Re: hovering diversity (was Re: Ornithurine diversity)

[Michael Habib <mhabib5@jhmi.edu>]

> I fear that either your misunderstanding Dann or the birds know
> something you don't.  The phenomenon is called "kiting", and I see it
> all the time on the bluffs above the beaches near Santa Barbara.  The
> first time I saw it I was blown away because the bird really looks
> like a kite on a string...  absolutely motionless.

I am also quite familiar with kiting, and Jim is correct: a simple 
headwind will not allow the animal to maintain a constant altitude.  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.  Thus, it's not a true 
hovering dynamic.

Of course, the conditions used for kiting are often not a constant 
wind, but can contain gusts - which contain energy that can be used to 
help keep the animal aloft.  Even in the most simple form of kiting 
(gliding into a steady, oncoming wind), many birds won't appear to be 
obviously sinking because they have quite good glide ratios.  Gannets, 
for example, use the technique regularly to loiter over fish schools 
(to line up a plunge dive).  With an aspect ratio of about 14, they 
have a very good glide ratio - you simply won't notice the sink rate 
unless you watch one for a long time (and it'll probably adjust or dive 
before it ever moves downward enough to be perceptible).

Cheers,

--Mike


Michael Habib, M.S.
PhD. Candidate
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280 0181
habib@jhmi.edu