Strangely after 4 hours, (and now 2 days) this appears not to have gone
through. I'll send it again. Apologies if it now comes to you twice. I'm not
sure if Crispin = Peter.
Crispin,
In short this is called cranial kinesis. If you were here and I had some birds'
skulls I could demonstrate it. Most (maybe even all?) birds can do it to
varying extents. It depends on how flexible the upper mandible is and where the
bendable part is. However in most birds the point of flexion is at the base of
the upper mandible, so that the whole of the upper mandible can be lifted and
lowered, independent of opening the lower mandible. This is really obvious in
parrots. In waders (I don't know if it is unique to waders, it may be but I
wonder if it may also be in hummingbirds and other long-beaked birds) the point
of flexion is midway or otherwise distally along the upper mandible. So this is
a modification that the waders have of the basic condition (hinge at the base).
In all cases the movement is achieved by the quadrate bone tipping forward (and
back) and the palatine bone sliding back and forward along the jugal bone. To
varying extents this movement can occur separate from opening the beak from the
main hinge at the quadrate bone. I presume the difference in waders would be
that the quatratojugal is especially long and the forward connecting point is
well forward onto the premaxilla compared to other birds. I have not looked at
a long beaked wader skull to verify this. We as mammals, are unusual creatures
and are unable to do this, as our quadrate bone has become an inner ear bone
and our upper mandible is solidly part of the skull. We have only a single
dentary-squamosal jaw hinge.
Philip
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