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Re: four winged Archaeopteryx
What I should probably have said was something like "expecting a
immobile tail surface to work for bird flight is a very strong claim";
fixed frond tail would have uniform drag and be a uniform contribution
to flight stability.
I understand what you are getting at, but that is only true if the tail
itself is kept in a uniform position and the angle of the body is also
kept uniform. In fact, because the airflow from the wings would
interact flows around the tail, the wings would technically have to be
static as well (though that is a more minor consideration). The tail
feathers could have limited (or absent) mobility at the feather base,
and yet the tail would still be adjustable because the tail can move at
its base. Certainly, adjusting angle of attack of the tail would be
relatively simple and well within known anatomical limits.
So you either have an Archie that can't fly with lost feathers/feather
damage to the tail, maybe can't turn without heroic effort, and is in
serious trouble with gusty winds (or anything else which applies an
assymetric airflow to the tail), or you've got an Archie with some
ability to adjust the tail surface to cope with conditions.
Modern birds have only limited motion at the feather base of most of
their remiges. The primaries have fine control, but the secondaries
and tertiaries are mostly mobile by virtue of attachment to a mobile
lever arm (ie. the ulna and humerus). Since Archaeopteryx could most
likely alter its tail position to some degree, it would easily account
for changes in airflow over its tail along roughly analogous lines.
Even if it could not move its tail (highly improbable), Archaeopteryx
would not be prevented from turning, or from flying with feather damage
to the tail. In both cases, the wings (being the primary lift
surfaces) would adjust accordingly. The total forces associated with
the wings would have exceeded those of the tail.
There are, in fact, examples of extant bird species that deal with a
problem similar to a rigid, feathered tail: both quetzals and peacocks
(or rather, the males of each) have long trains of feathers grown from
the back. Because the trains are not actually part of the tail, they
do not have the same mobility as retrices. Peacocks can lift their
train for display, but have little fine control of their display
feathers in flight. In both cases, while the display feathers
certainly have aerodynamic costs, the birds can compensate quite well.
In the case of Archaeopteryx, the tail would actually be
aerodynamically useful, so Archaeopteryx would have been in better
shape than a modern Quetzal, even if it had no tail mobility (which,
again, I doubt was the case).