The ability of most birds to protract the upper mandible, and the
structure of the skull which enables them to do so are responsible for
common reference to the skull as "kinetic" (Beecher, 1951a:412; Fisher,
1955:175). The movement is effected by muscular action on a series of
movable bones that exert their forward force on the upper mandible,
which in turn swings on a horizontal hinge, the "naso-frontal hinge,"
at the base of the beak. The bone initiating the movement is the
quadrate, which is hinged posteriorly by its otic process and which
ordinarily swings up or down depending on the muscle or muscles being
contracted at any given moment. The upward swing of the quadrate pushes
the jugal bar, which is attached to its lateral tip, along its
longitudinal axis, in an anterodorsal direction, and the force is
transferred to the upper mandible, which is thereby elevated. A
synergetic mechanism is simultaneously initiated by the same bone--the
quadrate. Since the quadrate body articulates with the pterygoid, the
upward movement forces the pterygoid to slide along a ridge in the
ventral midline of the cranium, the sphenoidal rostrum, thus pushing
the palatine forward and exerting an upward push on the upper mandible.
In the columbids the quadrate has a bifurcated otic process that
functions as the hinge. The posterior tips of the forks are situated
almost vertically (one above the other) and the movement of the
quadrate is not so much up and down, or vertical, as it is horizontal
(fig. 12). When the quadrate moves medially the upper mandible is
protracted; a lateral movement results in retraction. There is a
slight, almost negligible, upward movement of the quadrate. The
movements of the various bony elements were observed on a skull that
had been made flexible by boiling in water for a minute as suggested by
Beecher (1951a:412).
Also in the columbids the naso-frontal hinge is not constructed in the
same manner as it is in many other birds as there is not a simple hinge
across the entire base of the beak. In fact, there is no true hinge at
all in the area of the nasals, but those bones are extremely thin and
they bend or flex under pressure. Actually, the hinge is double or
divided. One part is on either side of the nasals. The hinges are
situated at the posterodorsal tips of two thin processes of the
maxillary bones and the appearance is not unlike that of half a span of
a suspension bridge having the hinges at the tops of the towers.
Several other species of birds share this type of hinge construction
with columbids.
The movement of the lower jaw is, of course, the primary operation
involved in opening the mouth. The lower jaw possesses a deep fossa at
its posterior end, or on its posterodorsal surface, which articulates
with the body of the quadrate bone. The length of that part of the
mandible extending behind the articulation is directly correlated with
the resistance offered the mandible in opening, since it is on the
posterior extension that the depressor of the lower mandible inserts.
The larger the muscle the more surface is needed for attachment. Also
the added length of the mandible posterior to the articulation serves
as a lever in opening the mandible, and the fulcrum is moved relatively
farther forward.
In birds lacking resistance to abduction of the lower mandible, as in
doves, it is nevertheless necessary for a slight postarticular process
to remain for the insertion of a small depressor muscle which, as
mentioned previously, is necessary to counteract the relaxed adductor
muscles of the lower jaw.
There are many exceptions to the rule that birds have kinetic skulls,
and usually a secondary fusion and reinforcement of bones around the
hinge has limited or eliminated all movement. Sims (1955) describes the
Hawfinch's immobile upper jaw, which is used as a powerful press in
cracking the stones of fresh fruit. Skulls of woodpeckers have been
modified somewhat in the same manner as a result of their foraging and
nesting habits (Burt, 1930).
The two most distantly related members of the genera under
investigation are the White-winged Dove, _Zenaida asiatica_, and the
Mourning Dove, _Zenaidura macroura_. They were chosen to demonstrate
differences and likenesses in proportions of members of the genera.
Ten measurements were taken on each skull, but simple observation
reveals that, in relation to total length of the skull, the beak of the
White-winged Dove is longer than that of the Mourning Dove. Tip of
upper mandible to base of beak averaged 48.6 and 42.9 per cent of the
total length of the skull in the White-winged Dove and Mourning Dove,
respectively. The position of the jugal bar has remained about the same
with respect to the cranial part of the skull, and the entire cranial
part of the skull is almost the same shape in the species studied.
Likewise, in the White-winged Dove the distance from the anterior tip
of the lower mandible to the anterior part of _M. adductor mandibulae
externus_ is relatively longer in relation to the length of the lower
mandible than in the Mourning Dove. Finally, the position of the jugal
with respect to the naso-frontal hinge is about the same in the two
species.
Measurements and calculations indicate that the longer beak of the
White-winged Dove as compared with the Mourning Dove is a function of
the beak itself, not of differences in other parts of the skull.
Measurements of skulls of Eared and Zenaida doves support this view.