Re: large fossil birds

[jrc <jrccea@bellsouth.net>]

----- Original Message ----- 
From: "Michael Habib" <mhabib5@jhmi.edu>
To: "jrc" <jrccea@bellsouth.net>; <dinosaur@usc.edu>
Sent: Tuesday, November 29, 2005 9:29 PM
Subject: Re: large fossil birds


> I have an average measure of 8.7 for the aspect ratio of whooper swans. As 
> birds go, this is reasonably high

Correct.  And as pterosaurs go, it is extraordinarily low.

> While whoopers don't have low AR's for birds overall, it is a bit on the 
> low side compared to other (smaller) anatids.  So it may turn out that 
> there is a negative relationship between aspect ratio and size to a degree 
> if one uses contrast analysis instead of raw species values (at least for 
> some groups).

Perhaps.  I tend to think aspect ratio is not strongly related to size in 
birds (except perhaps within species).  Teratorns and albatrosses show two 
different trends at work and their aspect ratios are more a function of the 
sources of the energy they extract from the atmosphere.
There is a positive relationship between aspect ratio and size in 
pterosaurs, at least within species.  Based on the allometric ratio of the 
wingbones between the two types of Quetz, aspect ratio increases with 
increasing span.  This is partly because the radius/ulna becomes relatively 
shorter in the larger animals, and because the radius/ulna is directed 
fairly far forward in pterosaurs (it is usually drawn in too spanwise an 
orientation with hyperextended elbow and wrist).

> Sounds very reasonable to me, for whatever that's worth.  It also sets up 
> a rather interesting possible tradeoff situation between launch/landing 
> system and locomotor bimodality (ie. birds are more limited in launch and 
> landing in some cases, but have the advantage of multiple locomotor 
> modules).

Yes.  I think the differing sets of tradeoffs between birds, bats, and 
pterosaurs is fascinating (and illuminating).

> 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).  As a somewhat related 
aside, the mean annual windfield near the western margin of the WIS about 
75-80 mya has been estimated at about 7 knots.  I don't know how much faith 
I put in that estimate, but it is one heckuvan average wind.  It would have 
existed for some distance inland, and if true, should have left some traces 
in the geologic record -- I wonder if anyone has searched for evidence of 
it?  For inland soaring, when the wind isn't there, convection will work (or 
micro-turbulence).  The micro-turbulence in today's cloud streets are 
sufficient to allow a Carbon Dragon to fly cross country for several hundred 
miles non-stop.  This is interesting because the Carbon Dragon has almost 
exactly the same span, aspect ratio, gross weight, and flight performance as 
Q northropi.  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. 
Remember that for a given planform, gliding range does not decrease as 
weight is increased.  A heavy animal with a given wing can glide just as far 
as a light animal with the same wing, and the heavy one will get to that 
destination sooner.

>  I agree with your point about moving between updrafts and the advantages 
> of high aspect ratios over land.  I would add the caveat (that I know 
> you're well aware of, but others may not be) that riding inland thermals 
> will often require lower wing loading in order to maintain slow speeds 
> required for tight turning to stay in the thermal updraft.

That is true, and is one of the reasons for high aspect  ratio for inland 
travelers.  They are more suited for extracting energy from the 
microturbulence in cloudstreets.  Travelers are less prone to make extensive 
use of  thermal updrafts than loiterers are and consequently, aren't as 
bound by the restrictions that are associated with tight turns and small 
diameter updrafts.  I've got some wingtip video of Gary Osoba flying a 
Windrose (similar in size, performance, and wingloading to Qn) crosscountry 
by means of cloudstreets rather than thermals.  Watching Gary's control 
movements is impressive.  Very small and precise, but also very sharply 
defined and extraordinarily quick and abrupt.  I can visualize a pterosaur 
making minute quick abrupt motions for the same reasons.  Gary also used the 
Windrose to simulate a skimming run and did about 600 yards before running 
out of energy.

> Thus, there is some give and take in wing loading, especially for species 
> that are very reliant on smaller thermals for gaining altitude.

Agreed.  But when you have a high aspect ratio, you're less likely to rely 
on smaller thermals for gaining altitude, and much less likely to rely on 
them for maintaining it.

>  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. 
Tipslots weren't an option for pterosaurs, and all pterosaurs have larger 
aspect ratios than pelicans do.  I don't think that is coincidental. (keep 
in mind when reading my conclusions that my own take on typical pterosaur 
gross planform is very similar to that of Padian, and rather similar to 
Bennett, but very different from Unwin).  Given different assumptions, one 
would draw different inferences.

> (which matches the wide range of loadings actually demonstrated by large 
> inland birds).

Yes.