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Re: Great in the air, not so good underwater

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,

PHASIANIDAE (AVES) The Journal of Experimental Biology 203: 3319–3332