Re: large fossil birds

[Michael Habib <mhabib5@jhmi.edu>]

> >  Thus, I wouldn't expect overland flyers to always be selected for 
> > high wing loading
>
> Nor would I.  It depends upon the specific niche they are filling and 
> their usual source for atmospheric energy extraction.  Birds are more 
> versatile in that regard than pterosaurs were.  Birds exhibit 
> variations on more than one theme.  Pterosaurs exhibit variations upon 
> a single theme.  You've probably noticed that birds with low to 
> intermediate aspect ratios find tipslots advantageous while higher 
> aspect ratios shift to triangular wingtips. Pelicans are right on the 
> morphological dividing line between the two forms, and as far as I 
> recall, are the highest aspect ratio birds to make use of tipslots.  
> For birds with aspect ratios less than pelicans, there can be an 
> advantage to reducing wingloading.  For birds with larger aspect 
> ratios than pelicans, the advantage would usually seem to be toward 
> increasing it...

I am not aware of any birds with AR's higher than pelicans that utilize 
tipslots either.  In fact, a fair number of birds with AR's 
significantly lower than pelicans have tapered wingtips without slots 
(falcons, for example, though they are not exactly typical birds).  
There are also differences in how the wings are held across birds with 
different AR's.  For example, those with high aspect ratios tend to 
hold the tips of the wings angled slightly downward and back 
(especially by marine birds during rapid soaring).  If I remember 
correctly, this serves to form more favorable vortex patterns at the 
distal portions of the wing and thus reduce drag near the tips, but my 
memory may not be serving me correctly on that one.

In fact, wing shape changes during different flight modes are often 
ignored (as far as I can tell) in discussions of flight.  For example, 
the 'classic' picture of vultures (which is the image generally 
illustrated) is one with the wings held out at maximum span and the 
tipslots fully open.  However, this is mostly the case during loitering 
and landing.  If you can catch vultures moving between updrafts you'll 
notice that they bend the wings and close their tipslots.  So, in that 
case, wing loading is behaviorally increased during travel between 
updrafts.  I'm not sure how the mechanics of wing shape changes during 
flight would work for pterosaurs with their unique wing membrane.  
Would they be limited in their ability to fly with shortened wingspans 
due to disruption of the wing surface by membrane folding?

> > I didn't realize lake effects were so strong; that's really quite 
> > cool.
>
> They are only strong given a wind plus a vertical discontinuity along 
> the shoreline (either bank slope or treeline, or both)... Note that 
> the only reason for maximising chord and minimising wingloading in 
> convective lift is to assure that the average minimum sink rate is 
> less than the average convective updraft.  Once that condition is met, 
> the advantage shifts to the highest aspect ratio and highest 
> wingloading that will meet that restriction, because the latter two 
> characteristics help in traversing the downdrafts between updrafts....

Again, a need anecdote from good ol' vultures: during some walks along 
a very large lake with a distinct treeline I noticed not only a very 
strong wind (even at ground level), but had the enjoyable experience of 
watching about 6 or 7 vultures riding the wind shears.  Not 
surprisingly, they were using the flexed, more highly loaded wing 
configuration I mentioned above (and were really moving hard...using a 
couple of known landmarks I got a rough estimation of 40-60 mph for a 
couple of them.  Considering how one usually catches turkey vultures 
moving, that's really cookin').

Cheers,

--Mike