Re: Thread of Ptero size thread

[Michael Habib <mhabib5@jhmi.edu>]

> 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.

Cheers,

--Mike H.