Operculum

The nares of some birds are covered by an operculum (plural opercula), a membraneous, horny or cartilaginous flap. In diving birds, the operculum keeps water out of the nasal cavity; when the birds dive, the impact force of the water closes the operculum.Some species which feed on flowers have opercula to help to keep pollen from clogging their nasal passages, while the opercula of the two species of Attagis seedsnipe help to keep dust out. 

Nails

All birds of the family Anatidae (ducks, geese, and swans) have a nail, a plate of hard horny tissue at the tip of the beak. This shield-shaped structure, which sometimes spans the entire width of the beak, is often bent at the tip to form a hook. It serves different purposes depending on the bird's primary food source.

Gonys

The gonys is the ventral ridge of the lower mandible, created by the junction of the bone's two rami, or lateral plates. The proximal end of that junction—where the two plates separate—is known as the gonydeal angle or gonydeal expansion. In some gull species, the plates expand slightly at that point, creating a noticeable bulge; the size and shape of the gonydeal angle can be useful in identifying between otherwise similar species. Adults of many species of large gulls have a reddish or orangish gonydeal spot near the gonydeal expansion.This spot triggers begging behavior in gull chicks. The chick pecks at the spot on its parent's bill, which in turn stimulates the parent to regurgitate food.

Culmen

The culmen is the dorsal ridge of the upper mandible. Likened by ornithologist Elliott Coues to the ridge line of a roof, it is the "highest middle lengthwise line of the bill" and runs from the point where the upper mandible emerges from the forehead's feathers to its tip.^[28] The bill's length along the culmen is one of the regular measurements made during bird banding (ringing),^[29] and is particularly useful in feeding studies. 

The shape or color of the culmen can also help with the identification of birds in the field. For example, the culmen of the Parrot Crossbill is strongly decurved, while that of the very similar Red Crossbill is more moderately curved.

Commissrue

Depending on its usage, commissure may refer to the junction of the upper and lower mandibles,or alternately, to the full-length apposition of the closed mandibles, from the corners of the mouth to the tip of the beak.

cere

Birds from a handful of families—including raptors, owls, skuas, parrots, turkeys and curassows—have a waxy structure called a cere which covers the base of their bill.

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