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