Re: four winged Archaeopteryx

[Michael Habib <mhabib5@jhmi.edu>]

> What I should probably have said was something like "expecting a
> immobile tail surface to work for bird flight is a very strong claim"; 
> a
> 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).

Cheers,

--MH