Hummingbirds (was Re: Thread of Ptero size thread)

[Michael Habib <mhabib5@jhmi.edu>]

> Mike is saying that they can find one suitible for their 
> specialization, but
> it is nowhere near optimal for flapping cruise because they are unable 
> to
> retract the wing significantly during the upstroke.  That implies an
> unavoidable, relatively low thrust coefficient in forward flight.
>
> ---------------- Actually, I get that on an "intertaxic" comparison 
> basis. I meant I don't understand why a gait within the taxa that is 
> less energy-intensive than "forward hovering" is not kinematically 
> feasible. He was speaking intra-specifically, right? If he was, then I 
> guess you just told me again, and I'll just have to wait for 
> comprehension. Maybe I will get some for Christmas.

No problem; I can explain my reasoning easily enough.  Basically, if a 
hummingbird were to shift to a stroke more kinematically similar to 
"standard" forward flight, it would do so very inefficiently, because 
it would not be able to minimize the upstroke significantly.  Given 
that the wing must go through an upstroke phase regardless, and given 
that hummingbirds have full pronation ability (though at the glenoid, 
not the distal forelimb), it is more energetically efficient for them 
to use the upstroke to produce "useful" (ie. positive lift vector) 
momentum flux than to leave the arm unreversed.  I don't see them 
having a less energy-intensive forward flight than "forward hovering" 
because they would simply get a lower thrust coefficient without much 
in return.  It is most efficient for them to reverse the wing on the 
upstroke and generate a positive force vector than to reverse their 
momentum somewhat on each upstroke.

I suspect the kinematic difference that you're hearing (nice use of the 
audio cues, btw) is either bounding flight, or a shift in wingbeat 
amplitude.  It could also be a difference in downstroke/upstroke timing 
(ie. faster reversals on one stroke or the other to make them 
asymmetrical in rate).  Hummingbirds already have a somewhat 
asymmetrical wing cycle in terms of power; Tobalske et al. showed that 
hummingbirds generate more than half of the total lift in each cycle 
off of the downstroke while hovering (presumably because the pec. major 
muscle is still bigger than pec. minor).

Cheers,

--Mike H.