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Re: Thread of Ptero size thread

OK. This is personal observation, and I have no written reference. Watching hummingbirds over the years I seen them fly (or thought I've seen) what appears to be forward flight fairly frequently.

They do use forward flight, of course. But they continue to use a full pronation/supination reversal stroke in which both the upstroke and downstroke impart useful momentum (ie. positive lift vector). They hover by angling this stroke in a roughly horizontal plane. By angling forward, a component of the vector imparts forward momentum, and this is how they move in forward flight. I recall reading this in Alexander (2002), Gill (1990), and I believe Tobalske's paper on hummingbird launch, as well.

It follows from what you say that 1) they _hover_ when migrating across the Gulf of Mexico,

No, but they do use the stroke system I detailed above, which means that the kinematic is very similar to hovering and has no stroke reduction (instead utilizing full stroke reversal). I suspect that their wings are uncambered, or that camber reverses. I'll check on that.

2) they use less energy hovering than level flap-flying (if they can)

No, hovering still takes more energy per unit time, just as it does on all animals that hover. The gap is not as large for hummingbirds because the stroke kinematic is essentially identical during forward flight (except for axis alteration). That has been measured, but I cannot remember by whom; I'll see if I can pull it out of my reference library.

and 3) the hovering failure density is indeed lower than their flapping failure density. Can you confirm? Got refs /obs on migration flight style?

I was saying that hummingbirds would be proportionately more heavily affected by density changes than larger birds, sorry for the confusion. They would have trouble hovering in a low density medium before they had trouble in forward flight, but the margin of difference would not be very large, given that the forward flight kinematic is essentially the same.

Also any hummingbird watchers out there?

I am, but I only get to see hovering and forward flight, not long distance flight. I rely on the high speed video and photos of others, as well as the literature, for my hummingbird kinematic information. I don't have any way of visualizing the stroke pattern on my own (ie. I don't have a high-speed camera).

Personally, I would have thought that 1) the (minimum) size constraint in hummingbirds would be from thermal relations rather than flight

Quite possible, though they might also be constrained by stroke frequency. Insects manage at smaller sizes, obviously, but they use exoskeletal elastic recoil with indirect muscle actions such that the wings don't have to be directly pulled into every stroke.

2) although their maximum size is currently constrained by hovering, they would essentially be unconstrained in the sense of evolutionary potential relative to flapping flight; indeed free to evolve to whatever max bird size potential is, if they shift gears to soaring flight.

In theory, yes, but hummingbirds are rather constrained now from evolving standard flapping flight. In particular, because the wrist and elbow are immobile, and the humerus, radius, and ulna so heavily shortened, they are limited in the amount of upstroke minimization that they can manage. This works great when utilizing full supination/pronation with upstrokes that produce useful momentum. However, at larger sizes that kinematic would not be feasible, and that would probably limit the evolution of larger size in hummingbirds. Swifts have similar forelimb changes, and they manage with a more typical flapping mode, so it must be possible to some point. Swifts are small-bodied, however, and also do not launch well from the ground. Other than the fact that they use a very interesting leading-edge vortex, I don't know that much about swift kinematics. I suspect that they still have some upstroke reduction, which hummingbirds largely lack.


--Mike H.