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Re: Senter 2006, Confuciusornis, and humeral mobility
On Mar 27, 2011, at 2:14 PM, Don Ohmes wrote:
> IIRC, Carl Koford states among other things -- CA Condors will not seek
> food in weather not conducive to their flight abilities, even to feed
> juveniles that are deprived* -- and are EXTREMELY shy about feeding on
> carrion on flat ground, consistently being the first to appear, yet the
> last to feed on carcasses -- and often cannot take off w/ a full crop
> even in non-dead air and on non-level ground.
> Such cautious behavior, whether instinctive or learned, is logically and
> observationally indicative of flight abilities critically affected by
> commonly occurring conditions, in my opinion.
It could be indicative of such constraints; yes. It could also be related to
other ecological or behavioral factors, however. That's what makes such
behavioral observations difficult to assess with confidence. It *could* be
that CA Condors have a reduced ability to launch, but there really aren't good
data on it, which is why I cautioned against taking this at face value.
Furthermore, if it turns out that the condors do require gusts to launch, it
must be for reasons beyond simple size, as plenty of birds considerably larger
than CA Condors do not have such constraints. In fact, the constraints and
variables associated with launch from a morphological standpoint are not
terribly difficult to work with in a quantitative framework, and so we know,
with confidence, that CA Condors are nowhere near any kind of general size
limit for avian flight - they could, however, be near some local limit enforced
by a quirk of their morphology that we are unaware of.
> That said, it has been _many_ years since I read Koford, and I might be
> misremembering his work, being somewhat biased toward the idea that
> takeoffs and landings do indeed tend to be more difficult for large
> birds as opposed to small ones.
Required launch speed is related to wing loading (primarily), and the available
launch power is dependent on the hind limbs. Therefore, large birds with small
wings and weak legs will have more constraints on their ability to launch than
small ones. However, in practice, most large birds have very strong hind limbs
and therefore sufficient power to launch without special conditions. Many of
them also have disproportionately broad wings, which breaks the influence of
size on wing loading (Wing loading increases by the 3/2 power only under
geometric similarity, after all). The tradeoff between hind limb dominated
takeoff power and airborne propulsive power does mean that large birds will hit
a mechanical limit as you scale them up - but the general limit for an avian
body plan is way above anything alive today. Specific morphologies have their
own limits, however, and most birds do not fit the "general limit" model.
Hence why a particular 2 kg bird might need cliffs to launch (frigate) while an
animal 6x its size vertical launches from the ground (turkey).
> I do not consider his observations (whatever they really may be) to be
> anecdotal, however -- and further take all responsibility should I
> indeed be misquoting him.
> OTOH, I would be quite cautious personally about deducing airflow and
> ground orientation from a video, particularly in a mountain environment,
> should such be the setting.
I agree completely - I was only pointing out the uncertainty regarding the
takeoff abilities in condors. My anecdotal accounts derived from videos are no
better than the anecdotes I was questioning - the thing is, most of the
anecdotes you read about aren't any better, either. I'd consider Koford more
reliable than that general body of anecdotal work, but we both noted that the
actual constraints responsible for his observations are uncertain.
Assistant Professor of Biology
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