more obvious than simply a wide bill base or sloped bill shape. While the exact purpose of basal knobs is unknown, they may serve as an indicator of health or sexual maturity, particularly during the mating season when some knobs become larger or may take on different colors with breeding plumage.
These knobs can also be a good field mark for proper species identification, and in some cases, may be considered diagnostic markings. The overall size, shape and prominence of the basal knob often varies by gender, with males typically having larger and more obvious knobs.
Basal knobs vary considerably from other bill structures, such as wattles, which are often fleshy or may have multiple smaller knobs or bumps or may dangle from the bill or other parts of the face. Another similar structure is the cere, a patch of skin at the base of the bill covering the nares, but which rarely has the engorged shape of a basal knob and does not project away from the bill.
Photo – Domestic Chinese Goose Profile © Greg Goebel