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Re: hovering diversity (was Re: Ornithurine diversity)



Erik Boehm (erikboehm07@yahoo.com) writes in response to my
description of kiting:

> In this case the "something else going on" is the way the bluffs
> alter the airflow such that there is an upward component/lift

Yes, I can accept that the birds are making use of an updraft imposed
by the bluffs.  I wasn't thinking clearly enough about that aspect
when I responded to Jim -- that Jim's response to Dann assumed the
windspeed was purely horizontal.  I never thought it was an accident
that 90% of the time that I've seen the behavior has been at the
bluffs which do, of course, deflect the wind coming in from off the
ocean.  The birds are *always* slightly windward of the bluffs, but I
suspect that has more to do with their understanding of the best
places and altitudes from which to hunt than aerodynamic
considerations (i.e., though I'm sure someone will quickly tell me if
I'm wrong, I'm sure the region containing updraft extends a good bit
to the leeward side of the bluffs as well, but birds never kite
there). 

In response to the same message, Michael Habib (mhabib5@jhmi.edu)
writes:

] In a constant headwind, the bird is actually gliding - by matching
] glide speed to the wind, it can have a ground-speed of zero, but it
] will still sink over time, just like any glide.

For my observations, I've got to go with the updraft explanation.
Unless by "long time":

] you simply won't notice the sink rate unless you watch one for a
] long time

you mean something on the order of minutes, hours, or days...

] the conditions used for kiting are often not a constant wind, but
] can contain gusts

In what I've seen it appears the birds don't so much appreciate the
gusts.  It is, of course, gusty around here (I've had cause to worry
about getting blown off the bluffs...), but when it gusts is when you
do see the birds move.

And again for the same message, Jim Cunningham (jrccea@bellsouth.net)
writes:

} You just proved my point.

Glad to be of service :-)

} birds usually have to resort to the 'flutter' stroke to hover at
} constant altitude

Yes, I've always thought it ironic that that's by far the most common
mechanism by which actual kites hover.

-- 
Mickey P. Rowe     (mrowe@lifesci.ucsb.edu)