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In some birds, especially in the diurnal Birds-of-Prey
and in the Parrots, the greater portion of the distal end of the upper
beak is hard, while the basal portion is thick and soft-the so-called cere.
It is generally very sensitive, and encloses the nostrils. Though mostly
bare, it is in some Parrots thickly covered with feathers, and then approaches
in structure the ordinary skin.
The neighbourhood of the nostrils is often soft,
and produces an operculum by which, in some cases, the external nares can
apparently be closed, although no muscles seem to exist there. Such a soft
and swollen operculum is a prominent feature in Pigeons, and is very large
and curled in Rhinochetus (KAGU). In the Petrels each operculum forms
a more or less complete tube, which may or may not fuse with its counterpart
in the middle line, and thus produce an apparently single tube with a longitudinal
vertical septum, whence the name" Tubinares." A leathery operculum or valve
also occurs in Plovers, in Podargus, many Passeres (especially shown in
Meliphagidae), and in the Humming-birds, in the last being covered with
feathers. In Caprimulgus each nostril is produced into a short, narrow,
and quite soft tube.
Another differentiating feature in connection with
the nostrils and the rostrum is the presence or absence of a complete vertical
internasal septum. If the septum is complete, which seems to be
the primary condition, the right and left nasal cavities are completely
separated from each other, and birds having this structure are said to
possess nares imperviae.
The septum either remains cartilaginous,
or it ossifies to a variable extent. Consequently in macerated skeletons,
where only the bony parts remain, this character cannot be determined.
In comparatively few birds is the ossification complete, but this occurs
in the Owls, in Podargus, in some Accipitres, Parrots, and others. When
the septum is incomplete, the right and left nostrils communicate with
each other, forming nares perviae, as in Phaethon, among the Steganopodes,
in the Herons, Grebes, Divers, Grallae (except Rhinochetus), Gaviae, Limicolae,
Storks, Flamingos, Anseres, Cathartidae (but not in the Vulturidae and
Falconidae), and in many Passeres, especially in the Meliphagidae. In some Pelecaniformes (or
Steganopodes as they are also known), for instance in the cormorants, the nostrils are reduced
to narrow slits, and this condition is carried to an extreme in the Gannets,
the external nostrils being absolutely closed, and the greater portion
of the nasal cavity obliterated or filled with cancellated bony tissue;
however, the olfactory apparatus is well developed, the inner nostrils
or choanae being very wide, and in open communication with the mouth, enabling
the Gannet to smell its food when in the mouth.
Various parts of the rostrum have received special
names: culmen,
the dorsal ridge of the upper bill; apex or tip;
dertrum, in which it often terminates; gonys, or more correctly
genys, the prominent ridge formed by the united halves of the under
jaw, (eg, in Gulls; tornia, the cutting edges of the
bill).
The form of the bill exhibits almost infinite variations
in size, shape and structure, of which only the most striking modifications
can be detailled here. Generally shape and size stand in obvious correlation
with the mode of feeding, but gender selection seems to play a great part,
and leads to formations which it is often impossible to understand.
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