Re: large fossil birds

[jrc <jrccea@bellsouth.net>]

----- Original Message ----- 
From: "Michael Habib" <mhabib5@jhmi.edu>
To: <dinosaur@usc.edu>; "jrc" <jrccea@bellsouth.net>
Sent: Tuesday, November 29, 2005 1:11 PM
Subject: Re: large fossil birds


> My main point is that maximum span versus mass relationships may not have 
> been the defining difference in max size between birds and pterosaurs.

My perception is that the relationship between span, mass, and required 
power probably isn't as closely coupled in pterosaurs as it is in birds, due 
to a difference in launch and landing technique (mostly launch).

> The very largest flying birds by mass are swans and kori bustards.  The 
> former have relatively high aspect ratio wings, while bustards have very 
> low aspect ratios (because they only fly for short distances at low 
> speeds).

The largest individual bird that I know of that flies by means of continuous 
flapping flight is a male Whooper Swan that has been designated 'JAP' by the 
folks who study him (I don't know if he is still alive).  I'm probably too 
close to pterosaurs, because I think of him as being 'relatively' low aspect 
ratio.  Though he is quite high aspect ratio for a swan, it's only about 
half the aspect ratio of a pterosaur using either the Bennett or Padian 
planform.  It might be similar to the aspect ratio of the Unwin planform 
(don't remember off the top of my head).  As an aside, some years ago, 
during a gale off the coast of Iceland, JAP made one of the most remarkable 
emergency flights I've ever heard of.  He is or was a pilot's pilot.

> If anything, the situation is 'messy' enough that I probably shouldn't 
> have tried to make the comparison between chords in large avians and large 
> pterosaurs in the first place.  I guess I had seabirds on the brain at the 
> time.  Oh well, my mistake.

Not necessarily.  If you buy into the Unwin planform, what you said earlier 
might well be true.
> > > resulting in a more favorable wing loading:mass scaling relationship.

> I suppose I was mostly concerned with the thresholds of being able to take 
> off when I made that comment.

My impression for pterosaurs is that takeoff didn't involve a bunch of mad 
flapping or running.  My perception is that they sidestepped the power 
required versus power available issue that limits birds, and did so by using 
a launch technique that reduced the power requirements on the pectoral 
muscles.

With regard to landing, being quadrupedal on the ground was a huge asset in 
avoiding the pitfalls of the fanny over teakettle technique used by 
albatrosses.

>  With regards to the trend, I probably underestimated the loadings in 
> pterosaurs.  I really am much more familiar with the wing shape trends in 
> birds, after all.

I've recently done a volumetric mass estimate on A piscator.  For a given 
span, pterosaur mass doesn't appear to be much different than that for 
equivilently spanned birds.  Pterosaurs tend to have smaller bodies, but 
offset that with larger heads and necks.  And though pterosaur bones have 
thinner bone walls for a given diameter than birds do -- for a given span 
the gross size of the pterosaur bones tends to be much larger, with more 
area available for muscle attachment.  The preceding is a visual analogy 
that can be pushed too far, but since pterosaurs were soarers, and since 
high wingloading is an advantage for soarers that want the ability to cover 
distance as well as loiter, I don't think they were under much selective 
pressure to reduce mass.  Only to support it effectively, both on the ground 
and in the air.

> I have not done any calculations on that problem myself [insert -- # of 
> continuous beats], but it certainly sounds reasonable.  Incidentally, any 
> thoughts on the apparent adaptations for dynamic soaring in Qspp given 
> they show up in terrestrial deposits?

Sure.  Both Qsp and Qn appear to have been inland animals.  I used to do a 
lot of very low level flying along the margins of long, narrow lakes while 
doing search and rescue work.  When there was a wind, I noticed quite a bit 
of turbulence near the margins of those lakes that would have been quite 
usable for a pterosaur.  You get an updraft on the leeward side of the lake, 
and a vortex on the windward side.  Energy can be extracted from both.  I 
see flying on the windward side of a lake as being similar to the lee shear 
soaring that albatrosses use so effectively (making use of shoreline 
effects).  Flying on the leeward side of the lake (the windward side of the 
adjacent shoreline) is similar to slope soaring.

> There are vultures somewhat adapted for burst flapping, but they still use 
> convective soaring most of the time.

I know you're aware of this, but soaring animals of any aspect ratio can use 
convective soaring when it is available.

> I'm just forseeing problems with vertical wind shear gradients being too 
> low

Given a wind, they aren't too low along the margins of lakes.  And again, 
high aspect ratio animals can effectively use convective lift too.  In fact, 
they find it easier to travel inland than do lower aspect animals, with 
their lower lift/drag ratios.  As an oversimplification, about 20% of the 
sky is covered by updrafts, 80% with downdrafts.  You want to be able to 
move through the downdrafts between updraft areas as efficiently as 
possible, and high aspect ratio and high wing loading both facilitate that 
(particularly with the facility for occasional burst flapping).

> (unless they were actually coastal and just happened to die overland).

I'm personally convinced that they (Quetz) weren't coastal; that they made 
their living inland and did a lot of traveling in a generally north-south 
direction.

> I've only just begun learning the mechanical implications of 
> spanloading).

Paul MacCready has used spanloading to enormous benefit in several of his 
aircraft.  you can get a quick insight into spanloading by looking at some 
of them.

Jim