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Re: Hummingbirds (was Re: Thread of Ptero size thread)
----- Original Message ----
From: Michael Habib <firstname.lastname@example.org>
Sent: Tuesday, December 19, 2006 5:57:07 PM
Subject: Hummingbirds (was Re: Thread of Ptero size thread)
> 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
> 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.
--------------- Thanks for the clarification.
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).
------------ Good ideas re soundtrack change, especially w/b amp shift. I have
to insist here that this was the most casual of observations and no
significance whatsoever was attached at the time*; that said, anybody who wants
to keep an eye/ear peeled should probably do it a low pop situation. They
aren't likely to economize while hanging out w/ the cohort... which is the
opposite of the standard lab situation, if my limited reading is an indication.
BTW-- have you seen the paper(s?) about the altitude-correlated shift from
territorial defense to trap-line foraging strategy in Andes pops?
------------- * ...other than "Damn. Little tail-hole stopped hovering just
when I was going to snap the picture..."
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).