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# 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 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.`