Re: Great in the air, not so good underwater

[Michael Habib <mhabib5@jhmi.edu>]

> I have the feeling that for any flying bird, the maximum constraints 
> on flight are usually not wing loading, they are more likely to be 
> related to launch mechanism (and sometimes, landing).

This is generally how I feel about the problem as well, though I think 
it is important to add the caveat that wing loading is pretty 
intimately related to launch for most birds.  All avian launch modes 
have some sort of wing loading limitations, but these naturally vary 
depending the launch mode in question.  Of course, the actual launch 
mode varies by both planform and hind limb structure, so it gets very 
complicated very quickly.

So, for example, large seabirds are loading limited to the extent that 
they have to be able to hit steady state from a running start and they 
can only run so fast.  Granted, that's not really a pure wing loading 
problem (they could sidestep by improved cursor ability), but it is an 
implicit factor and I suspect that the largest pseudodontorns were near 
the size limit for their particular morphotype.

Burst-takeoff specialists (like galliforms), on the other hand, I would 
expect to be limited by wing oscillation frequency.  This is limited, 
in turn, by loading, mass-specific power, or wing inertia (or all 
three), which trade off within a given planform (which is then 
constrained largely by the specific speed drag regime and need for a 
short span).  Interestingly, Tobalske and Dial indeed found wingbeat 
frequency to be the likely limiting factor on launch performance in 
four taxa of burst fliers they tested (and linked this to power needs). 
 They discuss the apparent conflict with Marden's results, without a 
satisfying explanation for the difference in results.  My suspicion is 
that both authors were right to a point, but that the tradeoff systems 
(and thus, maximum sizes) change depending on launch specifics.

Just my two cents.  Here are the paper citations I just alluded to:

Marden, J. (1994). From damselflies to pterosaurs: how burst and
sustainable flight performance scale with size. Am J. Physiol. 266,
R1077–R1084.

Tobalske, B and K. Dial (2000). EFFECTS OF BODY SIZE ON TAKE-OFF FLIGHT 
PERFORMANCE IN THE
PHASIANIDAE (AVES) The Journal of Experimental Biology 203: 3319–3332