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Alimentary, my dear hoatzin (was Microraptor hanqingi, new species from China.)

Don Ohmes <d_ohmes@yahoo.com> wrote:

>> Again, I actually like the idea - since you're the one who pointed it out
>> here, I was hoping you had some suggestions for what we'd predict if
>> tree-roosting was indeed important to paravians.
> The idea was never advanced by me as a testable lifestyle for any particular
> fossil specimen -- that would be a "misunderstanding" -- but it falsifies
> the idea that an animal evolving flight in tree-down gliding-first mode will
> by evolutionary logic show "arboreal adaptations" in its skeleton -- beyond
> the basic capacity to climb a tree...
> I suppose predictions made by tree-roosting might include 1) limited
> upstroke early on (that is, even after the appearance of a sophisticated
> gliding wing), 2) that the perching foot would appear subsequent to powered
> flight (full upstroke), not before -- and 3) the capability of climbing
> vertically.
> One note -- controlled climbing up is easier than climbing down -- and in
> the hoatzin type wing-claw model, climbing down would appear to be
> problematic, which makes parachuting/gliding useful.

I agree with Don here: a theropod that could climb up trees (for
whatever reason) would not necessarily need to climb down, because the
feathered wings (and tail?) allowed it to return to the ground by
parachuting or gliding.  I also agree that a strict "ground-up"
scenario might predict a full wingstroke early on, even when the wings
are rudimentary.  Biomechanical studies appear to back this up: such
studies indicate that neither _Archaeopteryx_ or _Confuciusornis_ were
capable of a full wingstroke.

I'm not sure that the hoatzin is a suitable analog, though.  Because
the juvenile hoatzin uses its wing-claws to clamber through trees, it
has become the poster child for how early birds (like _Archaeopteryx_)
might have moved around in trees.  The behavior of juvenile hoatzins
is a consequence of the peculiar physiology of this species,
particularly the foregut digestion/fermentation of plant matter akin
to ruminants (unique among birds).  Aside from the fact that they are
both avialans, there is almost nothing in common between
_Archaeopteryx_ and the hoatzin.

When juvenile hoatzins move through trees, it's a pretty haphazard
process: bill, neck, wings and feet are all used to negotiate their
way among the branches.  The use of the small, hook-like wing-claws in
climbing is not unique among modern birds.  What makes the hoatzin so
unusual is that the hooks are kept by the juveniles for so long.
There appears to be two reasons for this: the hoatzin's food source is
so nutrient-poor that growth is slow; and the voluminous foregut means
that the flight abilities of the hoatzin are weak and develop fairly
late (meaning that young hoatzins have to use climbing rather than
flying to get around for a long time).  Nevertheless, the hoatzin's
feet do most of the work during climbing: the toes are large and have
superb anisodactyl grasping abilities, befitting a species that is
essentially obligately arboreal.  Hoatzin chicks do jump into the
water when threatened; but they are certainly not aquatic.

So in no way can the hoatzin be compared to _Archaeopteryx_.
Hoatzins, as juveniles and adults, are good perchers (though poor
climbers), and spend their entire lives in trees  - except when the
juveniles are forced to drop into water below.  Hoatzins studiously
avoid the ground, and rarely (if ever) use their hindlimbs for