Circling for rodents?

Vultures are amazing *pilots*. The ones around the Magalies gliding club in
SA are known to range as far as the Kruger National Park, over 200 km away.
I've flown with them at cloudbase as high as 14 000', had them formate on a
wingtip and once over the shoulder of a Blanik, peering into the rear
cockpit from less than a metre away.

Flying in the Drakensburg one can often end up with 30 or more birds in a
big thermal or fly in formation with a group of them along the cliff faces.

Ian





"Martin Gregorie" wrote in message
...
On 04 Jul 2004 18:48:04 GMT, ospam (Frostowits)
wrote:

Pardon this input from an uninformed intruder to this subject, but why
couldn't
birds simply sense how strong the lift is by the amount of stress it puts
on
their "airframe". When I pump iron, I'm all too aware of the amount of
effort
required. Surely birds can do the same.

Some seem to do just that. Kites in particular. I used to watch them a
lot in India and discovered that you can tell how strong the lift is
by looking at them - the stronger it is the more dihedral they use. If
its really strong they just bomb round with a steep V-form and their
tip feathers closed. Weaker lift gets more care and attention, less
dihedral and more open tip feathers. When they're really scratching
their wings are flat or even a little anhedralled and the tip feathers
are fully spread and up to give tip dihedral. They initiate a turn
with a big dab of negative in the inner tip and then control the turn
on tail tilt - the outer tail tip is raised, so you can tell that
they fly like we do with down force on the tail. Kites are easy to
read because they often work low altitude lift where you can see
exactly what they are doing and have big, long tails that are easy to
observe.

I wondered about how vultures fly but they were so seldom low enough
to really watch that I couldn't work out very much. Also, with much
shorter tails than kites its difficult to see whether they use tail
tilt at all or which way its applied.

The above is about all I know about soaring birds: I'm no
ornithologist or naturalist. My background is chemistry, competitive
free flight model flying and, latterly, soaring.

I've heard a number of theories about how birds detect thermals
including that they hear them. I'd well believe that, with a nerve on
each feather, they must *really* feel the air and all its
micro-turbulence. Maybe they can hear it too. However, that tells
something about how they work 'normal' thermals but not a lot about
how they can find and work the very weak, smooth lift you get early
and late in the day. We know that migratory birds have a excellent
directional sense so why shouldn't a soaring bird have a built-in
vario too? I'd love to know how it works.

If you haven't read it, find a copy of Philip Wills' "On Being A
Bird". There's a chapter about flying with vultures in South Africa
and how he worked just how vultures operate - altitudes, spacing, food
finding strategy etc. The whole book is a good read too.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :