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Re: Great in the air, not so good underwater

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.

No, they don't have to be able to hit steady state directly from a running start. All they have to hit is an anaerobic burst window, and have the ability to accelerate to steady state before the burst power runs out.

Point taken. Just the same, the running launch system has particular constraints and benefits. I should have said "to reach their burst window" rather than "to hit steady state".

And swans use a very different technique that involves (in visual extreme -- for purposes of qualitative illustration) a 'hovercrafting' technque combined with 'jet thrust augmentation' during a portion of the flapping cycle...

Yeah, swan takeoffs just blow my mind. Cool stuff. I was mostly referring to sulids and procellariiforms
when I mentioned running starts in seabirds before. I probably should have been more specific. Grebes and loons also use running starts, but those are different, as well.

However, my real point is that if flight is truly important to the animal, it or its decendants will partially sidestep the launch issue. A couple of ways to accomplish this are cliff launching and front limb assist during launching (I'm not sure how a bird would accomplish that latter, but bats and pterosaurs can). Given enough time for evolution to operate, I suspect there are hundreds of other ways as well.

I'm sure there are very many indeed. What launch modes are open seems to basically depend on the old standbys of historical constraint and selection. Constraint pretty clearly prohibits the forelimb-assisted launch cycle in birds, and thus I doubt they'll ever produce volant members as large as pterosaurs. Stabilizing selection is probably hard at work, too, though: despite denying some helpful launch modes, avian bipedality has some great advantages. In fact, being bipedal gives birds access to some launch solutions denied to pterosaurs and bats; the ones I have in mind work best at small to medium body sizes.

Still, I'll bet nothing beat seeing a Quetzalcoatlus catapault off the ground...makes me sad that they're gone.